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Title: Rock Blasting - A Practical Treatise on the Means Employed in Blasting - Rocks for Industrial Purposes
Author: André, Geo. G.
Language: English
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Transcriber’s Notes
Texts printed in italics in the source document have been transcribed
_between underscores_; small capitals have been replaced by ALL
CAPITALS. ^{T} represents a superscript T, ~T~ a T-shape part rather
than the letter T.
More Transcriber’s Notes will be found at the end of this text.
ROCK BLASTING.
ROCK BLASTING.
A
PRACTICAL TREATISE
ON THE
MEANS EMPLOYED IN BLASTING ROCKS
FOR INDUSTRIAL PURPOSES.
BY
GEO. G. ANDRÉ, F.G.S., ASSOC. INST. C.E.,
MINING CIVIL ENGINEER; MEMBER OF THE SOCIETY OF ENGINEERS.
[Illustration]
LONDON:
E. & F. N. SPON, 46, CHARING CROSS.
NEW YORK:
446, BROOME STREET.
1878.
PREFACE.
During the past decade, numerous and great changes have taken place in
the system followed and the methods adopted for blasting rocks in
industrial operations. The introduction of the machine drill led
naturally to these important changes. The system which was suitable to
the operations carried on by hand was inefficient under the requirements
of machine labour, and the methods which had been adopted as the most
appropriate in the former case were found to be more or less unsuitable
in the latter. Moreover, the conditions involved in machine boring are
such as render necessary stronger explosive agents than the common
gunpowder hitherto in use, and a more expeditious and effective means of
firing them than that afforded by the ordinary fuse. These stronger
agents have been found in the nitro-cotton and the nitro-glycerine
compounds, and in the ordinary black powder improved in constitution
and fired by detonation; and this more expeditious and effective means
of firing has been discovered in the convenient application of
electricity. Hence it is that the changes mentioned have been brought
about, and hence, also, has arisen a need for a work like the present,
in which the subjects are treated of in detail under the new aspects due
to the altered conditions.
GEO. G. ANDRÉ.
LONDON, 17, KING WILLIAM STREET, STRAND,
_January 1st, 1878._
CONTENTS.
CHAPTER I.
THE TOOLS, MACHINES, AND OTHER APPLIANCES USED IN ROCK BLASTING.
PAGE
Section I. _Hand-boring Tools._--Drills. Hammers. Auxiliary
Tools. Sets of Blasting Gear 1
Section II. _Machine-boring Tools._--Machine Rock-drills. Borer-
bits. Drill Carriages 23
Section III. _Appliances for firing Blasting Charges._--Squibs.
Safety Fuse. Electric Fuses. Cables. Detonators. Electric Firing-
Machines 42
CHAPTER II.
EXPLOSIVE AGENTS USED IN ROCK BLASTING.
Section I. _Phenomena accompanying an Explosion._--Nature of an
Explosion. Heat liberated by an Explosion. Gases generated by an
Explosion. Force developed by an Explosion 64
Section II. _Nature of Explosive Agents._--Mechanical Mixtures.
Chemical Compounds 76
Section III. _Relative Strength of the common Explosive Agents._--
Force developed by Gunpowder. Relative Force developed by
Gunpowder, Gun-cotton, and Nitro-Glycerine 88
Section IV. _Means of firing the common Explosive Agents._--Action
of Heat. Detonation 92
Section V. _Some Properties of the common Explosive Agents._--
Gunpowder, Gun-cotton, Dynamite. Firing Temperatures 97
Section VI. _Some Varieties of the Nitro-Cellulose and the Nitro-
Glycerine Compounds._--Nitrated Gun-cotton. Tonite, or Cotton-
Powder. Schultze’s Powder. Lithofracteur. Brain’s Powder.
Cellulose-Dynamite 103
CHAPTER III.
THE PRINCIPLES OF ROCK BLASTING.
Line of least Resistance. Force required to cause Disruption.
Conditions of Disruption. Example of a Heading. Economical
Considerations. Tamping 106
CHAPTER IV.
THE OPERATIONS OF ROCK BLASTING.
_Hand Boring._--Boring the Shot-holes. Charging the Shot-holes.
Firing the Charges 128
_Machine Boring._--Boring the Shot-holes. Charging and Firing.
Removing the dislodged Rock. Division of Labour 142
_Examples of Drivings._--The St. Gothard Tunnel. The Hoosac
Tunnel. The Musconetcong Tunnel. Headings at Marihaye, Anzin, and
Ronchamp 157
CHAPTER V.
SUBAQUEOUS BLASTING.
Preparation of the Charge. Boring under Water. Submarine Rocks.
Obstructions in Water-courses 164
ROCK BLASTING.
CHAPTER I.
THE TOOLS, MACHINES, AND OTHER APPLIANCES USED IN BLASTING ROCKS.
SECTION I.--HAND BORING.
_Drills._--The operations of blasting consist in boring suitable holes
in the rock to be dislodged, in inserting a charge of some explosive
compound into the lower portion of these holes, in filling up,
sometimes, the remaining portion of the holes with suitable material,
and in exploding the charge. The subjects which naturally first present
themselves for consideration are: the nature, form, and construction of
the tools, machines, and other appliances used. Of these tools, the
“drill” or “borer” constitutes the chief. To understand clearly the
action of the rock drill, we must consider the nature of the substance
which has to be perforated. He who has examined the mineral constitution
of rocks will have recognised the impossibility of _cutting_ them, using
that term in its ordinary acceptation, inasmuch as the rock constituents
are frequently harder than the material of the tools employed to
penetrate them. As a rock cannot be cut, the only way of removing
portions of it is to fracture or to disintegrate it by a blow delivered
through the medium of a suitable instrument. Each blow so delivered may
be made to chip off a small fragment, and by this means the rock may be
gradually worn away. To effect this chipping, however, the instrument
used must present only a small surface to the rock, in order to
concentrate the force, and that surface must be bounded by inclined
planes or wedge surfaces, to cause a lateral pressure upon the particles
of rock in contact with them. In other words, the instrument must be
provided with an edge similar to that possessed by an ordinary _cutting_
tool.
The conditions under which the instrument is worked are obviously such
that this edge will be rapidly worn down by attrition from the hard rock
material, and by fracture. To withstand these destructive actions, two
qualities are requisite in the material of which the instrument is
composed, namely, hardness and toughness. Thus there are three important
conditions concurring to determine the nature and the form of a cutting
tool to be used in rock boring--1, a necessity for a cutting edge; 2, a
necessity for a frequent renewal of that edge; and 3, a necessity for
the qualities of hardness and toughness in the material of the tool.
In very hard rock, a few minutes of work suffice to destroy the cutting
edge, and then the tool has to be returned to the smithy to be
re-sharpened. Hence it is manifest that the form of the edge should not
be one that is difficult to produce, since, were it so, much time would
be consumed in the labour of re-sharpening. Experience has shown that
the foregoing conditions are most fully satisfied in the steel rod
terminating in a simple chisel edge, now universally adopted.
This form of drill is exhibited in Fig. 1, which represents a common
“jumper” borer. It consists of a rod terminating at each end in a chisel
edge, and having a swell, technically described as the “bead,” between
the extremities to give it weight. The bead divides the jumper into two
unequal portions, each of which constitutes a chisel bit, with its shank
or “stock.” The shorter stock is used while the hole is shallow, and the
longer one to continue it to a greater depth.
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
With the jumper, the blow is obtained from the direct impact of the
falling tool. The mode of using the instrument is to lift it with both
hands to a height of about a foot, and then to let it drop. In lifting
the jumper, care is taken to turn it partially round, that the edge may
not fall twice in the same place. By this means, the edge is made to act
most favourably in chipping away the rock, and the hole is kept fairly
circular. So long as the holes are required to be bored vertically
downwards, the jumper is a convenient and very efficient tool, and hence
in open quarrying operations, it is very commonly employed. But in
mining, the shot-holes are more often required to be bored in some other
direction, or, as it is termed, “at an angle;” that is, at an angle with
the vertical. Or it may be that a shot-hole is required to be bored
vertically upward. It is obvious that in any one of these directions the
jumper is useless. To meet the requirements of such cases, recourse is
had to the hammer wherewith to deliver the blow, and the drill is
constructed to be used with the hammer. We have a suitable form of tool
for application in this wise when we cut out the bead of the jumper and
leave the ends flat for a striking face, as shown in Figs. 2 and 3. The
form of the two chisels thus obtained is that adopted for the ordinary
rock drill.
It will be understood from these descriptions that a rock drill consists
of the chisel edge or _bit_, the _stock_, and the _striking face_.
Formerly drills were made of wrought iron, and steeled at each end to
form the bit and the striking face. Now they are commonly made of cast
steel, which is supplied for that purpose in octagonal bars of the
requisite diameter. The advantages offered by steel stocks are numerous.
The superior solidity of texture of that material renders it capable of
transmitting the force of a blow more effectively than iron. Being
stronger than the latter material, a smaller diameter of stock, and,
consequently, a less weight, are sufficient. This circumstance also
tends to increase the effect of the blow by diminishing the mass through
which it is transmitted. On the other hand, a steel stock is more easily
broken than one of iron.
The cutting edge of a drill demands careful consideration. To enable the
tool to free itself readily in the bore-hole, and also to avoid
introducing unnecessary weight into the stock, the bit is made wider
than the latter; the difference in width may be as much as 1 inch. It is
evident that in hard rock, the liability of the edge to fracture
increases as the difference of width. The edge of the drill may be
straight or slightly curved. The straight edge cuts its way somewhat
more freely than the curved, but it is weaker at the corners than the
latter, a circumstance that renders it less suitable for very hard rock.
It is also slightly more difficult to forge. The width of the bit
varies, according to the size of the hole required, from 1 inch to 2½
inches. Figs. 4, 5, and 6 show the straight and the curved bits, and the
angles of the cutting edges for use in rock.
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
[Illustration: FIG. 6.]
The stock is octagonal in section; it is made in lengths varying from
20 inches to 42 inches. The shorter the stock the more effectively does
it transmit the force of the blow, and therefore it is made as short as
possible. For this reason, several lengths are employed in boring a
shot-hole, the shortest being used at the commencement of the hole, a
longer one to continue the depth, and a still longer one, sometimes, to
complete it. To ensure the longer drills working freely in the hole, the
width of the bit should be very slightly reduced in each length. It has
already been remarked that the diameter of the stock is less than the
width of the bit; this difference may be greater in coal drills than in
rock or “stone” drills; a common difference in the latter is ⅜ of an
inch for the longer. The following proportions may be taken as the
average adopted:--
+---------+---------+
| Width of| Diameter|
| the Bit.| of the |
| | Stock. |
+---------+---------+
|1 inch | ⅝ inch |
|1⅛ „ | ¾ „ |
|1¼ „ | ⅞ „ |
|1½ „ |1 „ |
|1¾ „ |1⅛ „ |
|2 inches|1⅜ „ |
|2¼ „ |1½ „ |
|2½ „ |1⅝ „ |
+---------+---------+
The striking face of the drill should be flat. The diameter of the face
is less than that of the stock in all but the smallest sizes, the
difference being made by drawing in the striking end. The amount of
reduction is greater for the largest diameters; that of the striking
face being rarely more than one-eighth of an inch.
The making and re-sharpening of rock drills constitute an extremely
important part of the labour of the mine smith. The frequent use of the
drill, and its rapid wear, necessitate a daily amount of work of no
trifling proportions, and the judgment and skill required in proper
tempering render some degree of intelligence in the workman
indispensable; indeed, so much depends upon the smith whose duty it is
to repair the miners’ tools, that no pains should be spared to obtain a
man capable of fulfilling that duty in the most efficient manner
possible.
When the borer-steel bars are supplied to the smith, he cuts them up, as
required, into the desired lengths. To form the bit, the end of the bar
is heated and flattened out by hammering to a width a little greater
than the diameter of the hole to be bored. The cutting edge is then
hammered up with a light hammer to the requisite angle, and the corners
beaten in to give the exact diameter of the bore-hole intended. As the
drills are made in sets, the longer stocks will have a bit slightly
narrower than the shorter ones, for reasons already given. The edge is
subsequently touched up with a file. In performing these operations,
heavy hammering should be avoided, as well as high heats, and care
should be taken in making the heat that the steel should be well covered
with coal, and far enough removed from the tuyere to be protected from
the “raw” air. Overheated or “burned” steel is liable to fly, and drills
so injured are useless until the burned portion has been cut away.
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
[Illustration: FIG. 9.]
Both in making and in re-sharpening drills, great care is required to
form the cutting edge evenly, and of the full form and dimensions. If
the corners get hammered in, as shown in Fig. 7, they are said to be
“nipped,” and the tool will not free itself in cutting. When a
depression of the straight, or the curved, line forming the edge occurs,
as shown in Fig. 8, the bit is said to be “backward,” and when one of
the corners is too far back, as in Fig. 9, it is spoken of as
“odd-cornered.” When either of these defects exist--and they are
unfortunately common--not only does the bit work less effectively on the
rock, but the force of the blow is thrown upon a portion only of the
edge, which, being thereby overstrained, is liable to fracture.
The hardening and tempering of steel is a matter requiring careful study
and observation. It is a well-known fact that a sudden and great
reduction of temperature causes a notable increase of hardness in the
metal. The reason of this phenomenon is not understood, but it is
certain that it is in some way dependent upon the presence of carbon.
The degree of hardness imparted to steel by this means depends upon the
amount of the reduction of the temperature, and the proportion of carbon
present in the metal, highly carburetted steel being capable of
hardening to a higher degree, under the same conditions, than steel
containing less carbon. Thus, for steel of the same quality, the wider
the range of temperature the higher is the degree of hardness. But here
we encounter another condition, which limits the degree of hardness
practically attainable.
The change which takes place among the molecules of the metal in
consequence of the change of temperature causes internal strains, and
thereby puts portions in a state of unequal tension. This state renders
the strained parts liable to yield when an additional strain is thrown
upon them while the tool is in use; in other words, the brittleness of
the steel increases with its hardness. Here again the proportion of
carbon present comes into play, and it must be borne in mind that for
equal degrees of hardness the steel which contains the least carbon
will be the most brittle. In hardening borer-steel, which has to combine
as far as possible the qualities of hardness and toughness, this matter
is one deserving careful attention. It is a remarkable fact, and one of
considerable practical value, that when oil is employed as the cooling
medium instead of water, the toughness of steel is enormously increased.
The tempering of steel, which is a phenomenon of a similar character to
that of hardening, also claims careful consideration. When a bright
surface of steel is subjected to heat, a series of colours is produced,
which follow each other in a regular order as the temperature increases.
This order is as follows: pale yellow, straw yellow, golden yellow,
brown, brown and purple mingled, purple, light blue, full clear blue,
and dark blue. Experience has shown that some one of these colours is
more suitable than the rest for certain kinds of tools and certain
conditions of working.
The selection of the proper colour constitutes a subject for the
exercise of judgment and skill on the part of the smith. For rock
drills, straw colour is generally the most suitable when the work is in
very hard rock, and light blue when the rock is only of moderate
hardness.
The processes of hardening and tempering drills are as follows: When the
edge of the bit has been formed in the manner already described, from 3
to 4 inches of the end is heated to cherry redness, and dipped in cold
water to a depth of about an inch to harden it. While in the water, the
bit should be moved slightly up and down, for, were this neglected, the
hardness would terminate abruptly, and the bit would be very liable to
fracture along the line corresponding with the surface of the water. In
cold weather, the water should be slightly warmed, by immersing a piece
of hot iron in it, before dipping the steel. When a sufficient degree of
hardness has been attained, the remainder of the hot portion is immersed
until the heat is reduced sufficiently for tempering. At this stage it
is withdrawn, and the colours carefully watched for. The heat which is
left in the stock will pass down to the edge of the bit, and as the
temperature increases in that part the colours will appear in regular
succession upon the filed surface of the edge. When the proper hue
appears, the whole drill is plunged into the water and left there till
cold, when the tempering is complete. When the edge is curved or
“bowed,” the colours will reach the corners sooner than the middle of
the bit. This tendency must be checked by dipping the corners in the
water, for otherwise the edge will not be of equal hardness throughout.
As the colour can be best observed in the dark, it is a good plan to
darken that portion of the smithy in which tempering is being carried
on.
The degree of temper required depends upon the quality of the steel and
the nature of the work to be performed. The larger the proportion of
carbon present in the metal, the lower must be the temper. Also the
state of the blunted edges, whether battered or fractured, will show
what degree of hardness it is desirable to produce. From inattention to
these matters, good steel is not unfrequently condemned as unsuitable.
To form the striking face, the end of the stock is heated to a dull red,
and drawn out by a hammer to form a conical head. The extremity is then
flattened to form a face from ½ inch to 1 inch in diameter. This head is
then annealed to a degree that will combine considerable toughness with
hardness. The constant blows to which the head is subjected tend to wear
it down very rapidly. There is great difference in the lasting qualities
of steel in this respect; some drills will wear away more quickly at the
striking than at the bit end.
A smith will, with the assistance of a striker, sharpen and temper about
thirty single-hand drills of medium size in an hour, or twenty
double-hand drills of medium size in the same time. Of course, much will
depend on the degree of bluntness in the cutting edge; but assuming the
drills to be sent up only moderately blunted, this may be taken as a
fair average of the work of two men.
It will be evident from the foregoing remarks, that to enable a drill to
stand properly it must be made of good material, be skilfully tempered
in the smithy, and provided with a cutting edge having an angle and a
shape suited to the character of the rock in which it is used. To these
conditions, may be added another, namely, proper handling; for if the
drill be carelessly turned in the hole so as to bring all the work upon
a portion only of the cutting edge, or unskilfully struck by the sledge,
fracture or blunting will speedily result. Improper handling often
destroys the edge in the first five minutes of using.
Drills, as before remarked, are used in sets of different lengths. The
sets may be intended for use by one man or by two. In the former case,
the sets are described as “single-hand” sets, and they contain a hammer
for striking the drills; in the latter case, the sets are spoken of as
“double-handed,” and they contain a sledge instead of a hammer for
striking. It may appear at first sight that there is a waste of power in
employing two men, or, as it is termed, the double set, for that two men
cannot bore twice as fast as one. This rate of speed can, however, be
obtained, and is due less to the greater effectiveness of the stroke
than to the fact that two men can, by repeatedly changing places with
each other, keep up almost without intermission a succession of blows
for an indefinite length of time; whereas, with the single set, the man
is continually obliged to cease for rest.
_Hammers._--To deliver the blow upon a rock drill, hammers and sledges
are used. The distinction between a hammer and a sledge is founded on
dimensions only: the hammer being intended for use in _one_ hand, is
made comparatively light and is furnished with a short handle, while the
sledge, being intended for use in _both_ hands, is furnished with a much
longer handle and is made heavier. The striking face of the blasting
sledge should be flat, to enable the striker to deliver a direct blow
with certainty upon the head of the drill; and to facilitate the
directing of the blow, as well as to increase its effect, the mass of
metal composing the head should be concentrated within a short length.
To cause the sledge to fly off from the head of the drill in the case of
a false blow being struck, and thereby to prevent it from striking the
hand of the man who holds the drill, the edges of the striking face
should be chamfered or bevelled down till the diameter is reduced by
nearly one-half. This requirement is, however, but seldom provided for.
[Illustration: FIG. 10.]
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
[Illustration: FIG. 13.]
The head of a sledge is of iron; it consists of a pierced central
portion called the “eye,” and two shanks or “stumps,” the steeled ends
of which form the striking faces or “panes.” The form of the head varies
in different localities, but whatever the variations may be, the form
may be classed under one of four types or “patterns.” A very common
form is that shown in Fig. 10 and known as the “bully” pattern. By
varying the width, as shown in Fig. 11, we obtain the “broad bully,” the
former being called for the sake of distinction the “narrow” bully.
Another common form is the “pointing” pattern, represented in Fig. 12.
The form shown in Fig. 13 is designated as the “bloat” pattern; and that
given in Fig. 14 the “plug” pattern. Each of these forms possesses
peculiar merits which renders it more suitable for certain uses than
the others. The same forms are used for hammers. The eye is generally
made oval in shape, but sometimes, especially with the bloat pattern, it
is made circular, as shown in Fig. 13. The weight of a sledge head may
vary from 5 lb. to 10 lb., but a common and convenient weight is 7 lb.
The length of the helve varies from 20 inches to 30 inches; a common
length for blasting sledges is 24 inches. The average weight of hammer
heads is about 3 lb., and the average length of the helve 10 inches.
[Illustration: FIG. 14.]
[Illustration: FIG. 15.]
Fig. 15 represents a blasting sledge used in South Wales. The stumps are
octagonal in section, and spring from a square block in the centre. The
panes or striking faces, however, are circular and flat. The length of
the head is 8¾ inches, and that of the helve 27 inches, and the weight
of the tool complete 7 lb.
[Illustration: FIG. 16.]
Fig. 16 represents a blasting sledge used in North Wales. The central
block is an irregular octagon in section, formed by slightly chamfering
the angles of a square section, and the stumps are chamfered down to
form a regular octagon at the panes, which are flat. The length of the
head is 7¾ inches, and that of the helve 22 inches, and the weight of
the tool complete 6 lb. 7 oz.
[Illustration: FIG. 17.]
The sledges used in the north of England have shorter heads, and are
lighter than the foregoing. Fig. 17 represents one of these blasting
sledges. The head is nearly square in section at the centre, and the
panes are flat. The length of the head is 5 inches, and that of the
helve 24½ inches, and the weight of the sledge complete 4 lb. 14 oz.
_Auxiliary Tools._--Besides the drill and the hammer, other tools are
needed in preparing the hole for the blasting charge. If the bore-hole
is inclined downwards, the débris or “bore-meal” made by the drill
remains on the bottom of the hole, where it is converted into mud or
“sludge” by the water there present. This sludge has to be removed as
the work progresses, to keep the rock exposed to the action of the
drill. The removal of the sludge is effected by a simple tool called a
“scraper.” It consists of a rod of iron from ¼ inch to ½ inch in
diameter, and of sufficient length to reach the bottom of the bore-hole.
One end of the rod is flattened out on the anvil and made circular in
form, and then turned up at right angles to the stem. The disc thus
formed must be less in diameter than the bore-hole, to allow it to pass
readily down. When inserted in the hole, the scraper is turned round
while it is being pressed to the bottom; on withdrawing the instrument,
the sludge is brought up upon the disc. The operation, two or three
times repeated, is sufficient to clear the bore-hole. The other end of
the scraper is sometimes made to terminate in a ring for convenience in
handling, as shown in Fig. 18. Instead of the ring, however, at one end,
a disc may be made at each end, as shown in Fig. 19, the discs in this
case being of different diameter, to render the scraper suitable for
different size bore-holes. Sometimes the scraper is made to terminate
in a spiral hook or “drag-twist,” as represented in Fig. 20. The use of
the drag is to thoroughly cleanse the hole before inserting the charge.
A wisp of hay is pushed down the hole, and the drag end of the scraper
introduced after it, and turned round till it has become firmly
entangled. The withdrawal of the hay by the drag wipes the bore-hole
clean. Instead of the twist drag, the “loop” drag is frequently
employed. This consists of a loop or eye, through which a piece of rag
or tow is passed. The rag or tow is used for the same purpose as the
hay, namely, to thoroughly cleanse and dry the bore-hole previous to the
introduction of the charge. Very frequently the “swab-stick” is used
instead of the scraper to clear out the bore-hole. This is simply a deal
rod bruised at one end by blows with a hammer until the fibres separate
to form a kind of stumpy brush or “swab.” When this is pushed down the
hole, the sludge passes up around and between the fibres, which are then
spread out by being pressed against the bottom of the hole. On
withdrawing the swab, the sludge is brought out with it.
[Illustration: FIG. 18.]
[Illustration: FIG. 19.]
[Illustration: FIG. 20.]
When the charge has been placed in the bore-hole, and the fuse laid to
it, the hole needs to be tamped, that is, the portion above the charge
has to be filled up with some suitable substance. For this purpose, a
“rammer,” “stemmer,” or “tamping iron,” as the instrument is variously
called, is required. This instrument is illustrated in Fig. 21. It
consists of a metal bar, the tamping end of which is grooved to receive
the fuse lying against the side of the bore-hole. The other end is flat,
to afford a pressing surface for the hand, or a striking face for the
hammer when the latter is needed. To prevent the danger of accidental
ignition from sparks caused by the friction of the metal against
silicious substances, the employment of iron stemmers has been
prohibited by law. They are usually made of copper or phosphor-bronze,
the latter substance being more resisting than the former.
[Illustration: FIG. 21.]
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
Sometimes in wet ground it becomes necessary to shut back the water from
the bore-hole before introducing the charge of gunpowder. This happens
very frequently in shaft sinking. The method employed in such cases is
to force clay into the interstices through which the water enters. The
instrument used for this purpose is the “claying-iron” or “bull,”
represented in Fig. 22. It consists of a round bar of iron, called the
stock or shaft, a little smaller in diameter than the bore-hole, and a
thicker portion, called the head or poll, terminating in a striking
face. The lower end of the shaft is pointed, to enable it to penetrate
the clay, and the head is pierced by a hole about an inch in diameter to
receive a lever. Clay in a plastic state having been put into the
bore-hole, the bull is inserted and driven down by blows with the
sledge. As the shaft forces its way down, the clay is driven into the
joints and crevices of the rock on all sides. To withdraw the bull, a
bar of iron is placed in the eye and used as a lever to turn it round to
loosen it; the rod is then taken by both hands and the bull lifted out.
To allow the bull to be withdrawn more readily, the shaft should be made
with a slight taper and kept perfectly smooth. As the bull is subjected
to a good deal of heavy hammering on the head, the latter part should be
made stout. This tool, which should be considered as an extra instrument
rather than as an essential part of a blasting set, is a very
serviceable one, and should always be at hand in wet ground when loose
gunpowder is employed.
Another instrument of this auxiliary character is the beche, Fig. 23,
used for extracting a broken drill. It consists of an iron rod of nearly
the diameter of the bore-hole, and hollow at the lower end. The form of
the aperture is slightly conical, so that the lower end may easily pass
over the broken stock of the drill, and, on being pressed down with
some force, may grasp the stock in the higher portion of the aperture
with sufficient firmness to allow of the two being raised together. When
only a portion of the bit remains in the hole, it may often be extracted
by means of the drag-twist end of the scraper, or the swab-stick may be
driven down upon the broken portion, and latter withdrawn with the swab.
_Sets of Blasting Gear._--On Plates I., II., and III., will be found
three sets of blasting gear; a set of coal-blasting gear; a set of
single-hand stone-blasting gear; and a set of double-hand stone-blasting
gear. In the first set, the drill, shown in Fig. 1, is 22 inches in
length; the cutting edge is straight and 1½ inch wide, and the weight is
2½ lb. The other drill, Fig. 2, is 42 inches in length; it has a
straight cutting edge 1⁷/₁₆ inch wide, and weighs 4 lb. 10 oz. The
hammer used in this set and shown in Fig. 3 weighs 2 lb. 14 oz.; the
length of the head is 4½ inches, and that of the handle 7¾ inches. In
the second or single-hand stone set, the shorter drill, Fig. 6, Plate
II., is 22 inches in length; the cutting edge is strongly curved, and is
1½ inch in width, and the weight is 3 lb. 10 oz. The longer drill, Fig.
7, is 36 inches in length; the width of the cutting edge, which is
curved as in the shorter drill, is 1⁷/₁₆ inch, and the weight is 6 lb. 5
oz. The hammer used with this set, and represented in Fig. 8, weighs 3
lb. 6 oz.; the length of the head is 5 inches, and that of the handle
10 inches. In the third or double-hand stone set, Plate III., the first
or shortest drill, Fig. 12, is 18 inches in length, 1¾ inch wide on the
cutting edge, and weighs 4¼ lb. The second drill, Fig. 13, is 27 inches
in length, 1¹¹/₁₆ wide on the cutting edge, and weighs 6 lb. The third
or longest drill, Fig. 14, is 40 inches in length, 1⅝ inch wide on the
cutting edge, and weighs 9¼ lb. The cutting edges of all these drills
are strongly curved as in the preceding set. The sledge used with this
set, and represented in Fig. 15, weighs about 5 lb.
SECTION II.--MACHINE BORING.
_Machine Rock-Drills._--The most remarkable advance, which in recent, or
perhaps in any, times has been made in the practice of mining consists
in the substitution of machine for hand labour in rock boring. The
importance of this change is obvious, and very great. Not only is the
miner relieved thereby of the labour of boring, but the speed with which
the shot-holes may be bored is increased a hundredfold. This gain of
speed offers many practical advantages. The ability to sink a shaft or
to drive a heading rapidly may ensure the success of an undertaking, and
save indirectly the expenditure of large sums of money; and, in all
cases, it allows the time spent in preparatory work to be materially
shortened. Indeed, it would be difficult to over-estimate the magnitude
of the advantage accruing from the increased rate of progress due to
the substitution of machine power for hand labour, and in the future we
may expect to see its application greatly extended. In making this
substitution, numerous difficulties have had to be overcome, and in
encountering these many failures have had to be recorded. But it must
now be conceded by the most prejudiced that rock-boring machines have
successfully passed through what may be described as the tentative stage
of their existence, and have taken a foremost place among the mechanical
appliances which experience has shown to be capable of effectually
performing the work required of them. In the author’s work on ‘Mining
Engineering,’ the requirements of a rock drill will be found fully
discussed, and the principles and the construction of the most important
machines now in use carefully explained and described. In the present
work, only one example can be given.
Machine drills penetrate rock in the same way as the ordinary hand
drills already described, namely, by means of a percussive action. The
cutting tool is in most cases attached directly to the piston rod, with
which it consequently reciprocates. Thus the piston with its rod is made
to constitute a portion of the cutting tool, and the blow is then given
by the direct action of the steam, or the compressed air, upon the tool.
As no work is done upon the rock by the back stroke of the piston, the
area of the forward side is reduced to the dimensions necessary only to
lift the piston, and to overcome the resistance due to the friction of
the tool in the bore-hole. The piston is made to admit steam or air into
the cylinder, and to cut off the supply, and to open the exhaust, as
required, by means of tappet valves, or other suitable devices; and
provision is made to allow, within certain limits, a variation in the
length of the stroke. During a portion of the stroke, means are brought
into action to cause the piston to rotate to some extent, for the
purposes that have been already explained. To keep the cutting edge of
the tool up to its work, the whole machine is moved forward as the rock
is cut away. This forward or “feed” motion is usually given by hand, but
in some cases it is communicated automatically. The machine is supported
upon a stand or framing which varies in form according to the situation
in which it is to be used. This support is in all cases constructed to
allow of the feed motion taking place, and also of the cutting tool
being directed at any angle. The support for a rock drill constitutes an
indispensable and a very important adjunct to the machine, for upon the
suitability of its form, material, and construction, the efficiency of
the machine will largely depend.
The foregoing is a general description of the construction and mode of
action of percussive rock-drills. The numerous varieties now in use
differ from each other rather in the details of their construction than
in the principles of their action, and the importance of the difference
is, of course, dependent upon that of the details. It is but just to
remark here that the first really practical solution of the
rock-drilling problem is due to M. Sommeiller, whose machine was
employed in excavating the Mont Cenis tunnel.
_The Darlington Drill._--The machine which, in England, has stood the
test of experience most satisfactorily, and which, consequently, is
surely working itself into general favour in this country, and also in
some of the important mining districts of the Continent, is the
invention of John Darlington, and is known as the “Darlington drill.”
This drill is remarkable as the attainment of the highest degree of
simplicity of parts possible in a machine. The valve gear of a machine
drill is especially liable to derangement. It must necessarily consist
of several parts, and these parts must as necessarily be of a somewhat
fragile character. Besides this, when actuated by the piston through the
intervention of tappets, the violence of the blow delivered at each
stroke is such as to rapidly destroy the parts. In some machines, the
force of these blows and their destructive tendency have been reduced to
a minimum; but when every means of remedying the evil has been employed,
there remains a large amount of inevitable wear and tear, and a
liability to failure from fracture or displacement exists in a greater
or less degree. Moreover, as these effects are greatly intensified by
increasing the velocity of the piston, it becomes at least undesirable
to use a high piston speed. To remedy these defects, which are inherent
in the system, Darlington proposed to remove altogether the necessity
for a valve gear by radically changing the mode of admitting the motor
fluid to the cylinder. This proposal he has realized in the machine
which is illustrated on Plate IV.
The Darlington rock-drill consists essentially of only two parts: the
cylinder A, Figs. 20 and 21, with its cover; and the piston B, with its
rod. The cover, when bolted on, forms a part of the cylinder; the piston
rod is cast solid with the piston, and is made sufficiently large at its
outer end to receive the tool. These two parts constitute an engine, and
with less than one fixed and one moving part it is obviously impossible
to develop power in a machine by the action of an elastic fluid. The
piston itself is made to do the work of a valve in the following manner:
The annular space affording the area for pressure on the fore part of
the piston gives a much smaller extent of surface than that afforded by
the diameter of the cylinder, as shown in the drawing; and it is obvious
that by increasing or diminishing the diameter of the piston rod, the
area for pressure on the one side of the piston may be made to bear any
desired proportion to that on the other side. The inlet aperture, or
port C, being in constant communication with the interior of the
cylinder, the pressure of the fluid is always acting upon the front of
the piston, consequently when there is no pressure upon the other side,
the piston will be forced backward in the cylinder. During this backward
motion, the piston first covers the exhaust port D, and then uncovers
the equilibrium port E, by means of which communication is established
between the front and back ends of the cylinder, and, consequently, the
fluid is made to act upon both sides of the piston. The area of the back
face of the piston being greater than that of the front face by the
extent occupied by the piston rod, the pressure upon the former first
acts to arrest the backward motion of the piston, which, by its
considerable weight and high velocity, has acquired a large momentum,
and then to produce a forward motion, the propelling force being
dependent for its amount upon the difference of area on the two sides of
the piston. As the piston passes down, it cuts off the steam from the
back part of the cylinder and opens the exhaust. The length or thickness
of the piston is such that the exhaust port D is never open to its front
side, but, in the forward stroke, it is opened almost immediately after
the equilibrium port is closed, and nearly at the time of striking the
blow. It will be observed that the quantity of fluid expended is only
that which passes over to the back face of the piston, since that which
is used to effect the return stroke is not discharged.
The means employed to give a rotary motion to the tool are deserving of
special attention, as being simple in design, effective in action, and
well situate within the cylinder. These means consist of a spiral or
rifled bar H, having three grooves, and being fitted at its head with a
ratchet wheel G, recessed into the cover of the cylinder. Two detents J,
J, Fig. 22, also recessed into the cover, are made to fall into the
teeth of the ratchet wheel by spiral springs. These springs may, in case
of breakage, be immediately renewed without removing the cover. It will
be observed that this arrangement of the wheel and the detents allow the
spiral bar H to turn freely in one direction, while it prevents it from
turning in the contrary direction. The spiral bar drops into a long
recess in the piston, which is fitted with a steel nut made to
accurately fit the grooves of the spiral. Hence the piston, during its
instroke, is forced to turn upon the bar; but, during its outstroke, it
turns the bar, the latter being free to move in the direction in which
the straight outstroke of the piston tends to rotate it. Thus the
piston, and with it the tool, assumes a new position after each stroke.
The mode of fixing the cutting tool to the piston rod is a matter
deserving some attention. As the tool has to be changed more than once
during the progress of a bore-hole, it is important that the change
should be accomplished in as short a time as possible; and as the
vibration of the machine and the strain upon the tool are necessarily
great, it is equally important that the tool be firmly held. It is also
desirable that the mode of fixing the tool shall not require a shoulder
upon the latter, a slot in it, or any peculiarity of form difficult to
be made in the smithy. The Darlington machine fulfils the requirements
of expedition in fixing, firmness of retention, and simplicity of form
most satisfactorily. The means and the method are the following: The
outer end of the rod or holder is first flattened to afford a seat for
the nut, as shown in Figs. 21 and 25. The slot is then cut and fitted
tightly with a piece of steel K forged of the required shape for the
clamp, and the holder is afterwards bored to receive the tool while the
clamp is in place. This clamp K is then taken out, its fittings eased a
little, and its end screwed and fitted with a nut. When returned to its
place in the holder, the clamp, in consequence of the easing, can be
easily drawn tight against the tool, by which means it is firmly held in
position. The shank of the tool is turned to fit the hole easily, and
the end of it is made hemispherical to fit the bottom of the hole, upon
which the force of the reaction of the blow is received.
It would seem impossible to attain a higher degree of simplicity of
form, or to construct a machine with fewer parts. The absence of a valve
or striking gear of any kind ensures the utmost attainable degree of
durability, and allows a high piston speed to be adopted without risk or
injury. As the piston controls its own motion, there is no liability to
strike against the cylinder cover. The stroke may be varied in length
from half an inch to four inches, and as the machine will work
effectively with a pressure of 10 lb. to the inch, holes may be started
with the greatest ease. With a pressure of 40 lb., the machine makes
1000 blows a minute, a speed that may be attained without causing undue
strains or vibration. This alone constitutes a very great advantage. It
must indeed be conceded that an unprejudiced consideration of the merits
of this drill shows it to be admirably adapted to the work required of
it.
_Borer-Bits._--The form and the dimensions of the cutting tools,
variously described as “drills,” “borers,” and “bits,” used with machine
rock-perforators are matters of great practical importance. The
dimensions are determined mainly by two conditions, namely, the
necessity for sufficient strength in the shank of the tool, and the
necessity for sufficient space between the shank and the sides of the
hole to allow the débris to escape. Experience has shown that the latter
condition is best fulfilled when the distance between the sides of the
hole and the shank of the tool is from ³/₁₆ inch to ¼ inch, regard being
had to the former condition.
The form of the cutting edge is determined by several conditions, some
of which have been already discussed in relation to hand drills. The
form first adopted was naturally that possessed by the hand drill,
namely, the chisel edge. To increase the useful effect of the blow, the
cutting edge was subsequently doubled, the bit being formed of two
chisel edges crossing each other at right angles. This bit, which from
its form was called the “cross” bit, was found to penetrate the rock
more rapidly than the straight or chisel bit. The gain in speed was very
marked at the commencement of the hole; but it diminished gradually as
the hole progressed in depth, owing to the difficulty with which the
débris escaped. To remedy this defect, the cutting edges were next made
to cross each other obliquely, so as to form the letter X. In this way,
the two chisel edges were retained, while the breadth of the bit was
considerably reduced. This form, described as the X bit, cleared the
hole much more effectively than the cross, but not in a manner that was
altogether satisfactory. Another modification of the form was,
therefore, made, and this time that of the Z was adopted, the upper and
the lower portions of which were arcs of circles struck from the centre
of the bit in the direction contrary to that of the rotation.
This form of tool, which is known as the Z bit, readily cleared itself
of the débris. But besides this advantage, it was found to possess
others of an important character. With the chisel-edge forms, the
corners of the bit were rapidly worn off by friction against the sides
of the hole. With the Z form, this wearing no longer occurred, by
reason of the large surface exposed to friction. Another advantage of
the Z form of bit lies in its tendency to bore the hole truly circular.
Generally then, it may be stated that this form satisfies most fully the
determining conditions. The form of bit, however, that is most suitable
in a given case will, in some degree, be determined by particular
circumstances. Of these, the nature and the character of the rock will
operate most strongly to influence the choice. Thus the cross bit will
generally be found the most suitable in fissured rock, while the single
chisel edge may be used with advantage in rock of a very solid and hard
character. Indeed, on the judicious selection of the most suitable form
of cutting edge, the success of machine boring largely depends. The
chisel bit, the cross bit, the X bit, and the Z bit, are shown in Figs.
24 to 27.
[Illustration: FIG. 24.]
[Illustration: FIG. 25.]
[Illustration: FIG. 26.]
[Illustration: FIG. 27.]
The sharpening of bits of a form other than that of the chisel is done
by means of “swages.” The tempering is effected in the way already
described in reference to hand drills. As in the latter case, the
degree of temper must be suited to the hardness of the rocks to be
penetrated. Generally the straw colour will be found to be the best
degree. It is a remarkable fact that the wear of the cutting edge of a
machine drill is, for a given length of boring, five or six times less
than that of a hand drill. Steel of the best quality should always be
used.
As in the case of hand boring, each successive length of drill must
diminish slightly in the width of its cutting edge; a diminution of
about ¹/₃₂ inch may be considered sufficient. Care should, however, be
taken to ensure the proper dimensions being given to the edge, and it
will be found advantageous to have at hand an accurate gauge through
which the tool may be passed previously to its being fixed to the
machine. It is important that the tool be truly “centred,” that is, the
centres of the edge of the bit, of the shank, and of the piston rod,
should be perfectly coincident.
_Rock-Drill Supports._--A machine rock-drill may satisfy every
requirement, and yet, by reason of the defective character of the
support to which it is attached, it may be unsuitable to the work
required of it. Hence it becomes desirable to carefully study the design
and construction of a drill support, and to consider the requirements
which it is needful to fulfil. Assuming the necessity for a high degree
of strength and rigidity in the support, a primary condition is that it
shall allow the machine to be readily adjusted to any angle, so that the
holes may be bored in the direction and with the inclination required.
When this requirement is not fulfilled, the machine is placed, in this
respect, at a great disadvantage with hand labour. If a machine drill
were not capable of boring in any position and in any direction, hand
labour would have to be employed in conjunction with it, and such
incompleteness in the work of a machine would constitute a serious
objection to its adoption.
Besides allowing of the desired adjustment of the machine, the support
must be itself adjustable to uneven ground. The bottom of a shaft which
is being sunk, or the sides, roof, and floor of a heading which is being
driven, present great irregularities of surface, and, as the support
must of necessity in most cases be fixed to these, it is obvious that
its design and construction must be such as will allow of its ready
adjustment to these irregularities. The means by which the adjustment is
effected should be few and simple, for simplicity of parts is important
in the support as well as in the machine, and for the same reasons. A
large proportion of the time during which a machine drill is in use is
occupied in shifting it from one position or one situation to another;
this time reduces, in a proportionate degree, the superiority of machine
over hand labour, in respect of rapidity of execution, and it is
evidently desirable that it should be shortened as far as possible.
Hence the necessity for the employment of means of adjustment which
shall be few in number, rapid in action, and of easy management.
For reasons similar to the foregoing, the drill support must be of small
dimensions, and sufficiently light to allow of its being easily
portable. The limited space in which rock drills are used renders this
condition, as in the case of the machine itself, a very important one.
It must be borne in mind that, after every blast, the dislodged rock has
to be removed, and rapidity of execution requires that the operations of
removal should be carried on without hindrance. A drill support that
occupies a large proportion of the free space in a shaft or a heading is
thus a cause of inconvenience and a source of serious delay. Moreover,
as it has to be continually removed from one situation to another, it
should be of sufficiently light weight to allow of its being lifted or
run along without difficulty. In underground workings, manual power is
generally the only power available, and therefore it is desirable that
both the machine and its support should be of such weight that each may
be lifted by one man. Of course, when any endeavour is made to reduce
the weight of the support, the necessity for great strength and rigidity
must be kept in view.
In spacious headings, such as are driven in railway tunnel work,
supports of a special kind may be used. In these situations, the
conditions of work are different from those which exist in mines. The
space is less limited, the heading is commenced at surface, and the
floor is laid with a tramway and sidings. In such a case, the support
may consist of a more massive structure mounted upon wheels to run upon
the rails. This support will carry several machines, and to remove it
out of the way when occasion requires, it will be run back on to a
siding; but for ordinary mining purposes, such a support is suitable.
_The Stretcher Bar._--The simplest kind of support is the “stretcher
bar.” This consists essentially of a bar so constructed that it may be
lengthened or shortened at pleasure, by means of a screw. It is fixed in
position by screwing the ends into firm contact with the sides, or with
the roof and the floor, of a heading. The machine is fixed to this bar
by means of a clamp, which, when loosened, slides along the bar, and
allows the drill to be placed in the required position, and to be
directed at the required angle. The bar illustrated in Fig. 26, Plate
V., is that which is used with the Darlington drill; in it, lightness
and rigidity are combined in the highest possible degree by the adoption
of the hollow section. The mode of setting the bar in a heading is shown
in the drawing; the end claws are set against pieces of wood on the
floor and the roof, and are tightened by turning the screw with a common
bar.
The simple stretcher bar is frequently used in narrow drivings and in
shafts of small diameter. But a more satisfactory support in drivings is
afforded by a bar suitably mounted upon a carriage designed to run upon
rails. The carriage consists simply of a trolly, to the fore part of
which the bar is fixed usually by some kind of hinge-joint. It is
obvious that the details of the construction of this support may be
varied greatly, and numerous designs have been introduced and adopted.
In Figs. 27 and 28, Plate VI., is shown a support of this character
designed by J. Darlington. A single vertical bar is carried on the fore
part of the trolly, and fixed, by the usual means, against the centre of
the roof. This vertical bar carries an arm, which is capable of turning
upon it, as upon a centre, and of sliding up and down it. This arm
carries the drill. The central bar having been fixed in position, the
arm is slid up to the highest position required, and fixed against the
side of the heading. A row of holes are then bored from this arm. When
these are completed, the arm is lowered the requisite distance, and
another row of holes are bored. This is continued until all the holes
are bored over one-half the face. The arm is then swung round, and fixed
against the other side of the heading, and the holes are bored over that
half the face in like manner. In this way, one-half the heading is kept
clear to allow the operations of removing the dislodged rock to be
carried on at the same time. If desired, two arms may be used. This
arrangement gives undoubtedly great facilities for working the drill,
and leaves the heading comparatively unencumbered.
In shaft sinking, the same support, slightly modified, is used without
the trolly. The arrangement adopted in this case is shown in Fig. 29,
Plate VII. The central bar is held firmly in its position by a cross
stretcher bar set against the sides of the shaft. The arms are made to
revolve upon this bar to allow the holes to be bored in the positions
required. When all the holes have been bored, the support, with the
machines, is hauled up, by means of a chain attached to the central bar,
out of the way of the blast. With this support, the time of fixing,
raising, and lowering is reduced to a minimum; while the facility with
which the machines may be slid along and fixed to the arm, and the
positions of the latter changed, allows the boring to be carried on
rapidly.
For open work, as in quarrying, where the stretcher bar cannot be used,
the tripod stand is adopted.
_The Dubois-François Carriage._--The support commonly used in France and
in Belgium consists of a kind of carriage carrying bars upon which the
drills are set. This carriage is used in drivings of all kinds; but it
is particularly suitable for tunnelling. It has been adopted, with but
slight modification, in the St. Gothard tunnel, and in several other
important works of the like character.
A modification of the carriage is shown in Figs. 30 and 31. Being
designed for ordinary mining operations, it carries but two machines;
but it will be readily perceived that, by increasing the number of
vertical screws, the same support may be made to carry a larger number.
It consists essentially of a vertical frame of flat bar iron _a b c d_,
8 feet in length, and 4 feet 9 inches in height above the rails, the
hinder portion of which rests upon a cast-iron plate _e f g h_, carried
upon two wheels; on this are fixed the two uprights _l_, _l′_, which,
being bound to the upper part by a transverse bar _m m′_, form a framing
to serve as a support to the two vertical screws _p′_, _q′_. The front
framing is formed of two longitudinals _b c_ and _b′ c′_ and the
uprights _a_, _a′_, and the vertical screws _p_, _q_, which are
connected to the upper part by the single piece _a d_. This framing is
supported below upon a small trolly with four wheels, connected to the
two longitudinals of the framing by a pivot bolt _n_ of ~T~ form, the
bar of the ~T~ being inserted into the elongated openings _o_ cut
through the middle of the curved portion of the longitudinals. The
cast-iron plate behind, the use of which is only to give stability to
the carriage, carries above it, by means of the two curved pieces _h_,
_h′_, a wrought-iron plate V, upon which the small tools needed for
repairs are kept. Two screws, _s_, _s′_, carried by lugs cast upon the
back of the plate, serve, by turning them down upon the rails, to fix
the carriage, the latter being slightly lifted by the screws.
Each machine is supported at two points. Behind, the point of support is
given by a cast-iron bracket _t_, having a projecting eye which enters
between the two cheeks formed at the back end of the machine by the
continuations of the two longitudinals of the framing. A pin bolt,
carried by the machine, allows the latter to be fixed to the bracket,
while leaving sufficient freedom of motion to allow of its being
directed at the required angle. This bracket, shown in plan in Fig. 33,
is supported by a kind of nut, Fig. 32, having two handles whereby it
may be easily turned. By raising or lowering this, the hinder support of
the drill may be brought to the requisite height. To prevent it turning
upon the screw, a pin is passed through the hole _o_, which pin forms a
stop for the handles aforementioned. The rotation of the bracket itself
is rendered impossible by the form of the vertical screw upon which it
is set, as shown in Fig. 33. In front, the support is a fork, the shank
of which slides along in the piece U, Figs. 30 and 31. This support,
which is not screwed on the inside, rests upon a nut of the same form as
that already described, and the same means are employed to prevent
rotation as in the case of the hinder supports.
SECTION III.--APPLIANCES FOR FIRING BLASTING CHARGES.
In the foregoing sections, the machines and tools used in rock boring
have been treated of. It now remains to describe those which are
employed in firing the charges after they have been placed in the
bore-holes. In this direction, too, great progress has been made in
recent times. With the introduction of new explosive agents, arose the
necessity for improved means of firing them. Attention being thus
directed to the subject, its requirements were investigated and its
conditions observed, the outcome being some important modifications of
the old appliances and the introduction of others altogether new. Some
of the improvements effected are scarcely less remarkable than the
substitution of machine for hand boring.
[Illustration: FIG. 28.]
[Illustration: FIG. 29.]
The means by which the charge of explosive matter placed in the
bore-hole is fired constitute a very important part of the set of
appliances used in blasting. The conditions which any such means must
fulfil are: (1) that it shall fire the charge with certainty; (2) that
it shall allow the person whose duty it is to explode the charge to be
at a safe distance away when the explosion takes place; (3) that it
shall be practically suitable, and applicable to all situations; and (4)
that it shall be obtainable at a low cost. To fulfil the second and most
essential of these conditions, the means must be either slow in
operation, or capable of being acted upon at a distance. The only known
means possessing the latter quality is electricity. The application of
electricity to this purpose is of recent date, and in consequence of the
great advantages which it offers, its use is rapidly extending. The
other means in common use are those which are slow in operation, and
which allow thereby sufficient time to elapse between their ignition and
the explosion of the charge for a person to retire to a safe distance.
These means consist generally of a train of gunpowder so placed that the
ignition of the particles must necessarily be gradual and slow. The old,
and in some parts still employed, mode of constructing this train was as
follows: An iron rod of small diameter and terminating in a point,
called a “pricker,” was inserted into the charge and left in the
bore-hole while the tamping was being rammed down. When this operation
was completed, the pricker was withdrawn, leaving a hole through the
tamping down to the charge. Into this hole, a straw, rush, quill, or
some other like hollow substance filled with gunpowder, was inserted. A
piece of slow-match was then attached to the upper end of this train,
and lighted.
The combustion of the powder confined in the straw fired the charge, the
time allowed by the slow burning of the match being sufficient to
enable the man who ignited it to retire to a place of safety. This
method of forming the train does not, however, satisfy all the
conditions mentioned above. It is not readily applicable to all
situations. Moreover, the use of the iron pricker may be a source of
danger; the friction of this instrument against silicious substances in
the sides of the bore-hole or in the tamping has in some instances
occasioned accidental explosions. This danger is, however, very greatly
lessened by the employment of copper or phosphor-bronze instead of iron
for the prickers. But the method is defective in some other respects.
With many kinds of tamping, there is a difficulty in keeping the hole
open after the pricker is withdrawn till the straw can be inserted. When
the holes are inclined upwards, besides this difficulty, another is
occasioned by the liability of the powder constituting the train to run
out on being ignited. And in wet situations, special provision has to be
made to protect the trains. Moreover, the manufacture of these trains by
the workmen is always a source of danger. Many of these defects in the
system may, however, be removed by the employment of properly
constructed trains. One of these trains or “squibs” is shown full size
in Fig. 28.
_Safety Fuse._--Many of the defects pertaining to the system were
removed by the introduction of the fuse invented by W. Bickford, and
known as “safety fuse.” The merits of this fuse, which is shown full
size in Fig. 29, are such as to render it one of the most perfect of the
slow-action means that have yet been devised. The train of gunpowder is
retained in this fuse, but the details of its arrangement are changed so
as to fairly satisfy the conditions previously laid down as necessary.
It consists of a flexible cord composed of a central core of fine
gunpowder, surrounded by hempen yarns twisted up into a tube, and called
the countering. An outer casing is made of different materials,
according to the circumstances under which it is intended to be used. A
central touch thread, or in some cases two threads, passes through the
core of gunpowder. This fuse, which in external appearance resembles a
piece of plain cord, is tolerably certain in its action; it may be used
with equal facility in holes bored in any direction; it is capable of
resisting considerable pressure without injury; it may be used without
special means of protection in wet ground; and it may be transported
from place to place without risk of damage.
In the safety fuse, the conditions of slow burning are fully satisfied,
and certainty is in some measure provided for by the touch thread
through the centre of the core. As the combustion of the core leaves, in
the small space occupied by it, a carbonaceous residue, there is little
or no passage left through the tamping by which the gases of the
exploding charge may escape, as in the case of the squibs. Hence results
an economy of force. Another advantage offered by the safety fuse is,
that it may be made to carry the fire into the centre of the bursting
charge if it be desired to produce rapid ignition. This fuse can be also
very conveniently used for firing charges of compounds other than
gunpowder, by fixing a detonating charge at the end of it, and dropping
the latter into the charge of the compound. This means is usually
adopted in firing the nitro-glycerine compounds, the detonating charge
in such cases being generally contained within a metallic cap. In using
this fuse, a sufficient length is cut off to reach from the charge to a
distance of about an inch, or farther if necessary, beyond the mouth of
the hole. One end is then untwisted to a height of about a quarter of an
inch, and placed to that depth in the charge. The fuse being placed
against the side of the bore-hole with the other end projecting beyond
it, the tamping is put in, and the projecting end of the fuse slightly
untwisted. The match may then be applied directly to this part. The
rate of burning is about two and a half feet a minute.
Safety fuse is sold in coils of 24 feet in length. The price varies
according to the quality, and the degree of protection afforded to the
train.
_Electric Fuses._--The employment of electricity to fire the charge in
blasting rock offers numerous and great advantages. The most important,
perhaps, is the greatly increased effect of the explosions when the
charges are fired simultaneously. But another advantage, of no small
moment, lies in the security from accident which this means of firing
gives. When electricity is used, not only may the charge be fired at the
moment desired, after the workmen have retired to a place of safety, but
the danger due to a misfire is altogether avoided. Further, the facility
afforded by electricity for firing charges under water is a feature in
this agent of very great practical importance. It would therefore seem,
when all these advantages are taken into account, that electricity is
destined to become of general application to blasting purposes in this
country, as it is already in Germany and in America.
An electric fuse consists of a charge of an explosive compound suitably
placed in the circuit of an electric current, which compound is of a
character to be acted upon by the current in a manner and in a degree
sufficient to produce explosion. The mode in which the current is made
to act depends upon the nature of the source of the electricity. That
which is generated by a machine is of high tension, but small in
quantity; while that which is generated by a battery is, on the
contrary, of low tension, but is large in quantity. Electricity of high
tension is capable of leaping across a narrow break in the circuit, and
advantage is taken of this property to place in the break an explosive
compound sufficiently sensitive to be decomposed by the passage of the
current. The electricity generated in a battery, though incapable of
leaping across a break in the circuit, is in sufficient quantity to
develop a high degree of heat. Advantage is taken of this property to
fire an explosive compound by reducing the sectional area of the wire
composing a portion of the circuit at a certain point, and surrounding
this wire with the compound. It is obvious that any explosive compound
may be fired in this way; but for the purpose of increasing the
efficiency of the battery, preference is given to those compounds which
ignite at a low temperature. Hence it will be observed that there are
two kinds of electric fuses, namely, those which may be fired by means
of a machine, and which are called “tension” fuses, and those which
require a battery, and which are known as “quantity” fuses.
In the tension, or machine fuses, the circuit is interrupted within the
fuse case, and the priming, as before remarked, is interposed in the
break; the current, in leaping across the interval, passes through the
priming. In the quantity or battery fuses, the reduction of the
sectional area is effected by severing the conducting wire within the
fuse case, and again joining the severed ends of the wire by soldering
to them a short piece of very fine wire. Platinum wire, on account of
its high resistance and low specific heat, is usually employed for this
purpose. The priming composition is placed around this fine wire, which
is heated to redness by the current as soon as the circuit is closed.
The advantages of high tension lie chiefly in the convenient form and
ready action of the machines employed to excite the electricity. Being
of small dimensions and weight, simple in construction, and not liable
to get quickly out of order, these sources of electricity are
particularly suitable for use in mining operations, especially when
these operations are entrusted, as they usually are, to men of no
scientific knowledge.
Another advantage of high tension is the small effect of line resistance
upon the current, a consequence of which is that mines may be fired at
long distances from the machine, and through iron wire of very small
section. A disadvantage of high tension is the necessity for a perfect
insulation of the wires.
When electricity of low tension is employed, the insulation of the
wires needs not to be perfect, so that leakages arising from injury to
the coating of the wire are not of great importance. In many cases, bare
wires may be used. Other advantages of low tension are the ability to
test the fuse at any moment by means of a weak current, and an almost
absolute certainty of action. For this reason, it is usually preferred
for torpedoes and important submarine work. On the other hand, the
copper wires used must be of comparatively large section, and the
influence of line resistance is so considerable that only a small number
of shots can be fired simultaneously when the distance is great.
[Illustration: FIG. 30.]
[Illustration: FIG. 31.]
[Illustration: FIG. 32.]
In Fig. 30 is shown an external view of an electric tension fuse. It
consists of a metal cap containing a detonating composition, upon the
top of which is placed the priming to be ignited by the electric spark.
The ends of two insulated wires project into this priming, which is
fired by the passage of the spark from one of these wires to the other.
The insulated wires are sufficiently long to reach a few inches beyond
the bore-hole.
Sometimes the fuse is attached to the end of a stick, and the wires are
affixed to the latter in the manner shown in Fig. 31. The rigidity of
the stick allows the fuse to be readily pushed into the bore-hole. When
the ground is not very wet, bare wires are, for cheapness, used, and the
stick is in that case covered with oiled paper, or some other substance
capable of resisting moisture. These “blasting sticks,” as they are
called, are extensively used in Germany. When heavy tamping is employed,
the stick is not suitable, by reason of the space which it occupies in
the bore-hole.
A mode of insulating the wires, less expensive than the guttapercha
shown in Fig. 30, is illustrated in Fig. 32. In this case, the wires are
cemented between strips of paper, and the whole is dipped into some
resinous substance to protect it from water. These “ribbon” wires may be
used in ground that is not very wet. They occupy little or no space in
the bore-hole, and therefore are suitable for use with tamping.
To connect the fuses with the machine or the battery, two sets of wires
are required when a single shot is fired, and three sets may be needed
when two or more shots are fired simultaneously. Of these several sets
of wires, the first consists of those which are attached to the fuses,
and which, by reason of their being placed in the shot-hole, are called
the “shot-hole wires.” Two shot-hole wires must be attached to each
fuse, and they must be of such a length that, when the fuse has been
placed in its proper position in the charge, the ends may project a few
inches from the hole. These wires must also be “insulated,” that is,
covered with a substance capable of preventing the escape of
electricity.
The second set of wires consists of those which are employed to connect
the charges one with another, and which, for this reason, are called
“connecting wires.” In connecting the charges in single circuit, the end
of one of the shot-hole wires of the first charge is left free, and the
other wire is connected, by means of a piece of this connecting wire, to
one of the shot-hole wires in the second hole; the other wire in this
second hole is then connected, in the same manner, to one of the wires
in the third hole; and so on till the last hole is reached, one
shot-hole wire of which is left free, as in the first. Whenever the
connecting wires can be kept from touching the rock, and also from
coming into contact one with another--and in most cases this may be
done--bare wire may be used, the cost of which is very little. But when
this condition cannot be complied with, and, of course, when blasting in
water, the connecting wires, like the shot-hole wires, must be
insulated. When guttapercha shot-hole wires are used, it is best to have
them sufficiently long to allow the ends projecting from one hole to
reach those projecting from the next hole. This renders connecting wire
unnecessary, and moreover saves one joint for each shot.
[Illustration: FIG. 33.]
[Illustration: FIG. 34.]
_Cables._--The third set of wires required consists of those used to
connect the charges with the machine or the battery. These wires, which
are called the “cables,” consist each of three or more strands of copper
wire well insulated with guttapercha, or better, indiarubber, the
coating of these materials being protected from injury by a sheathing of
tape or of galvanized iron wire underlaid with hemp. Two cables are
needed to complete the circuit; the one which is attached to the
positive pole of the machine, that is, the pole through which the
electric current passes out, is distinguished as the “leading cable,”
and the other, which is attached to the negative pole, that is, the pole
through which the current returns to the machine, is described as the
return cable. Sometimes both the leading and the return cables are
contained within one covering. When a cable having a metallic sheathing
is used, the sheathing may be made to serve as a return cable, care
being taken to make good metallic contact with the wires that connect
the sheathing to the fuses and to the terminal of the machine. The best
kind of unprotected cable consists of a three-strand tinned copper wire,
each 0·035 inch in diameter, insulated with three layers of indiarubber
to 0·22 inch diameter, and taped with indiarubber-saturated cotton to
0·24 inch diameter, as shown in Fig. 33. The best protected cable
consists of a similar strand of copper wire, covered with guttapercha
and tarred jute, and sheathed with fifteen galvanized iron wires of 0·08
inch diameter each, to a total diameter of 0·48 inch, as shown in Fig.
34.
_Detonators._--The new explosives of the nitro-cotton and
nitro-glycerine class cannot be effectively fired by means of safety or
other fuse alone. To bring about their instantaneous decomposition, it
is necessary to produce in their midst the explosion of some other
substance. The force of this initial explosion causes the charge of
gun-cotton, or dynamite, as the case may be, to detonate. It has been
found that the explosion of the fulminate of mercury brings about this
result most effectively and with the greatest certainty; and this
substance is therefore generally used for the purpose. The charge of
fulminate is contained in a copper capsule about a quarter of an inch in
diameter, and from 1 inch to 1¼ inch in length. These caps, with their
charge of fulminate, which are now well known to users of the
nitro-compounds, are called “detonators.” It is of the highest
importance that these detonators should contain a sufficiently strong
charge to produce detonation, for if too weak, not only is the whole
force of the explosive not developed, but a large quantity of noxious
gas is generated. Gun-cotton requires a much stronger charge of
fulminate than dynamite.
[Illustration: FIG. 35.]
In the electric fuses illustrated, the metal case shown is the
detonator, the fuse being placed inside above the fulminate. When safety
fuse is used, the end is cut off clean and inserted into the cap, which
is then pressed tightly upon the fuse by means of a pair of nippers, as
shown in Fig. 35. When water tamping is used, and when, with ordinary
tamping, the hole is very wet, a little white-lead or grease must be put
round the edge of the cap as a protection. The electric fuses are always
made waterproof; consequently, they are ready for use under all
circumstances. When the safety fuse burns down into the cap, or when, in
the other case, the priming of the electric fuse is fired, the fulminate
explodes and causes the detonation of the charge in which it is placed.
_Firing Machines and Batteries._--The electrical machines used for
firing tension fuses are of two kinds. In one kind, the electricity is
excited by friction, and stored in a condenser to be afterwards
discharged by suitable means provided for the purpose. In the other
kind, the electricity is excited by the motion of an armature before the
poles of a magnet. The former kind are called “frictional electric”
exploders; the latter kind are known as “magneto-electric” exploders.
When a magneto-electric machine contains an electro-magnet instead of a
permanent magnet, it is described as a “dynamo-electric exploder.”
Frictional machines act very well as exploders so long as they are kept
in a proper state. But as they are injuriously affected by a moist
atmosphere, and weaken rapidly with use by reason of the wearing away of
the rubbers, it is necessary to take care that they be in good
electrical condition before using them for firing. Unless this care be
taken, the quantity of electricity excited by a given number of
revolutions of the plate will be very variable, and vexatious failures
will ensue. If, however, the proper precautions be observed, very
certain and satisfactory results may be obtained. In Germany and in
America, frictional exploders are generally used.
Magneto-electric machines possess the very valuable quality of
constancy. They are unaffected, in any appreciable degree, by
atmospheric changes, and they are not subject to wear. These qualities
are of inestimable worth in an exploder used for ordinary blasting
operations. Moreover, as they give electricity of a lower tension than
the frictional machines, defects of insulation are less important. Of
these machines, only the dynamo variety are suitable for industrial
blasting. It is of primary importance that an exploder should possess
great power. The mistake of using weak machines has done more than
anything else to hinder the adoption of electrical firing in this
country.
[Illustration: FIG. 36.]
The machine most used in Germany is Bornhardt’s frictional exploder,
shown in Fig. 36. This machine is contained in a wooden case 20 inches
in length, 7 inches in breadth, and 14 inches in depth, outside
measurement. The weight is about 20 lb.
To fire the charges by means of this exploder, the leading wire is
attached to the upper terminal B, and the return to the lower terminal
C, the other ends of these wires being connected to the fuses. The
handle is then turned briskly from fifteen to thirty times, according to
the number of the fuses and the state of the machine, to excite the
electricity. The knob A is then pressed suddenly in, and the discharge
takes place. To ascertain the condition of the machine, a scale of
fifteen brass-headed nails is provided on the outside, which scale may
be put in communication with the poles B and C by means of brass chains,
as shown in the drawing. If after twelve or fourteen turns, the spark
leaps the scale when the knob is pushed in, the machine is in a
sufficiently good working condition. To give security to the men
engaged, the handle is designed to be taken off when the machine is not
in actual use; and the end of the machine into which the cable wires are
led is made to close with a lid and lock, the key of which should be
always in the possession of the man in charge of the firing operations.
[Illustration: FIG. 37.]
[Illustration: FIG. 38.]
In America, there are two frictional exploders in common use. One, shown
in Fig. 37, is the invention of H. Julian Smith. The apparatus is
enclosed in a wooden case about 1 foot square and 6 inches in depth.
The handle is on the top of the case, and is turned horizontally. This
handle is removable, as in Bornhardt’s machine. The cable wires having
been attached to the terminals, the handle is turned forward a certain
number of times to excite the electricity, and then turned a quarter of
a revolution backward to discharge the condenser and to fire the blast.
By this device, the necessity for a second aperture of communication
with the inside is avoided, an important point in frictional machines,
which are so readily affected by moisture. The aperture through which
the axis of the plate passes, upon which axis the handle is fixed, is
tightly closed by a stuffing-box. A leathern strap on one end of the
case allows the machine to be easily carried. The weight of this
exploder is under 10 lb.
The other exploder used is that designed by G. Mowbray. This machine,
which is shown in Fig. 38, is contained in a wooden barrel-shaped case,
and is known as the “powder-keg” exploder, the form and dimensions of
the case being those of a powder-keg. The action is similar to that of
the machine last described. The cable wires having been attached to the
terminals at one end of the keg, the handle at the other end is turned
forward to excite the electricity, and the condenser is discharged by
making a quarter turn backward, as in Smith’s machine. The handle is in
this case also removable. The weight of the powder-keg exploder is about
26 lb.
Both of these machines are very extensively used, and good results are
obtained from them. They stand well in a damp atmosphere, and do not
quickly get out of order from the wearing of the rubbers. They are also,
especially the former, very easily portable.
[Illustration: FIG. 39.]
The machine commonly used in England is the dynamo-electric exploder of
the Messrs. Siemens. This machine, which is the best of its kind yet
introduced for blasting purposes, is not more than half the size of
Bornhardt’s frictional exploder; but it greatly exceeds the latter in
weight, that of Siemens’ being about 55 lb. The apparatus, which is
contained within the casing shown in Fig. 39, consists of an ordinary
Siemens’ armature, which is made, by turning the handle, to revolve
between the poles of an electro-magnet. The coils of the electro-magnet
are in circuit with the wire of the armature; the residual magnetism of
the electro-magnet cores excites, at first, weak currents; these pass
into the coils, thereby increasing the magnetism of the cores, and
inducing still stronger currents in the armature wire, to the limit of
magnetic saturation of the iron cores of the electro-magnets. By the
automatic action of the machine, this powerful current is, at every
second turn of the handle, sent into the cables leading to the fuses.
To fire this machine, the handle is turned gently till a click is heard
from the inside, indicating that the handle is in the right position to
start from. The cable wires are then attached to the terminals, and the
handle is turned quickly, but steadily. At the completion of the second
revolution, the current is sent off into line, as it is termed, that is,
the current passes out through the cables and the fuses. As in the case
of the frictional machines, the handle is, for safety, made removable.
This exploder is practically unaffected by moisture, and it is not
liable to get out of order from wear.
Induction coils have been used to fire tension fuses; but it is
surprising that they have not been more extensively applied to that
purpose. A coil designed for the work required of it is a very effective
instrument. If constructed to give a spark not exceeding three inches in
length, with comparatively thick wire for quantity, it makes a very
powerful exploder. An objection to its use is the necessity for a
battery. But a few bichromate of potash cells, provided with spiral
springs to hold the zincs out of the liquid, and designed to be set in
action by simply pressing down the zincs, give but little trouble, so
that the objection is not a serious one. The writer has used an
induction exploder in ordinary mining operations without experiencing
any difficulty or inconvenience. It is cheap, easily portable, and
constant in its action.
Batteries are used to fire what are known as “quantity” or “low tension”
fuses. Any cells may be applied to this purpose; but they are not all
equally suitable. A firing battery should require but little attention,
and should remain in working order for a long time. These conditions are
satisfactorily fulfilled by only two cells, namely, the Léclanché and
the Bichromate of Potash. The latter is the more powerful, and generally
the more suitable. The Léclanché is much used in this country for firing
purposes, under the form known as the “Silvertown Firing Battery.” This
battery consists of a rectangular teak box, containing ten cells. Two,
or more, of these may be joined up together when great power is
required. In France, the battery used generally for firing is the
Bichromate. This battery is much more powerful than the Léclanché, and
as no action goes on when the zincs are lifted out of the liquid, it is
equally durable. It is moreover much cheaper. At the suggestion of the
writer, Mr. Apps, of the Strand, London, has constructed a bichromate
firing battery of very great power. It is contained in a box of smaller
dimension than the 10-cell Silvertown. The firing is effected by simply
lowering the zincs, which rise again automatically out of the liquid, so
that there is no danger of the battery exhausting itself by continuous
action in case of neglect. Externally, this battery, like the
Silvertown, appears a simple rectangular box, so that no illustration is
needed. With either of these, the usual objections urged against the
employment of batteries, on the ground of the trouble involved in
keeping them in order, and their liability to be injured by ignorant or
careless handling, do not apply, or at least apply in only a very
unimportant degree.
To guard against misfires, the machine or the battery used should be
constructed to give a very powerful current. If this precaution be
observed, and the number of fuses in circuit be limited to one-half that
which the machine is capable of firing with a fair degree of certainty,
perfectly satisfactory results may be obtained. The employment of weak
machines and batteries leads inevitably to failure. In the minds of
those who have hitherto tried electrical blasting in this country, there
seems to be no notion of any relation existing between the work to be
done and the force employed to do it. The electrical exploder is
regarded as a sort of magic box that needs only to be set in action to
produce any required result. Whenever failure ensues, the cause is
unhesitatingly attributed to the fuses.
CHAPTER II.
EXPLOSIVE AGENTS USED IN BLASTING ROCKS.
SECTION I.--PHENOMENA ACCOMPANYING AN EXPLOSION.
_Nature of an Explosion._--The combination of oxygen with other
substances for which it has affinity is called generally “oxidation.”
The result of this combination is a new substance, and the process of
change is accompanied by the liberation of heat. The quantity of heat
set free when two substances combine chemically is constant, that is, it
is the same under all conditions. If the change takes place within a
short space of time, the heat becomes sensible; but if the change
proceeds very slowly, the heat cannot be felt. The same _quantity_,
however, is liberated in both cases. Thus, though the quantity of heat
set free by a chemical combination is under all conditions the same, the
degree or _intensity_ of the heat is determined by the rapidity with
which the change is effected.
When oxidation is sufficiently rapid to cause a sensible degree of heat,
the process is described as “combustion.” The oxidation of a lump of
coke in the furnace, for example, is effected within a short space of
time, and, as the quantity of heat liberated by the oxidation of that
weight of carbon is great, a high degree results. And it is well known
and obvious that as combustion is quickened, or, in other words, as the
time of change is shortened, the intensity of the heat is proportionally
increased. So in the case of common illuminating gas, the oxidation of
the hydrogen is rapidly effected, and, consequently, a high degree of
heat ensues.
When oxidation takes place within a space of time so short as to be
inappreciable to the senses, the process is described as “explosion.”
The combustion of a charge of gunpowder, for example, proceeds with such
rapidity that no interval can be perceived to intervene between the
commencement and the termination of the process. Oxidation is in this
case, therefore, correctly described as an explosion; but the combustion
of a train of gunpowder, or of a piece of quick-match, though
exceedingly rapid, yet, as it extends over an appreciable space of time,
is not to be so described. By analogy, the sudden change of state which
takes place when water is “flashed” into steam, is called an explosion.
It may be remarked here that the application of this expression to the
bursting of a steam boiler is an abuse of language; as well may we speak
of an “explosion” of rock.
From a consideration of the facts stated in the foregoing paragraphs,
it will be observed that oxidation by explosion gives the maximum
intensity of heat.
_Measure of Heat, and specific Heat._--It is known that if a certain
quantity of heat will raise the temperature of a body one degree, twice
that quantity will raise its temperature two degrees, three times the
quantity, three degrees, and so on. Thus we may obtain a measure of heat
by which to determine, either the temperature to which a given quantity
of heat is capable of raising a given body, or the quantity of heat
which is contained in a given body at a given temperature. The quantity
of heat requisite to produce a change of one degree in temperature is
different for different bodies, but is practically constant for the same
body, and this quantity is called the “specific heat” of the body. The
standard which has been adopted whereby to measure the specific heat of
bodies is that of water, the unit being the quantity of heat required to
raise the temperature of 1 lb. of water through 1° Fahr., say from 32°
to 33°. The quantity of heat required to produce this change of
temperature in 1 lb. of water is called the “unit of heat,” or the
“thermal unit.” Having determined the specific heat of water, that of
air may in like manner be ascertained, and expressed in terms of the
former. It has been proved by experiments that if air be heated at
constant pressure through 1° Fahr., the quantity of heat absorbed is
0·2375 thermal units, whatever the pressure or the temperature of the
air may be. Similarly it has been shown that the specific heat of air at
constant volume is, in thermal units, 0·1687; that is, if the air be
confined so that no expansion can take place, 0·1687 of a thermal unit
will be required to increase its temperature one degree.
_Heat liberated by an Explosion._--In the oxidation of carbon, one atom
of oxygen may enter into combination with one atom of that substance;
the resulting body is a gas known as “carbonic oxide.” As the weight of
carbon is to that of oxygen as 12 is to 16, 1 lb. of the former
substance will require for its oxidation 1⅓ lb. of the latter; and since
the two enter into combination, the product, carbonic oxide, will weigh
1 + 1⅓ = 2⅓ lb. The combining of one atom of oxygen with one of carbon
throughout this quantity, that is, 1⅓ lb. of oxygen, with 1 lb. of
carbon, generates 10,100 units of heat. Of this quantity, 5700 units are
absorbed in changing the carbon from the solid into the gaseous state,
and 4400 are set free. The quantity of heat liberated, namely, the 4400
units, will be expended in raising the temperature of the gas from 32°
Fahr., which we will assume to be that of the carbon and the oxygen
previous to combustion, to a much higher degree, the value of which may
be easily determined. The 4400 units would raise 1 lb. of water from 32°
to 32 + 4400 = 4432°; and as the specific heat of carbonic oxide is
0·17 when there is no increase of volume, the same quantity of heat will
raise 1 lb. of that gas from 32° to
4400
32 + ---- = 25,914°.
0·17
But in the case under consideration, we have 2⅓ lb. of the gas, the
resulting temperature of which will be
25,914
------ = 9718°.
2⅓
In the oxidation of carbonic oxide, one atom of oxygen combines with one
atom of the gaseous carbon; the resulting body is a gas known as
“carbonic acid.” Since 2⅓ lb. of carbonic oxide contains 1 lb. of
carbon, that quantity of the oxide will require 1⅓ lb. of oxygen to
convert it into the acid, that is, to completely oxidize the original
pound of solid carbon. By this combination, 10,100 units of heat are
generated, as already stated, and since the carbon is now in the gaseous
state, the whole of that quantity will be set free. Hence the
temperature of the resulting 3⅔ lb. of carbonic acid will be
4400 + 10,100
32 + ------------- = 23,516°.
0·17 × 3·667
It will be seen from the foregoing considerations that if 1 lb. of pure
carbon be burned in 2⅔ lb. of pure oxygen, 3⅔ lb. of carbonic acid is
produced, and 14,500 units of heat are liberated; and further, that if
the gas be confined within the space occupied by the carbon and the
oxygen previously to their combination, the temperature of the product
may reach 23,516° Fahr.
In the oxidation of hydrogen, one atom of oxygen combines with two atoms
of the former substance; the resulting body is water. As the weight of
hydrogen is to that of oxygen as 1 is to 16, 1 lb. of the former gas
will require for its oxidation 8 lb. of the latter; and since the two
substances enter into combination, the product, water, will weigh 1 + 8
= 9 lb. By this union, 62,032 units of heat are generated. Of this
quantity, 8694 are absorbed in converting the water into steam, and
53,338 are set free. The specific heat of steam at constant volume being
0·37, the temperature of the product of combustion, estimated as before,
will be
53,338
32 + -------- = 16,049°.
0·37 × 9
Hence it will be observed that if 1 lb. of hydrogen be burned in 8 lb.
of oxygen, 9 lb. of steam will be produced, and 53,338 units of heat
will be liberated; and further, that the temperature of the product may
reach 16,049°.
_Gases generated by an Explosion._--It was shown in the preceding
paragraph that in the combustion of carbon, one atom of oxygen may unite
with one atom of carbon to form carbonic oxide, or two atoms of oxygen
may unite with one atom of carbon to form carbonic acid. When the
combination takes place according to the former proportions, the
reaction is described as “imperfect combustion,” because the carbon is
not fully oxidized; but when the combination is effected in the latter
proportions, the combustion is said to be “perfect,” because no more
oxygen can be taken up. The products of combustion are in both cases
gaseous. Carbonic oxide, the product of imperfect combustion, is an
extremely poisonous gas; it is this gas which is so noisome in close
headings, and in all ill-ventilated places, after a blast has been
fired. A cubic foot of carbonic oxide, the specific gravity of which is
0·975, weighs, at the mean atmospheric pressure, 0·075 lb., so that 1
lb. will occupy a space of 13·5 cubic feet. Thus 1 lb. of carbon
imperfectly oxidized will give 2⅓ lb. of carbonic oxide, which, at the
mean atmospheric pressure of 30 inches and the mean temperature of 62°
Fahr., will occupy a space of 13·5 × 2⅓ = 31·5 cubic feet. The product
of perfect combustion, carbonic acid, is a far less noxious gas than the
oxide, and it is much more easily expelled from confined places, because
water possesses the property of absorbing large quantities of it. In an
ill-ventilated but wet heading, the gas from a blast is soon taken up.
Carbonic acid is a comparatively heavy gas, its specific gravity
relatively to that of common air being 1·524. Hence a cubic foot at the
ordinary pressure and temperature will weigh 0·116 lb., and 1 lb. of the
gas under the same conditions will occupy a space of 8·6 cubic feet.
Thus if 1 lb. of carbon be completely oxidized, there will result 3⅔ lb.
of carbonic acid, which will fill a space of 8·6 × 3⅔ = 31·5 cubic feet.
It will be observed that, though an additional pound of oxygen has been
taken up during this reaction, the product occupies the same volume as
the oxide. In complete combustion, therefore, a contraction takes place.
In the oxidation of hydrogen, as already pointed out, one atom of oxygen
combines with two atoms of the former substance to form water. In this
case, the product is liquid. But the heat generated by the combustion
converts the water into steam, so that we have to deal with this product
also in the gaseous state, in all considerations relating to the effects
of an explosion. A cubic foot of steam, at atmospheric pressure and a
temperature of 212° Fahr., weighs 0·047 lb.; 1 lb. of steam under these
conditions will, therefore, occupy a space of 21·14 cubic feet. Thus the
combustion of 1 lb. of hydrogen will produce 9 lb. of steam, which,
under the conditions mentioned, will fill a space of 21·14 × 9 = 190·26
cubic feet.
Usually in an explosion a large quantity of nitrogen gas is liberated.
This gas, which is not in itself noxious, has a specific gravity of
0·971, so that practically a cubic foot will weigh 0·075 lb., and 1 lb.
will occupy a space of 13·5 cubic feet, which are the weight and the
volume of carbonic oxide. Other gases are often formed as products of
combustion; but the foregoing are the chief, viewed as the results of an
explosion, since upon these the force developed almost wholly depends.
_Force developed by an Explosion._--A consideration of the facts
enunciated in the foregoing paragraphs will show to what the tremendous
energy developed by an explosion is due. It was pointed out that the
combustion of 1 lb. of carbon gives rise to 31·5 cubic feet of gas. If
this volume of gas be compressed within the space of 1 cubic foot it
will obviously have a tension of 31·5 atmospheres; that is, it will
exert upon the walls of the containing vessel a pressure of 472 lb. to
the square inch. If the same volume be compressed into a space
one-eighth of a cubic foot in extent, say a vessel of cubical form and 6
inches side, the tension will be 31·5 × 8 = 252 atmospheres, and the
pressure 472 × 8 = 3776 lb. to the square inch. Assuming now the oxygen
to exist in the solid state, and the two bodies carbon and oxygen to
occupy together a space of one-eighth of a cubic foot, the combustion of
the carbon will develop upon the walls of an unyielding containing
vessel of that capacity a pressure of 252 atmospheres. Also the
combustion of 1 lb. of hydrogen gives rise, as already remarked, to
190·26 cubic feet of steam; and if combustion take place under similar
conditions with respect to space, the pressure exerted upon the
containing vessel will be 22,830 lb., or nearly 10·5 tons, to the
square inch, the tension being 190·26 × 8 = 1522 atmospheres.
The force thus developed is due wholly to the volume of the gas
generated, and by no means represents the total amount developed by the
explosion. The volume of the gases evolved by an explosion is estimated
for a temperature of 62°; but it was shown in a former paragraph that
the temperature of the products of combustion at the moment of their
generation is far above this. Now it is a well-known law of
thermo-dynamics that, the volume remaining the same, the pressure of a
gas will vary directly as the temperature; that is, when the temperature
is doubled, the pressure is also doubled. By temperature is understood
the number of degrees measured by Fahrenheit’s scale on a perfect gas
thermometer, from a zero 461°·2 below the zero of Fahrenheit’s scale,
that is, 493°·2 below the freezing point of water. Thus the temperature
of 62° for which the volume has been estimated is equal to 461·2 + 62 =
523°·2 absolute.
It was shown that the temperature of the product of combustion when
carbon is burned to carbonic oxide is 9718° Fahr., which is equivalent
to 10179°·2 absolute. Hence it will be observed that the temperature has
been increased
10179°·2
-------- = 19·45 times.
523°·2
According to the law above enunciated, therefore, the pressure will be
increased in a like ratio, that is, it will be, for the volume and the
space already given, 3776 × 19·45 = 73,443 lb. = 32·8 tons to the square
inch.
When carbon is burned to carbonic acid, the temperature of the product
was shown to be 23,516° Fahr., which is equivalent to 23977·2 absolute.
In this case, it will be observed that the temperature has been
increased
23977·2
------- = 45·83 times.
523·2
Hence the resulting pressure will be 3776 × 45·83 = 173,154 lb. = 77·3
tons to the square inch. It will be seen from these pressures that when
combustion is complete, the force developed is 2·36 greater than when
combustion is incomplete; and also that the increase of force is due to
the larger quantity of heat liberated, since the volume of the gases is
the same in both cases. If we suppose the carbon burned to carbonic
oxide in the presence of a sufficient quantity of oxygen to make
carbonic acid, we shall have 31·5 cubic feet of the oxide + 15·7 cubic
feet of free oxygen, or a total volume of 42·7 cubic feet of gases. If
this volume be compressed within the space of one-eighth of a cubic
foot, it will have a tension of 42·7 × 8 = 341·6 atmospheres, and will
exert upon the walls of the containing vessel a pressure of 5124 lb. to
the square inch. The temperature of the gases will be
4400
32 + ------------- = 6347° Fahr. = 6808°·2 absolute,
0·190 × 3·667
the mean specific heat of the gases being 0·190; whence it will be seen
that the temperature has been increased
6808°·2
------- = 13·01 times.
523·2
According to the law of thermo-dynamics, therefore, the pressure under
the foregoing conditions will be 5124 × 13·01 = 66,663 lb. = 29·8 tons
to square inch. So that, under the conditions assumed in this case, the
pressures developed by incomplete and by complete combustion are as 29·8
to 77·3, or as 1 to 2·59.
Similarly, when hydrogen is burned to water, the temperature of the
product will be, as shown in a former paragraph, 16,049 Fahr. = 16510·2
absolute; and the pressure will be
16510·2
22,830 × ------- = 720,286 lb. = 321·1 tons to the square inch.
523·2
It will be observed, from a consideration of the foregoing facts, that a
very large proportion of the force developed by an explosion is due to
the heat liberated by the chemical reactions which take place. And hence
it will plainly appear that, in the practical application of explosive
agents to rock blasting, care should be taken to avoid a loss of the
heat upon which the effects of the explosion manifestly so largely
depend.
SECTION II.--NATURE OF EXPLOSIVE AGENTS.
_Mechanical Mixtures._--In the preceding section, it was shown that an
explosion is simply the rapid oxidation of carbon and hydrogen. To form
an explosive agent, the problem is, how to bring together in a
convenient form the combustible, carbon or hydrogen, and the oxygen
required to oxidize it. Carbon may be obtained pure, or nearly pure, in
the solid form. As wood charcoal, for example, that substance may be
readily procured in any needful abundance; but pure oxygen does not
exist in that state, and it is hardly necessary to point out that only
the solid form is available in the composition of an explosive agent. In
nature, however, oxygen exists in the solid state in very great
abundance in combination with other substances. Silica, for example,
which is the chief rock constituent, is a compound of silicon and
oxygen, and the common ores of iron are made up chiefly of that metal
and oxygen. The elementary constituents of cellulose, or wood fibre, are
carbon, hydrogen, and oxygen; and the body known as saltpetre, or
nitrate of potash, is compounded of potassium, nitrogen, and oxygen. But
though oxygen is thus found in combination with many different
substances, it has not the same affinity for all. When it is combined
with a substance for which its affinity is strong, as in the silica and
the iron oxide, it cannot be separated from that substance without
difficulty; but if the affinity be weak, dissociation may be more easily
effected. The former combination is said to be “stable,” and the latter
is, in contradistinction, described as “unstable.” It will be evident on
reflection that only those compounds in which the oxygen exists in
unstable combination can be made use of as a constituent part of an
explosive agent, since it is necessary that, when required, the oxygen
shall be readily given up. Moreover, it will also appear that when one
of these unstable oxygen compounds and carbon are brought together the
mixture will constitute an explosive agent, since the oxygen which is
liberated by the dissociation of the unstable compound will be taken up
by the carbon for which it has a stronger affinity. Saltpetre is one of
those compounds, and a mixture of this body with charcoal constitutes
gunpowder. The means employed to dissociate the elements of saltpetre is
heat. It is obvious that other compounds of oxygen might be substituted
for the saltpetre, but this body being easily procurable is always
employed. The chlorate of potash, for example, is less stable than the
nitrate, and therefore an explosive mixture containing the former
substance will be more violent than another containing the latter. For
the violence of an explosion is in a great measure determined by the
readiness with which the oxygen is given up to the combustible. But the
chlorate is much more costly than the nitrate. As, however, the force
developed is greater, the extra cost would perhaps be compensated by the
increased effect of the explosion. But the instability of the chlorate
is such that friction or a moderately light blow will produce explosion
in a mixture containing that substance, a circumstance that renders it
unfit to be the oxidizer in an explosive agent in common use. The
nitrate is therefore preferred on the ground of safety. Saltpetre, or
nitrate of potash, consists, as already pointed out, of the metal
potassium in combination with the substances nitrogen and oxygen. Of
these, the last only is directly concerned in the explosion; but the two
former, and especially the nitrogen, act indirectly to intensify its
effects in a manner that will be explained hereafter.
The chemical formula for nitrate of potash is KNO₃, which signifies that
three atoms of oxygen exist in this body in combination with one atom of
nitrogen and one atom of kalium or potassium. As the atomic weights of
these substances are 16, 14, and 39 respectively, the weight of the
molecule is 101, that is, in 101 lb. of nitrate of potash there are 39
lb. of potassium, 14 lb. of nitrogen, and (16 × 3) = 48 lb. of oxygen.
Hence the proportion of oxygen in nitrate of potash is by weight 47·5
per cent. It will be seen from this proportion that to obtain 1 lb. of
oxygen, 2·1 lb. of the nitrate must be decomposed.
The carbon of gunpowder is obtained from wood charcoal, the light woods,
such as alder, being preferred for that purpose. The composition of the
charcoal varies somewhat according to the degree to which the burning
has been carried, the effect of the burning being to drive out the
hydrogen and the oxygen. But, generally, the composition of gunpowder
charcoal is about 80 per cent. carbon, 3·25 per cent. hydrogen, 15 per
cent. oxygen, and 1·75 per cent. ash. Knowing the composition of the
charcoal, it is easy to calculate the proportion of saltpetre required
in the explosive mixture.
Thus far we have considered gunpowder as composed of charcoal and
saltpetre only. But in this compound, combustion proceeds too slowly to
give explosive effects. Were the chlorate of potash used instead of the
nitrate, the binary compound would be sufficient. The slowness of
combustion in the nitrate mixture is due to the comparatively stable
character of that body. To accelerate the breaking up of the nitrate, a
quantity of sulphur is mixed up with it in the compound. This substance
possesses the property of burning at a low temperature. The proportion
of sulphur added varies from 10 per cent. in powder used in fire-arms,
to 20 per cent. in that employed for blasting purposes. The larger the
proportion of sulphur, the more rapid, within certain limits, is the
combustion. Thus ordinary gunpowder is a ternary compound, consisting of
charcoal, saltpetre, and sulphur.
As the composition of charcoal varies, it is not practicable to
determine with rigorous accuracy the proportion of saltpetre required in
every case; a mean value is therefore assumed, the proportions adopted
being about--
Charcoal 15
Saltpetre 75
Sulphur 10
---
100
---
With these proportions, the carbon should be burned to carbonic acid,
and the sulphur should be all taken up by the potassium. Powder of this
composition is used for fire-arms. For blasting purposes, as before
remarked, the proportion of sulphur is increased at the expense of the
saltpetre, in order to quicken combustion and to lessen the cost, to 20
per cent. as a maximum. With such proportions, some of the carbon is
burned to carbonic oxide only, and some of the sulphur goes to form
sulphurous acid, gases that are particularly noisome to the miner.
It is essential to the regular burning of the mixture that the
ingredients be finely pulverized and intimately mixed. The manufacture
of gunpowder consists of operations for bringing about these results.
The several substances are broken up by mechanical means, and reduced to
an impalpable powder. These are then mixed in a revolving drum, and
afterwards kneaded into a paste by the addition of a small quantity of
water. This paste is subjected to pressure, dried, broken up, and
granulated; thus, the mixing being effected by mechanical means, the
compound is called a mechanical mixture. It will be observed that in a
mechanical mixture the several ingredients are merely in contact, and
are not chemically united. They may therefore be separated if need be,
or the proportions may be altered in any degree. Mechanical mixtures,
provided the bodies in contact have no chemical action one upon another,
are stable, that is, they are not liable, being made up of simple
bodies, to decompose spontaneously.
_Chemical Compounds._--In a mechanical mixture, as we have seen, the
elements which are to react one upon another are brought together in
separate bodies. In gunpowder, for example, the carbon is contained in
the charcoal, and the oxygen in the saltpetre. But in a chemical
compound, these elements are brought together in the same body. In a
mechanical mixture, we may put what proportion of oxygen we please. But
elements combine chemically only in certain definite proportions, so
that in the chemical compound we can introduce only a certain definite
proportion of oxygen. The oxygen in saltpetre is in chemical combination
with the potassium and the nitrogen, and, as we have already seen, these
three substances hold certain definite proportions one to another. That
is, to every atom of potassium, there are one atom of nitrogen and
three atoms of oxygen. Or, which amounts to the same thing, in 1 lb. of
saltpetre, there are 0·386 lb. of potassium, 0·139 lb. of nitrogen, and
0·475 lb. of oxygen. Moreover, these elements occupy definite relative
positions in the molecule of saltpetre. But in the mechanical mixture,
the molecules of which it is made up have no definite relative
positions. Even if the three substances--charcoal, saltpetre, and
sulphur--of which gunpowder is composed, could be so finely divided as
to be reduced to their constituent molecules, the relative position of
these would be determined by the mixing, and it would be impossible so
to distribute them that each should find itself in immediate proximity
to those with which it was to combine. But so far are we from being able
to divide substances into their constituent molecules, that when we have
reduced them to an impalpable powder, each particle of that powder
contains a large number of molecules. Thus, in a mechanical mixture, we
have groups of molecules of one substance mingled irregularly with
groups of molecules of another substance, so that the atoms which are to
combine are not in close proximity one to another, but, on the contrary,
are, many of them, separated by wide intervals. In the chemical
compound, however, the atoms are regularly distributed throughout the
whole mass of the substance, and are, relatively to one another, in the
most favourable position for combining. Viewed from this point, the
chemical compound may be regarded as a perfect mixture, the mechanical
mixture being a very imperfect one. This difference has an important
influence on the effect of an explosion. All the atoms in a chemical
compound enter at once into their proper combinations, and these
combinations take place in an inconceivably short space of time, while,
in a mechanical mixture, the combinations are less direct, and are much
less rapidly effected. This is the reason why the former is more violent
in its action than the latter. The one is crushing and shattering in its
effects, the other rending and projecting. The compound gives a sudden
blow; the mixture applies a gradually increasing pressure. It is this
sudden action of the compound that allows it to be used effectively
without tamping. The air, which rests upon the charge, and which offers
an enormous resistance to motion at such inconceivably high velocities,
serves as a sufficient tamping.
Gun-cotton may be taken as an example of a chemical compound. The woody
or fibrous part of plants is called “cellulose.” Its chemical formula is
C₆H₁₀O₅, that is, the molecule of cellulose consists of six atoms of
carbon in combination with ten atoms of hydrogen and five atoms of
oxygen. If this substance be dipped into concentrated nitric acid, some
of the hydrogen is displaced and peroxide of nitrogen is substituted for
it. The product is nitro-cellulose, the formula of which is
C₆H₇(NO₂)₃O₅. If this formula be compared with the last, it will be
seen that three atoms of hydrogen have been eliminated and their place
taken by three molecules of the peroxide of nitrogen NO₂; so that we now
have a compound molecule, which is naturally unstable. The molecules of
the peroxide of nitrogen are introduced into the molecule of cellulose
for the purpose of supplying the oxygen needed for the combustion of the
carbon and the hydrogen, just as the groups of molecules of saltpetre
were introduced into the charcoal of the gunpowder for the combustion of
the carbon and the hydrogen of that substance. Only, in the former case,
the molecules of the peroxide are in chemical combination, not merely
mixed by mechanical means as in the latter. The compound molecule of
nitro-cellulose may be written C₆H₇N₃O₁₁, that is, in 297 lb. of the
substance, there are (6 × 12) 72 lb. of carbon, (7 × 1) 7 lb. of
hydrogen, (3 × 14) 42 lb. of nitrogen, and (11 × 16) 176 lb. of oxygen;
or 24·2 per cent. carbon, 2·3 per cent. hydrogen, 14·1 per cent.
nitrogen, and 59·4 per cent. oxygen. When the molecule is broken up by
the action of heat, the oxygen combines with the carbon and the
hydrogen, and sets the nitrogen free. But it will be observed that the
quantity of oxygen present is insufficient to completely oxidize the
carbon and the hydrogen. This defect, though it does not much affect the
volume of gas generated, renders the heat developed, as shown in a
former section, considerably less than it would be were the combustion
complete, and gives rise to the noxious gas carbonic oxide.
Cotton is one of the purest forms of cellulose, and, as it may be
obtained at a cheap rate, it has been adopted for the manufacture of
explosives. This variety of nitro-cellulose is known as “gun-cotton.”
The raw cotton made use of is waste from the cotton mills, which waste,
after being used for cleaning the machinery, is swept from the floors
and sent to the bleachers to be cleaned. This is done by boiling in
strong alkali and lime. After being picked over by hand to remove all
foreign substances, it is torn to pieces in a “teasing” machine, cut up
into short lengths, and dried in an atmosphere of 190° F. It is then
dipped into a mixture of one part of strong nitric acid and three parts
of strong sulphuric acid. The use of the sulphuric acid is, first, to
abstract water from the nitric acid, and so to make it stronger; and,
second, to take up the water which is formed during the reaction. After
the dipping, it is placed in earthenware pots to digest for twenty-four
hours, in order to ensure the conversion of the whole of the cotton into
gun-cotton. To remove the acid, the gun-cotton is passed through a
centrifugal machine, and subsequently washed and boiled. It is then
pulped, and again washed with water containing ammonia to neutralize
any remaining trace of acid. When rendered perfectly pure, it is
compressed into discs and slabs of convenient dimensions for use.
Another important chemical compound is nitro-glycerine. Glycerine is a
well-known, sweet, viscous liquid that is separated from oils and fats
in the processes of candle-making. Its chemical formula is: C₃H₈O₃; that
is, the molecule is composed of three atoms of carbon, in combination
with eight atoms of hydrogen, and three atoms of oxygen. In other words,
glycerine consists of carbon 39·1 per cent., hydrogen 8·7 per cent., and
oxygen 52·2 per cent. When this substance is treated, like cellulose,
with strong nitric acid, a portion of the hydrogen is displaced, and
peroxide of nitrogen is substituted for it; thus the product is:
C₃H₅(NO₂)₃O₃, similar, it will be observed, to nitro-cellulose. This
product is known as nitro-glycerine. The formula may be written
C₃H₅N₃O₉. Hence, in 227 lb. of nitro-glycerine, there are (3 × 12) 36
lb. of carbon; (5 × 1) 5 lb. of hydrogen; (3 × 14) 42 lb. of nitrogen;
and (9 × 16) 144 lb. of oxygen; or 15·8 per cent. is carbon, 2·2 per
cent. hydrogen, 18·5 per cent. nitrogen, and 63·5 per cent. oxygen. When
the molecule is broken up by the action of heat, the oxygen combines
with the carbon and the hydrogen, and sets the nitrogen free. And it
will be seen that the quantity of oxygen present is more than sufficient
to completely oxidize the carbon and the hydrogen. In this, the
nitro-glycerine is superior to the nitro-cotton. In both of these
compounds, the products of combustion are wholly gaseous, that is, they
give off no smoke, and leave no solid residue.
In the manufacture of nitro-glycerine, the acids, consisting of one part
of strong nitric acid and two parts of strong sulphuric acid, are mixed
together in an earthenware vessel. When quite cold, the glycerine is run
slowly into this mixture, which, during the process, is kept in a state
of agitation, as heat is developed in the process; and, as the
temperature must not rise above 48° F., the vessels are surrounded with
iced water, which is kept in circulation. When a sufficient quantity of
glycerine has been run into the mixture, the latter is poured into a tub
of water. The nitro-glycerine being much heavier than the dilute acid
mixture, sinks to the bottom; the acid liquid is then poured off, and
more water added, this process being repeated until the nitro-glycerine
is quite free from acid.
Nitro-glycerine is, at ordinary temperatures, a clear, nearly
colourless, oily liquid, having a specific gravity of about 1·6. It has
a sweet, pungent taste, and if placed upon the tongue, or even if
allowed to touch the skin in any part, it causes a violent headache.
Below 40° F. it solidifies in crystals.
Dynamite is nitro-glycerine absorbed in a silicious earth called
kieselguhr. Usually it consists of about 75 per cent. nitro-glycerine
and 25 per cent. kieselguhr. The use of the absorbent is to remove the
difficulties and dangers attending the handling of a liquid. Dynamite is
a pasty substance of the consistence of putty, and is, for that reason,
very safe to handle. It is made up into cartridges, and supplied for use
always in that form.
SECTION III.--RELATIVE STRENGTH OF THE COMMON EXPLOSIVE AGENTS.
_Force developed by Gunpowder._--In the combustion of gunpowder, the
elements of which it is composed, which elements, as we have seen, are
carbon, hydrogen, nitrogen, oxygen, potassium, and sulphur, combine to
form, as gaseous products, carbonic acid, carbonic oxide, nitrogen,
sulphuretted hydrogen, and marsh gas or carburetted hydrogen, and, as
solid products, sulphate, hyposulphite, sulphide, and carbonate of
potassium. Theoretically, some of these compounds should not be
produced; but experiment has shown that they are. It has also been
ascertained that the greater the pressure, the higher is the proportion
of carbonic acid produced, so that the more work the powder has to do,
the more perfect will be the combustion, and, consequently, the greater
will be the force developed. This fact shows that overcharging is not
only very wasteful of the explosive, but that the atmosphere is more
noxiously fouled thereby. The same remark applies even more strongly to
gun-cotton and the nitro-glycerine compounds.
The careful experiments of Messrs. Noble and Abel have shown that the
explosion of gunpowder produces about 57 per cent. by weight of solid
matters, and 43 per cent. of permanent gases. The solid matters are, at
the moment of explosion, in a fluid state. When in this state, they
occupy 0·6 of the space originally filled by the gunpowder, consequently
the gases occupy only 0·4 of that space. These gases would, at
atmospheric pressure and 32° F. temperature, occupy a space 280 times
that filled by the powder. Hence, as they are compressed into 0·4 of
that space, they would give a pressure of
280
--- × 15 = 10,500 lb.,
0·4
or about 4·68 tons to the square inch. But a great quantity of heat is
liberated in the reaction, and, as it was shown in a former section,
this heat will enormously increase the tension of the gases. The
experiments of Noble and Abel showed that the temperature of the gases
at the instant of explosion is about 4000° F. Thus the temperature of
32° + 461°·2 = 493°·2 absolute, has been raised
4000
------ = 8·11 times,
493°·2
so that the total pressure of the gases will be 4·68 × 8·11 = 42·6 tons
to the square inch. And this pressure was, in the experiments referred
to, indicated by the crusher-gauge. When, therefore, gunpowder is
exploded in a space which it completely fills, the force developed may
be estimated as giving a pressure of about 42 tons to the square inch.
_Relative Force developed by Gunpowder, Gun-cotton, and
Nitro-glycerine._--Unfortunately no complete experiments have hitherto
been made to determine the absolute force developed by gun-cotton and
nitro-glycerine. We are, therefore, unable to estimate the pressure
produced by the explosion of those substances, or to make an accurate
evaluation of their strength relatively to that of gunpowder. It should,
however, be borne in mind that a correct estimate of the pressure
produced to the square inch would not enable us to make a full
comparison of the _effects_ they were capable of causing. For though, by
ascertaining that one explosive gives twice the pressure of another, we
learn that one will produce twice the effect of another; yet it by no
means follows from that fact that the stronger will produce no more than
twice the effect of the weaker. The rending effect of an explosive
depends, in a great measure, on the rapidity with which combustion takes
place. The force suddenly developed by the decomposition of the chemical
compounds acts like a blow, and it is a well-known fact that the same
force, when applied in this way, will produce a greater effect than when
it is applied as a gradually increasing pressure. But some calculations
have been made, and some experiments carried out, which enable us to
form an approximate estimate of the relative strength of these explosive
substances.
Messrs. Roux and Sarrau give the following as the result of their
investigations, derived from a consideration of the weight of the gases
generated and of the heat liberated. The substances are simply exploded,
and the strength of gunpowder is taken as unity.
---------------+---------+-------------+---------
|Relative |Heat in Units|
Substance. | Weight | liberated |Relative
|of Gases.| from 1 lb. |Strength.
---------------+---------+-------------+---------
Gunpowder | 0·414 | 1316 | 1·00
Gun-cotton | 0·850 | 1902 | 3·00
Nitro-glycerine| 0·800 | 3097 | 4·80
---------------+---------+-------------+---------
The relative strength is that due to the volume of the gases and the
heat, no account being taken of the increased effect due to the rapidity
of the explosion.
Alfred Noble has essayed to appreciate the effects of these different
explosives by means of a mortar loaded with a 32-lb. shot and set at an
angle of 10°, the distances traversed by the shot being taken as the
results to be compared. Considered, weight for weight, he estimates as
follows the relative strengths of the substances compared, gunpowder
being again taken as unity:--
Gunpowder 1·00
Gun-cotton 2·84
Dynamite 2·89
Nitro-glycerine 4·00
The relative strength, bulk for bulk, is, however, of greater importance
in rock blasting. This is easily computed from the foregoing table and
the specific gravity of the substances, which is 1·00 for gunpowder and
compressed gun-cotton, 1·60 for nitro-glycerine, and 1·65 for dynamite.
Compared in this way, bulk for bulk, these explosives range as
follows:--
Gunpowder 1·00
Gun-cotton 2·57
Dynamite 4·23
Nitro-glycerine 5·71
Hence, for a given height of charge in a bore-hole, gun-cotton exerts
about 2½ times the force of gunpowder, and dynamite about 4¼ times that
force.
SECTION IV.--MEANS OF FIRING THE COMMON EXPLOSIVE AGENTS.
_Action of Heat._--We have seen that the oxygen required for the
combustion of the carbon in gunpowder is stored up in the saltpetre. So
long as the saltpetre remains below a certain temperature, it will
retain its oxygen; but when that temperature is reached, it will part
with that element. To fire gunpowder, heat is therefore made use of to
liberate the oxygen, which at once seizes upon the carbon with which it
is in presence. The means employed to convey heat to an explosive have
been described in the preceding chapter. It is necessary to apply heat
to one point only of the explosive; it is sufficient if it be applied to
only one grain. That portion of the grain which is thus raised in
temperature begins to “burn,” as it is commonly expressed, that is, this
portion enters at once into a state of combustion, the saltpetre giving
up its oxygen, and the liberated oxygen entering into combination with
the carbon. The setting up of this action is called “ignition.” The hot
gases generated by the combustion set up ignite other grains surrounding
the one first ignited; the gases resulting from the combustion of these
ignite other grains; and, in this way, ignition is conveyed throughout
the mass. Thus the progress of ignition is gradual. But though it takes
place, in every case, gradually, if the gases are confined within the
space occupied by the powder, it may be extremely rapid. It is easy to
see that the gases evolved from a very small number of grains are
sufficient to fill all the interstices, and to surround every individual
grain of which the charge is composed. But besides this ignition from
grain to grain, the same thing goes on from the outside to the inside of
each individual grain, the grain burning gradually from the outside to
the inside in concentric layers. The successive ignitions in this
direction, however, of layer after layer, is usually described as the
progress of combustion. Thus the time of an explosion is made up of that
necessary for the ignition of all the grains, and of that required for
their complete combustion.
The time of ignition is determined in a great measure by the proportion
which the interstices, or empty spaces between the grains, bear to the
whole space occupied by the powder. If the latter be in the form of an
impalpable dust, ignition cannot extend throughout the mass in the
manner we have described; but we shall have merely combustion proceeding
from grain to grain. If, on the contrary, the powder be in large
spherical grains or pellets, the interstices will be large, and the
first gases formed will flash through these, and ignite all the grains
one after another with such rapidity that ignition may be regarded as
simultaneous. Thus the time of ignition is shortened by increasing the
size of the grains and approximating the latter to the spherical form.
But the time of combustion is determined by conditions contrary to
these. As combustion proceeds gradually from the outside to the inside
of a grain, it is obvious that the larger the grain is, the longer will
be the time required to burn it in. Also it is evident that if the grain
be in the form of a thin flake, it will be burned in a much shorter time
than if it be in the spherical form. Thus the conditions of rapid
ignition and rapid combustion are antagonistic. The minimum time of
explosion is obtained when the grains are irregular in shape and only
sufficiently large to allow a fairly free passage to the hot gases.
There are other conditions which influence the time of combustion; among
them is the _density_ of the grain. This is obvious, since the denser
the grain, the greater is the quantity of material to be consumed. But
besides this, combustion proceeds more slowly through a dense grain than
through an open one. The presence of moisture also tends to retard
combustion.
The progress both of ignition and of combustion is accelerated, not
uniform. In proportion as the grains are ignited, the gases evolved
increase in volume, and as the progress of combustion continues to
generate gases, the tension of these increases, until, as we have seen,
the pressure rises as high as 42 tons to the square inch. As the
pressure increases, the hot gases are forced more and more deeply into
the grains, and combustion, consequently, proceeds more and more
rapidly.
_Detonation._--By detonation is meant the simultaneous breaking up of
all the molecules of which the explosive substance is composed. Properly
the term is applicable to the chemical compounds only. But it is applied
to gunpowder to denote the simultaneous ignition of all the grains. The
mode of firing by detonation is obviously very favourable to the rending
effect required of blasting powder, since it reduces to a minimum the
time of explosion. It is brought about, in all cases, by means of an
initial explosion. The detonator, which produces this initial
explosion, consists of an explosive compound, preferably one that is
quick in its action, contained within a case sufficiently strong to
retain the gases until they have acquired a considerable tension. When
the case bursts, this tension forces them instantaneously through the
interstices of the powder, and so produces simultaneous ignition. A
pellet of gun-cotton, or a cartridge of dynamite, the latter especially,
makes a good detonator for gunpowder. Fired in this way, very much
better effects may be obtained from gunpowder than when fired in the
usual manner. Indeed, in many kinds of rock, more work may be done with
it than with gun-cotton or with dynamite.
The action of a detonator upon a chemical compound is different. In this
case, the explosion seems to be due more to the vibration caused by the
blow than by the heat of the gases from the detonator. Probably both of
these causes operate in producing the effect. However this may be, the
fact is certain that under the influence of the explosion of the
detonator, the molecules of a chemical compound, like nitro-glycerine,
are broken up simultaneously, or at least, so nearly simultaneously,
that no tamping is needed to obtain the full effect of the explosion.
Dynamite is always, and gun-cotton is usually, fired by means of a
detonator. A much larger quantity of explosive is needed to detonate
gunpowder than is required for dynamite, or gun-cotton, since, for the
former explosive, a large volume of gases is requisite. Dynamite
detonators usually consist of from six to nine grains of fulminate of
mercury contained in a copper cap, as described in the preceding
chapter. Gun-cotton detonators are similar, but have a charge of from
ten to fifteen grains of the fulminate. An insufficient charge will only
scatter the explosive instead of firing it, if it be unconfined, and
only explode it without detonation, if it be in a confined space.
SECTION V.--SOME PROPERTIES OF THE COMMON EXPLOSIVE AGENTS.
_Gunpowder._--The combustion of gunpowder, as we have seen, is gradual
and comparatively slow. Hence its action is rending and projecting
rather than shattering. This constitutes one of its chief merits for
certain purposes. In many quarrying operations, for instance, the
shattering action of the chemical compounds would be very destructive to
the produce. In freeing blocks of slate, or of building stone, a
comparatively gentle lifting action is required, and such an action is
exerted by gunpowder. Moreover, this action may be modified by using
light tamping, or by using no tamping, a mode of employing gunpowder
often adopted in slate quarries. The effect of the violent explosives
cannot be modified in this way.
Gunpowder is injured by moisture. A high degree of moisture will destroy
its explosive properties altogether, so that it cannot be used in water
without some protective covering. Even a slight degree of moisture, as
little as one per cent. of its weight, materially diminishes its
strength. For this reason, it should be used, in damp ground, only in
cartridges. This is, indeed, the most convenient and the most economical
way of using gunpowder in all circumstances. It is true that there is a
slight loss of force occasioned by the empty space around the cartridge,
in holes that are far from circular in shape. But at least as much will
be lost without the cartridge from the moisture derived from the rock,
even if the hole be not wet. But in all downward holes, the empty spaces
may be more or less completely filled up with dry loose sand.
The products of the explosion of gunpowder are partly gaseous, partly
solid. Of the former, the most important are carbonic acid, carbonic
oxide, and nitrogen. The sulphuretted and the carburetted hydrogen are
formed in only small quantities. The carbonic oxide is a very noxious
gas; but it is not formed in any considerable quantity, except in cases
of overcharging. The solid products are compounds of potassium and
sulphur, and potassium and carbon. These constitute the smoke, the dense
volumes of which characterize the explosion of gunpowder. This smoke
prevents the immediate return of the miner to the working face after
the blast has taken place.
_Gun-cotton._--The combustion of gun-cotton takes place with extreme
rapidity, in consequence of which its action is very violent. Its effect
is rather to shatter the rock than to lift it out in large blocks. This
quality renders it unsuitable to many quarrying operations. In certain
kinds of weak rock, its disruptive effects are inferior to those
produced by gunpowder. But in ordinary mining operations, where strong
tough rock has to be dealt with, its superior strength and quickness of
action, particularly the latter quality, produce much greater disruptive
effect than can be obtained from gunpowder. Moreover, its shattering
action tends to break up into small pieces the rock dislodged, whereby
its removal is greatly facilitated.
Gun-cotton may be detonated when in a wet state by means of a small
quantity of the dry material. This is a very important quality, inasmuch
as it allows the substance to be used in a wet hole without protection,
and conduces greatly to the security of those who handle it. When in the
wet state, it is uninflammable, and cannot be exploded by the heaviest
blows. Only a powerful detonation will bring about an explosion in it
when in the wet state. It is, therefore, for safety, kept and used in
that state. Since it is insensible to blows, it may be rammed tightly
into the bore-hole, so as to fill up all empty spaces. The primer of
dry gun-cotton, however, which is to detonate it, must be kept perfectly
dry, and handled with caution, as it readily detonates from a blow.
Gun-cotton, when ignited in small quantities in an unconfined space,
burns fiercely, but does not explode.
The products of the combustion of gun-cotton are:--carbonic acid,
carbonic oxide, water, and a little carburetted hydrogen or marsh-gas.
On account of the insufficiency of oxygen, already pointed out, a
considerable proportion of carbonic oxide is formed, which vitiates the
atmosphere into which it is discharged. Overcharging, as in the case of
gunpowder, causes an abnormal quantity of the oxide to be formed.
_Dynamite._--As combustion takes place more rapidly in nitro-glycerine
than in gun-cotton, the effects of dynamite are more shattering than
those of the latter substance. Gun-cotton holds, indeed, a mean position
in this respect between dynamite, on the one hand, and gunpowder on the
other. Dynamite is, therefore, even less suitable than gun-cotton for
those uses which are required to give the produce in large blocks. But
in very hard and tough rock, it is considerably more effective than
gun-cotton, and, under some conditions, it will bring out rock which
gun-cotton fails to loosen.
Dynamite is unaffected by water, so that it may be used in wet holes;
indeed, water is commonly used as tamping, with this explosive. In
upward holes, where water cannot, of course, be used, dynamite is
generally fired without tamping, its quick action rendering tamping
unnecessary.
The pasty form of dynamite constitutes a great practical advantage,
inasmuch as it allows the explosive to be rammed tightly into the
bore-hole so as to fill up all empty spaces and crevices. This is
important, for it is obvious that the more compactly the charge is
placed in the hole, the greater will be the effect of the explosion.
Moreover, this plastic character renders it very safe to handle, as
blows can hardly produce sufficient heat in it to cause explosion. If a
small quantity of dynamite be placed upon an anvil and struck with a
hammer, it explodes readily; but a larger quantity so struck does not
explode, because the blow is cushioned by the kieselguhr. If ignited in
small quantities in an unconfined space, it burns quietly without
explosion.
If dynamite be much handled out of the cartridges, it causes violent
headaches; and the same effect is produced by being in a close room in
which there is dynamite in the unfrozen state.
Dynamite possesses one quality which places it at a disadvantage with
respect to other explosives, namely, that of freezing at a comparatively
high temperature. At about 40° F. the nitro-glycerine solidifies, and
the dynamite becomes chalky in appearance. In this state, it is exploded
with difficulty, and, consequently, it has to be thawed before being
used. This may be safely done with hot water; performed in any other way
the operation is dangerous.
The products of the combustion of dynamite are carbonic acid, carbonic
oxide, water, and nitrogen. As, however, there is more than a
sufficiency of oxygen in the compound, but little of the oxide is formed
when the charge is not excessive. If, therefore, dynamite be properly
detonated, and overcharging be avoided, its explosion will not greatly
vitiate the atmosphere. But if it be only partially detonated
hypo-nitric fumes are given off, which have a very deleterious effect
upon the health. It is, thus, of the highest importance that complete
detonation should be effected, not merely to obtain the full effect of
the explosive, but to avoid the formation of this noxious gas. This may
be done by using a detonator of sufficient strength, and placing it well
into the primer.
_Firing Points of the Common Explosive Compounds._--The following table
shows the temperatures at which the commonly used compounds explode:--
-------------------+-----------+-----------------
|When slowly| When suddenly
| Heated. | Heated.
+-----------+-----------------
Gunpowder | .. |from 500° to 540°
Gun-cotton | 360° | 482°
Kieselguhr dynamite| 356° | 446°
Cellulose dynamite | 342° | 446°
-------------------+-----------+-----------------
Cotton powder explodes at the same temperatures as gun-cotton, and
lithofracteur at the same temperature as kieselguhr dynamite.
SECTION VI.--SOME VARIETIES OF THE NITRO-CELLULOSE AND THE
NITRO-GLYCERINE COMPOUNDS.
_Nitrated Gun-cotton._--It has been shown that gun-cotton contains an
insufficient quantity of oxygen for its complete combustion. To furnish
that which is wanting, gun-cotton has sometimes incorporated with it a
certain proportion of nitrate of potash, or of nitrate of baryta. This
compound, which, it will be observed, is at once a chemical compound and
a mechanical mixture, is known as “nitrated gun-cotton.”
_Cotton Powder, or Tonite._--The explosive which is now well known as
“tonite” or “cotton powder,” is essentially nitrated gun-cotton. It is
produced in a granulated form, and is compressed into cartridges of
various dimensions to suit the requirements of practice. The convenient
form in which tonite is made up, ready to the miner’s hand, has greatly
contributed towards bringing it into favour. But irrespective of this,
the fact of its being so highly compressed as to give it a density
equal, or nearly equal, to dynamite gives it a decided advantage over
the other nitro-cotton compounds as they are at present used.
_Schultze’s Powder._--In Schultze’s powder, the cellulose is obtained
from wood. The wood is first sawn into sheets, about ¹/₁₆ inch thick,
and then passed through a machine, which punches it up into grains of a
uniform size. These are deprived of their resinous matters by a process
of boiling in carbonate of soda, and are further cleansed by washing in
water, steaming, and bleaching by chloride of lime. The grains, which
are then pure cellulose, are converted into nitro-cellulose in the same
way as cotton, namely, by being treated with a mixture of nitric and
sulphuric acids. The nitro-cellulose thus produced is subsequently
steeped in a solution of nitrate of potash. Thus the finished compound
is similar in character to nitrated gun-cotton.
_Lithofracteur._--Lithofracteur is a nitro-glycerine compound in which a
portion of the base is made explosive. In dynamite, the base, or
absorbent material, is, as we have said, a silicious earth, called
“kieselguhr.” In lithofracteur, the same substance is used; but in
addition, a mixture of nitrate of baryta and charcoal, a kind of
gunpowder, is introduced. The object of employing this explosive mixture
is to increase the force of the explosion, the kieselguhr being an inert
substance. Obviously this object would be attained if the explosive
mixture possessed the same absorbent power as the kieselguhr. But
unfortunately it does not, and, as a consequence, less nitro-glycerine
is used. Thus what is gained in the absorbent is lost in the substance
absorbed. The composition of lithofracteur varies somewhat; but its
average proportion of ingredients are the following:--
Nitro-glycerine 52·50
Nitrate of baryta 16·40
Charcoal 2·85
Sulphur 25·75
Kieselguhr 22·50
------
100·00
------
_Brain’s Powder._--Brain’s powder is a nitro-glycerine compound, similar
in character to lithofracteur. The exact composition of the base has
never been published, so far as relates to the proportions of the
ingredients. But it is composed of chlorate of potash, charcoal, and
nitrated sawdust. The proportion of nitro-glycerine never exceeds 40 per
cent. Horseley’s powder contains about the same proportion of
nitro-glycerine in a base of chlorate of potash and nut-galls.
_Cellulose Dynamite._--In Germany, gun-cotton is used as an absorbent
for nitro-glycerine, the compound being known as “Cellulose dynamite.”
It is chiefly used for primers to explode frozen dynamite. It is more
sensitive to blows than the kieselguhr dynamite.
CHAPTER III.
THE PRINCIPLES OF ROCK BLASTING.
_Line of Least Resistance._--The pressure of a fluid is exerted equally
in all directions; consequently the surrounding mass subjected to the
force will yield, if it yield at all, in its weakest part, that is, the
part which offers least resistance. The line along which the mass
yields, or line of rupture, is called the “line of least resistance.” If
the surrounding mass were perfectly homogeneous, it would always be a
straight line, and it would be the shortest distance from the centre of
the charge to the surface. Such, however, is never the case, and the
line of rupture is, therefore, always a more or less irregular line, and
often much longer than that from the centre direct to the surface. It
will be obvious, on reflection, that the line of least resistance will
be greatly dependent upon (1) the texture of the rock, which may vary
from one point to another; (2) its structure, which renders it more
easily cleavable in one direction than in another; (3) the position,
direction, and number of the joints, which separate the rock into more
or less detached portions; and (4) the number and relative position of
the unsupported faces of the rock. All these circumstances must be
ascertained, and the position and the direction of the bore-hole
determined in accordance with them, in order to obtain the maximum
effect from a given quantity of explosive. It must not be supposed,
however, that this is a labour involving minute examination and long
consideration. On the contrary, a glance is generally sufficient to
enable the trained eye to estimate the value of those circumstances, and
to determine accordingly the most effective position for the shot. In
practice, the line of least resistance is taken as the shortest distance
from the centre of the charge to the surface of the rock, unless the
existence of joint planes, a difference of texture, or some other
circumstance, shows it to lie in some other direction.
_Force required to cause Disruption._--When the line of least resistance
is known, it remains to determine the quantity of the explosive compound
required to overcome the resistance along that line. This matter is one
of great importance, for not only is all excess waste, but this waste
will be expended in doing mischief. In mining operations, the dislodged
rock is violently projected, and the air is vitiated in an unnecessary
degree; and in quarrying, stones are shattered which it is desirable to
extract in a sound state. The evil effects of overcharging, in
occasioning the formation of noxious gases, was pointed out in the last
chapter. Of course it is not possible so to proportion a charge to the
resistance that the rock shall be just lifted out, and no more; because
neither the force developed by the charge, nor the value of the
resistance can be known with precision. But a sufficient approximation
may be easily arrived at to enable us to avoid the loud report that is
indicative of wasted force.
Charges of an explosive compound of uniform strength produce effects
that vary as the weight of those charges, that is, a double charge will
move a double mass. And, as homogeneous masses vary as the cube of any
similar line within them, the general rule is established that charges
of powder capable of producing the same effects are to each other as the
cubes of the lines of least resistance. Generally, the quantity of black
blasting powder requisite to overcome the resistance will vary from ¹/₂₀
to ¹/₃₀ of the cube of the line of least resistance, the latter being
measured in feet and the former in pounds. Thus, if the rock to be
blasted be moderately strong limestone, for example, and the shortest
distance from the centre of the charge to the surface of the rock be 3
feet, we shall have 3 × 3 × 3 = 27, the cube of the line, and ²⁷/₂₅ lb.
= 1²/₂₅ lb., or about 1 lb. 1 oz., as the weight of the powder required.
If dynamite be used, and we assume it to be four times as strong as
common black powder, of course, only one-fourth of this quantity will be
required. Also if gun-cotton, or cotton-powder, be used, and we assume
its strength to be three times that of black powder, one-third only
will be needed. Again, if Curtis’s and Harvey’s new extra-strong mining
powder fired by a detonator be employed, we may assume it to be twice as
strong as common black powder fired by the ordinary means, and
consequently we shall need only one-half the quantity indicated by the
formula.
It is neither practicable nor desirable that such calculations and
measurements as these should be made for every blast; their practical
value lies in this, namely, that if the principles involved in them be
clearly understood, the blaster is enabled to proportion his charges _by
sight_ to the resistance to be overcome, with a sufficient degree of
precision. A few experiments in various kinds of rock, followed by some
practice, will enable a man to acquire this power.
As it is a common and a convenient practice to make use of the bore-hole
as a measure of the quantity of explosive to be employed, we have
calculated the following table:--
---------+------------+----------+------------
Diameter | | |Dynamite (or
of |Black Powder|Gun-cotton| Tonite)
the Hole.| in 1 inch. |in 1 inch.| in 1 inch.
---------+------------+----------+------------
ins. | ozs. | ozs. | ozs.
1 | 0·419 | 0·419 | 0·670
1¼ | 0·654 | 0·654 | 1·046
1½ | 0·942 | 0·942 | 1·507
1¾ | 1·283 | 1·283 | 2·053
2 | 1·675 | 1·675 | 2·680
2¼ | 2·120 | 2·120 | 3·392
2½ | 2·618 | 2·618 | 4·189
2¾ | 3·166 | 3·166 | 5·066
3 | 3·769 | 3·769 | 6·030
---------+------------+----------+------------
[Illustration: FIG. 40.]
[Illustration: FIG. 41.]
[Illustration: FIG. 42.]
[Illustration: FIG. 43.]
_Conditions of Disruption._--Having explained the law according to which
the elastic gases evolved by an explosion act upon the surrounding rock,
and shown how the force required to cause disruption may be calculated,
it now remains to consider the conditions under which disruption may
take place. Suppose a block of unfissured rock detached on all sides, as
shown in plan, in Fig. 40, and a bore-hole placed in the centre of this
block. If a charge be fired in this position, the lines of rupture will
radiate from the centre towards any two, or towards all four of the
unsupported faces of the block, because the forces developed will act
equally in all directions, and the lines of rupture will be those of
least resistance. Evidently this is the most favourable condition
possible for the charge, since the rock offers an unsupported face on
every side; and it is evident that the line of rupture must reach an
unsupported face to allow of dislodgement taking place. Suppose, again,
as shown in Fig. 41, the block to be unsupported on three sides only,
and the charge placed at _h_. In this case, the lines of rupture may
run to any two, or to all three, of the unsupported faces; and hence
this will be the next most favourable condition for the action of the
charge. The greatest useful effect, however, will be obtained in this
case by placing the charge farther back at _h′_, when the lines of
rupture must necessarily run to the opposite faces _b c_, and,
consequently, the whole of the block will be dislodged. Assume another
case, in which the rock is unsupported upon only two sides, as shown in
Fig. 42, and the charge placed at _h_. In this case, the lines of
rupture must run to each of the unsupported faces _a b_. Thus, it is
evident that this condition, though still a favourable one for the good
effect of the charge, is inferior to the preceding. As rock is never
homogeneous in composition nor uniform in texture, the lines of rupture,
which, as before remarked, will be those of least resistance, may reach
the faces at any point, as at _m n_, _m′ n′_, or any point intermediate
between these. But it will be seen that the useful effect will be
greatest when these lines, radiating from the charge, make an angle of
180°, or, in other words, run in directly contrary directions, and that
the useful effect diminishes with the angle made by these lines of
rupture. Suppose, again, the rock to be unsupported upon one side only,
as shown in Fig. 43, and the charge placed at _h_. In this case, the
lines of rupture must run to the face _a_, and the condition must
therefore be considered as less favourable than the preceding. As in
those cases, the useful effect will depend upon the angle made by the
lines of rupture _h m_ and _h n_, which angle may be very small, and
which must necessarily be much less than 180°. A greater effect may be
obtained, under this condition, by firing several charges
simultaneously. If, for example, we have two charges placed, one at _h_,
and the other at _h′_, and fired successively, the lines of rupture will
run in or near the directions _h m_, _h n_, _h′ m′_, _h′ n′_, and the
portion of rock dislodged will be _m h n h′ n′_. But if these two
charges be fired simultaneously, the lines of rupture will be _h m_, _h
o_, _h′ o_, _h′ n′_, and the mass of rock dislodged will be _m h h′ n′_.
Simultaneous firing is in this way productive of a greatly increased
useful effect in numerous cases, and the mining engineer, and the
quarryman especially, will do well to direct their attention to this
source of economy. There is yet another case to be considered, in which
the conditions are still less favourable. Suppose two unsupported faces
at right angles to each other, and the charge placed at _h_, as shown in
Fig. 44. In this case, the lines of rupture will run to each of the two
unsupported faces; but as these lines must necessarily make a very small
angle with each other--for the length of the lines increases rapidly
with the angle--the useful effect will be less than in the last case. It
follows, therefore, that this is the most unfavourable condition
possible, and as such it should be avoided in practice.
[Illustration: FIG. 44.]
[Illustration: FIG. 45.]
[Illustration: FIG. 46.]
In the foregoing considerations, the holes have been assumed to be
vertical, and for this reason the unsupported face which is
perpendicular to the hole, that is, the face into which the hole is
bored, has been neglected. For it is evident that, under the conditions
assumed, the lines of rupture cannot reach this face, which, therefore,
has practically no existence. Suppose, for example, a bore-hole placed
at _h_, in Fig. 45, and the rock to be supported upon every side except
that at right angles to the hole. The forces acting perpendicularly to
the direction of the bore-hole are opposed on all sides by an infinite
resistance. Hence, in this case, either the tamping will be blown out,
or, if the forces developed are unequal to the work, no effect will be
produced beyond a slight enlargement of the hole at the base. This,
however, is a case of frequent occurrence in practice, and it becomes
necessary to adopt measures for making this unsupported face available.
Evidently this object can be attained only by so directing the bore-hole
that a line perpendicular to it may reach the face; that is, the line of
the bore-hole must make with the unsupported face an angle less than
90°. This direction of the bore-hole is shown in Fig. 46, which may be
regarded as a sectional elevation of Fig. 45. In this case, the lines of
rupture, which will run similarly to those produced in the case shown in
Fig. 43, will reach the unsupported face at _b_, and the length of these
lines, and consequently the depth of the excavation, for a given length
of bore-hole, will depend upon the angle which the latter makes with
the face. This mode of rendering a single exposed surface available is
called “angling the holes,” and it is generally resorted to in shaft
sinking and in driving headings. The conditions involved in “angling”
are favourable to the action of strong explosives.
_Example of a Heading._--To show how these principles are applied in
practice, we will take a typical case of a heading, 7 feet by 9 feet, as
shown in Fig. 47. In this case, we have at starting only one exposed
face, which is perpendicular to the direction of the driving. Hence it
is evident that we shall have to proceed by angling the holes. We might
begin in any part of the exposed face; but, as it will hereafter appear,
the most favourable position is the centre. We therefore begin at this
point by boring a series of holes, numbered 1 on the drawing. These
holes are angled towards each other; that is, the two sets of three
holes vertically above each other converge in the direction of their
lower ends, as shown in the sectional plan, Fig. 48. In this instance,
we have assumed six holes as necessary and sufficient. But it is obvious
that the number of holes, as well as their distance apart horizontally,
will be determined by their depth, the tenacity of the rock, and the
strength of the explosive used. When these holes are fired, a
wedge-shaped portion of the rock will be forced out, and this result
will be more effectually and certainly obtained if the charges be
fired simultaneously. The removal of this portion of the rock is called
“taking out the key.” The effect of removing this key is to leave the
surrounding rock unsupported on the side towards the centre; that is,
another face is formed perpendicular to the first.
[Illustration: FIG. 47.]
[Illustration: FIG. 48.]
[Illustration: FIG. 49.]
Having thus unkeyed the rock by the removal of this portion from the
centre, it will evidently be unnecessary, except for convenience or
increased effect, to angle any more of the shot-holes. The second series
therefore, numbered 2 in the drawing, may be bored perpendicularly to
the face of the heading. When this series is fired, the lines of rupture
will all run to the unsupported face in the centre--and from hole to
hole, if the shots be fired simultaneously--and the annular portion of
rock included between the dotted lines 1 and 2 will be removed. If the
shots be fired successively, the first will act under the condition of
one unsupported face, as illustrated in Fig. 43; but as another
unsupported face will be formed by the removal of the rock in front of
this charge, the succeeding shots will be subject to the more favourable
condition represented in Fig. 42. The firing of this second series of
shots still leaves the surrounding rock unsupported towards the centre,
and consequently the same conditions will exist for the third series,
numbered 3 on the drawing, the firing of which series will complete the
excavation. Fig. 49 shows the appearance of Fig. 48 after the firing of
the central holes.
It may be remarked here that, owing to the want of homogeneity in the
rock, and to the existence of joints and fissures, the outer line of
rupture will not, in practice, run so regularly as indicated, in this
assumed case, by the dotted lines. This circumstance will influence the
position of the holes, or the quantity of explosive, in the next series,
and furnish an opportunity for the exercise of judgment on the part of
the blaster.
There exist also other circumstances which will influence the position
and the number of the holes in a very important degree, and which
therefore must be taken fully into account at every advance. One of
these is the irregularity of the face of the excavation. Instead of
forming an unbroken plane at right angles to the direction of the
heading, or of the shaft, this face is broken up by projecting bosses
and more or less deep depressions. Obviously these protuberances and
cavities will influence, in no inconsiderable degree, the lines of least
resistance; the latter being lengthened or shortened, or changed in
direction, by the presence of the former, which give existence to
unsupported faces to which the lines may radiate. These conditions must,
in every case, be taken into account when determining the best position
for the bore-hole. Of yet greater importance, is the existence of joint
planes and bedding planes. A bed of rock may be, and frequently is, cut
up by these planes into detached blocks of greater or less dimensions,
according to the more or less perfect development of the different sets.
Hence it becomes necessary, in determining a suitable position for
blasting the charge, to consider such planes as unsupported faces, and
to ascertain the direction and length of the lines of resistance under
such conditions. If a charge be placed in close proximity to one of
these planes, not only may the lines of rupture run in unforeseen
directions, but the greater part of the force of the explosion will be
lost by the escape of the gases along the plane. The same loss of force
may be occasioned by the presence of a cavity, such as are of frequent
occurrence in cellular or vughy rock. When the joint planes are fully
developed, their existence can be ascertained by inspection; but when
their development is imperfect, there may be considerable difficulty in
discovering them. In such cases, the rock should be carefully inspected,
and sounded with a hammer or pick. When a cavity is bored into, it may
be rammed full of clay, and the boring continued through the clay; or if
sufficient depth has been obtained, the charge may be placed upon the
clay, which will prevent the wasteful dissipation of the gases. As none
of the aforementioned circumstances occur under precisely similar
conditions, no general rule of much service can be laid down; they are
matters upon which the blaster must be left to use his own judgment, and
to do this effectively, it is necessary that he possess some knowledge
of the materials with which he deals.
_Economical Considerations._--Besides the important economical
considerations involved in the foregoing, there are others which claim
attention. Foremost among these is the question whether, for a given
effect, it be better to augment or to diminish the individual importance
of the shots; that is, whether it be better to diminish the number of
the holes and to increase their diameter, or to diminish their diameter
and increase their number; or, again, to diminish their diameter and to
increase their depth, or to increase their diameter and to diminish
their number and their depth. It may be readily shown mathematically,
and the results are confirmed by experience, that there is an important
gain in reducing the diameter of the shot-holes to the lowest limit
allowed by the strength and the gravimetric density of the explosive,
and increasing their depth. The gain is mainly in the direction of a
saving of labour, and it is especially remarkable in the case of machine
boring. Here again we perceive the advantage of strength in the
explosive agent employed.
The simultaneous firing of the shots offers several important
advantages. It has already been shown how one charge aids another, under
such a condition, and in what way the line of rupture is affected by it.
When the shots are fired successively, each one has to _tear out_ the
portion of rock allotted to it; but when they are fired simultaneously,
their collective force is brought to bear upon the whole mass to be
dislodged. This is seen in the diagram, Fig. 43. When deep holes are
used, the greater useful effect caused by simultaneous firing becomes
very marked. Hence electricity associates itself naturally with machine
drills and strong explosives.
_Tamping._--To “tamp” a shot-hole is to fill it up above the charge of
explosive with some material, which, when so applied, is called the
“tamping.” The object of tamping is to oppose a resistance to the escape
of the gases in the direction of the bore-hole. Hence a primary
condition is that the materials used shall be of a strongly resisting
character. A second determining condition is that these materials shall
be of easy application. This condition precludes the use of all such
devices as plugs, wedges, and forms of a similar character, which have
been from time to time proposed.
The only material that, in practice, has been found to satisfactorily
fulfil the requirements, is rock in a broken, pulverulent, or plastic
state. As, however, all rock is not equally suitable, either from the
point of view of its resisting character, or from that of convenience of
handling, it becomes necessary to consider which satisfies the two
conditions in the most complete manner.
Though it is not easy to assign a perfectly satisfactory reason why one
kind of rock substance opposes a greater resistance to motion in a
bore-hole than another, yet it is certain that this resistance is mainly
due to the friction among the particles of that substance. If a column
of solid, hard rock, of the same diameter as the bore-hole, be driven
down upon the charge, the resistance opposed by the column to the
imprisoned gases will be, neglecting the weight of the former, that of
the friction between the sides of the column and those of the hole. But
if disintegrated rock be used, not only is an absolute motion imparted
to the particles, but, on account of the varying resistances, a relative
motion also. Consequently, friction occurs amongst the particles, and as
the number of these is immense, the sum of the slight friction of one
particle against another, and of the great friction of the outside
particles against the sides of the hole, amounts to a much greater value
than that of the outside particles of the solid column against the sides
of the bore-hole. If this view of the facts alone be taken, it follows
that dry sand is the most resistant material, and that the finer the
grains, the greater will be the resistance which it offers. In practice,
however, it has been found that though the resistance offered by sand
tamping is very great, and though also the foregoing inference is true
when the tamping is lifted by the pressure of a solid against it from
below, this substance is notably inferior to some others when acted upon
by an explosion of gases. The explanation of this apparent anomaly is
that the gases, under the enormous tension to which they are subjected
in the bore-hole, insinuate themselves between the particles, and so
prevent the friction which would otherwise take place. When the
readiness with which water, through the influence of gravity alone,
permeates even closely compacted sand, is borne in mind, there will be
no difficulty in conceiving a similar action on the part of more subtile
gases in a state of extreme tension. Under such conditions as these,
there is no resistance whatever due to friction, and the only resistance
opposed to the escape of the gases is that proceeding from the inertia
of the mass. How this resistance may be very great, we have shown in the
case of air tamping. Hence, it becomes necessary to have recourse to
some other material of a composition less liable to be thus acted upon,
or to seek means of remedying the defect which renders such action
possible.
Clay, dried either in the sun, or, preferably, by a fire, appears to
fulfil the requirements of a tamping material in the fullest degree.
This substance is composed of exceedingly minute grains of silicious
matters, bound together by an aluminous and calcareous or ferruginous
cement. Thus constituted, there are no voids between the particles, as
in porous substances, and, consequently, there is no passage for the
gases, the substance being impervious alike to water and gas. Hence,
when this material is employed as tamping, the forces act only upon the
lower surface, friction takes place among the particles, and the
requisite degree of resistance is produced. By reason of its possession
of this property, clay is generally used as the tamping material.
In rock blasting, it is usual to prepare the clay beforehand, and this
practice is conducive both to effective results and to rapidity of
tamping. The latter consideration is an important one, inasmuch as the
operation, as commonly performed, requires a good deal of time. To
prepare the pellets of clay, a lump is taken and rolled between the
palms of the hands until it has assumed the form of a sausage, from
three to four inches in length, and of the diameter of the bore-hole.
These pellets are then baked until they are thoroughly dry, when they
are ready for use. In making them up to the requisite diameter, a little
excess should be allowed for shrinkage, since it is essential that they
fit tightly into the hole. When the charge has been put in, and covered
with a wad of hay, or a handful of sand or rubbish, one of these pellets
is inserted and pushed home with a wooden rammer. Considerable pressure
should be applied to make the clay fill the hole completely, but blows
should be avoided. A second pellet is then pushed down in the same way,
and the operations are repeated until the whole of the hole is tamped.
To consolidate the whole, light blows may be applied to the outer
pellet. It will be found advantageous to place an undried pellet
immediately above the charge, because the plasticity of such a pellet
enables it to fill all the irregularities of the sides of the hole, and
to securely seal the passage between the sides and the tamping, along
which the gases might otherwise force their way. In coal blasting, soft
shale is always used for tamping, because it is ready at hand, and heavy
shots are not required.
Broken brick constitutes a fairly good tamping material, especially when
tempered with a little moisture; but as it is not readily procurable,
its application is necessarily limited. The dust and chippings of the
excavated rock are largely employed as tamping in quarries. This
material, however, has but little to recommend it for the purpose beyond
its readiness to hand.
It now remains to consider what means are available for remedying the
defect inherent in sand as a tamping material. This constitutes a very
important practical question, because if the defect can be removed, sand
will constitute by far the most suitable material whenever the bore-hole
has a downward direction. It can be everywhere obtained at a low cost;
it may be poured into the hole as readily as water; and its application
gives rise to no danger. Obviously the difficulty will be overcome if we
can find suitable means for preventing the gases from penetrating the
sand.
The end proposed may be successfully attained by means of the plastic
clay pellet applied in the following manner. Immediately above the
charge, place a handful of perfectly dry and very fine sand. This may be
obtained by sifting, if not otherwise procurable. Upon this sand, force
firmly down with a wooden rammer, so as to fill every irregularity, a
plastic clay pellet, about four inches in length, and of the same
diameter as the bore-hole, prepared by rolling between the hands in the
manner already described. Above this pellet, fill the hole with dry
sand. The impervious nature of the clay prevents the gases from reaching
the sand, except along the line of junction of the clay with the sides
of the hole. Tamped in this way, a resistance is obtained scarcely, if
at all, inferior to that opposed by the most carefully placed dried
clay.
By the employment of a detonator, the defect due to the porous character
of sand is not removed, but its influence is greatly diminished. When
detonation is produced in an explosive compound, the full force of the
elastic gases is developed instantaneously; and it has already been
shown that, under such conditions, the resistance occasioned by the
presence of any substance in the bore-hole, even the air alone, in the
case of nitro-glycerine, is sufficient to throw the chief portion of the
force upon the sides of the hole. Loose sand, therefore, may be
successfully employed as tamping under these conditions, since its
inertia will oppose a sufficient resistance to the escape of the gases.
But though the rock may be dislodged when light tampings are used with
detonation, there can be no doubt that a considerable proportion of the
force of the explosion is lost; and hence it will always be advantageous
to tamp securely by means of the clay pellet, as already described. The
highest degree of economy is to be obtained by detonating the charge,
and tamping in this manner.
CHAPTER IV.
THE OPERATIONS OF ROCK BLASTING.
_Hand Boring._--When the positions and the directions of the shot-holes
have been determined, the operations of blasting are begun by striking a
few blows with the hammer upon the spot from which the hole is to start,
for the purpose of preparing the surface to receive the drill. In some
cases, this preliminary operation will not be needed; but generally some
preparation is desirable, especially if the surface be smooth, and the
hole be to be bored at an angle with it. For the purpose of
illustration, we will take the case of a hole bored vertically
downwards, and will suppose the boring to be carried on by double-hand.
_Boring the Shot-holes._--The surface of the rock having been prepared
to receive the drill, one man sits down, and placing the shortest drill
between his knees, holds it vertically, with both hands. The other man,
who stands opposite, if possible, then strikes the drill upon the head
with the sledge, lightly at first, but more heavily when the tool has
fairly entered the rock. The man who holds the drill raises it a little
after each blow, and turns it partly round, the degree of turn usually
given being about one-eighth of a revolution. By this means, the hole is
kept circular, and the cutting edge of the drill is prevented from
falling twice in the same place. To keep the tool cool, and to convert
the dust and chippings into sludge, the hole is kept partially filled
with water, whenever it is inclined downwards. For this reason, downward
holes are sometimes described as “wet” holes, and upward holes as “dry”
holes. The presence of water greatly facilitates the work of boring. It
has been found by experience that the rate of boring in a dry and in a
wet hole varies as 1 : 1·5; that is, it takes one and a half times as
long to bore a dry hole as to bore a wet hole. Thus, by using water, the
time may be reduced by one-third. To prevent the water from spurting out
at each stroke and splashing the man who holds the drill, a kind of
leathern washer is placed upon the drill immediately above the hole, or
a band of straw is tied round it. When the hole has become too deep for
the short drill, the next length is substituted for it, which is in its
turn replaced by the third or longest drill as the depth becomes
greater. Each drill, on the completion of the length of hole for which
it is intended, is sent away to the smithy to be re-sharpened. In very
hard rock, the drills may have to be frequently changed, a circumstance
that renders it necessary to have several of the same length at hand.
The depth of shot-holes varies from 1 foot to 10 feet, according to the
nature of the rock, the character of the excavation, and the strength of
the explosive to be used. In shafts and in headings, the depth varies
generally between 2 feet 6 inches and 4 feet, a common depth being 3
feet.
The débris which accumulates at the bottom of the hole must be removed
from time to time to keep the rock exposed to the edge of the drill. The
removal of this sludge is effected by means of the tool called a
“scraper.” If the sludge is in too liquid a state to allow of its ready
removal by this means, a few handfuls of dust are thrown in to render
the mass more viscous. The importance of keeping the bore-hole clear of
sludge, and of shortening the time expended in using the scraper, has
led, in some localities, to the adoption of means for rendering the
sludge sufficiently viscous to adhere to the drill. When in this state,
the sludge accumulates around the tool rather than beneath it, the fresh
portion formed pushing the mass upward till it forms a thick coating
upon the drill throughout a length of several inches. When the tool is
withdrawn from the hole, this mass of débris is withdrawn with it; in
this way, the employment of a scraper is rendered unnecessary. This mode
of clearing the bore-hole is commonly adopted by the Hartz miners, who
use slaked lime for the purpose. This lime they reduce to the
consistency of thick paste by the addition of water, and they store it,
covered with water, in a small tin box, which they carry with them to
their work. To use this paste, they take a piece about the size of a
walnut, dilute it with water, and pour it into the bore-hole. This lime
paste is, for the purpose intended, very effective in friable rock,
especially if it be of a granular structure, as sandstone. As the grains
of sand resulting from the trituration of such rocks have no more
tendency to adhere to each other than to the drill, each of them becomes
covered with a coating of lime, which causes them to agglutinate into a
viscous mass possessing sufficient adhesiveness to enable it to cling to
the tool in the manner described.
When the hole has been bored to the required depth, it is prepared for
the reception of the charge. The sludge is all carefully scraped out to
clear the hole, and to render it as dry as possible. This is necessary
in all cases; but the subsequent operations will be determined by the
nature of the explosive, and the manner in which it is to be used. If
black powder be employed in a loose state, the hole must be dried. This
is done by passing a piece of rag, tow, or a wisp of hay, through the
eye of the scraper and forcing it slowly up and down the hole, to absorb
the moisture. If water is likely to flow into the hole from the top, a
little dam of clay is made round the hole to keep it back. When water
finds its way into the hole through crevices, claying by means of the
“bull” must be resorted to. In such cases, however, it is far more
economical of time and powder to employ the latter in waterproof
cartridges. Indeed, excepting a few cases that occur in quarrying,
gunpowder should always be applied in this way. For not only is a
notable saving of time effected by avoiding the operations of drying the
hole, but the weakening of the charge occasioned by a large proportion
of the grains being in contact with moist rock is prevented. But besides
these advantages, the cartridge offers security from accident, prevents
waste, and affords a convenient means of handling the explosive. It may
be inserted as easily into upward as into downward holes, and it allows
none of the powder to be lost against the sides of the hole, or by
spilling outside. These numerous and great advantages are leading to the
general adoption of the cartridge.
_Charging the Shot-holes._--When the hole is ready to receive the
explosive, the operations of charging are commenced. If the powder be
used loose, the required quantity is poured down the hole, care being
taken to prevent the grains from touching and sticking to the sides of
the hole. This precaution is important, since not only is the force of
the grains so lodged lost, but they might be the cause of a premature
explosion. As it is difficult to prevent contact with the sides when the
hole is vertical, and impossible when it is inclined, recourse is had to
a tin or a copper tube. This tube is rested upon the bottom of the
hole, and the powder is poured in at the upper end; when the tube is
raised, the powder is left at the bottom of the hole. In horizontal
holes, the powder is put in by means of a kind of spoon. In holes that
are inclined upwards, loose powder cannot be used. When the powder is
used in cartridges, the cartridge is inserted into the hole and pushed
to the bottom with a wooden rammer.
If the charge is to be fired by means of a squib, a pointed metal rod,
preferably of bronze, of small diameter, called a “pricker,” is placed
against the side of the bore-hole, with its lower pointed end in the
charge. The tamping is then put in, in small portions at a time, and
firmly pressed down with the tamping iron, the latter being so held that
the pricker lies in the groove. The nature of tamping has been already
fully described. When the tamping is completed, the pricker is
withdrawn, leaving a small circular passage through the tamping down to
the charge. Care must be taken in withdrawing the pricker not to loosen
the tamping, so as to close up this passage. A squib is then placed in
the hole thus left, and the charge is ready for firing.
If the charge is to be fired by means of safety fuse, a piece
sufficiently long to project a few inches from the hole is cut off and
placed in the hole in the same position as the pricker. When the powder
is in cartridges, the end of the fuse is inserted into the cartridge
before the latter is pushed into the bore-hole. The fuse is held in its
position during the operation of tamping by a lump of clay placed upon
the end which projects from the hole, this end being turned over upon
the rock. The tamping is effected in precisely the same manner as when
the pricker is used.
If the charge is to be fired by electricity, the fuse is inserted into
the charge, and the wires are treated in the same way as the safety
fuse. When the tamping is completed, the wires are connected for firing
in the manner described in a former chapter.
In all cases, before tamping a gunpowder charge placed loose in the
hole, a wad of tow, hay, turf, or paper is placed over the powder
previously to putting in the tamping. If the powder is in cartridges, a
pellet of plastic clay is gently forced down upon the charge. Heavy
blows of the tamping iron are to be avoided until five or six inches of
tamping have been put in.
When gun-cotton is the explosive agent employed, the wet material which
constitutes the charge is put into the shot-hole in cartridges, one
after another, until a sufficient quantity has been introduced. Each
cartridge must be rammed down tightly with a wooden rammer to rupture
the case and to make the cotton fill the hole completely. A length of
safety fuse is then cut off, and one end of it is inserted into a
detonator cap. This cap is fixed to the fuse by pressing the open end
into firm contact with the latter by means of a pair of nippers
constructed for the purpose. The cap, with the fuse attached, is then
placed into the central hole of a dry “primer,” which should be well
protected from moisture. When an electric fuse is used, the cap of the
fuse is inserted in the same way into the primer. The primer is put into
the shot-hole and pushed gently down upon the charge. As both the dry
gun-cotton and the detonator may be exploded by a blow, this operation
must be performed with caution.
Cotton-powder or tonite requires a somewhat different mode of handling.
It is made up in a highly compressed state into cartridges, having a
small central hole for the reception of the detonator cap. This cap,
with the safety fuse attached in the way described, or the cap of the
electric fuse, is inserted into the hole, and fixed there by tying up
the neck of the cartridge with a piece of copper wire placed round the
neck for that purpose. The cartridge is then pushed gently down the
shot-hole, or, if a heavier charge is required, a cartridge without a
detonator is first pushed down, and the “primed” cartridge put in upon
it. No ramming may be resorted to, as the substance is in the dry state.
When dynamite is the explosive agent used, a sufficient number of
cartridges is inserted into the shot-hole to make up the charge
required. Each cartridge should be rammed home with a moderate degree
of force to make it fill the hole completely. Provided a wooden rammer
be employed, there is no danger to be feared from explosion. A detonator
cap is fixed to the end of a piece of safety fuse, and, if water tamping
is to be used, grease, or white-lead, is applied to the junction of the
cap with the fuse. A “primer,” that is, a small cartridge designed to
explode the charge, is then opened at one end, and the detonator cap, or
the cap of the electric fuse, is pushed into the dynamite to a depth
equal to about two-thirds of its length, and the paper covering of the
primer is firmly tied to the cap with a string. If the cap be pushed too
far into the dynamite, the latter may be fired by the safety fuse, in
which case the substance is only burned, not detonated. With an electric
fuse this cannot occur. The same result ensues if the cap be not in
contact with the dynamite. The object of tying in the cap is to prevent
its being pulled out. The primer thus attached to the fuse is then
pushed gently down upon the charge in the shot-hole. It should be
constantly borne in mind that no ramming may take place after the
detonator is inserted.
Gun-cotton and tonite require a light tamping. This should consist of
plastic clay; or sand may be used in downward holes. The tamping should
be merely pushed in, blows being dangerous. A better effect is obtained
from dynamite when tamped in this way than when no tamping is used. In
downward holes, water is commonly employed as tamping for a dynamite
charge, especially in shaft sinking, when the holes usually tamp
themselves. But in other cases, it is a common practice to omit the
tamping altogether to save time.
_Firing the Charges._--When all the holes bored have been charged, or as
many of them as it is desirable to fire at one time, preparation is made
for firing them. The charge-men retire, taking with them the tools they
have used, and leaving only him of their number who is to fire the
shots, in the case of squibs or safety fuse being employed. When this
man has clearly ascertained that all are under shelter, he assures
himself that his own way of retreat is open. If, for example, he is at
the bottom of a shaft, he calls to those above, in order to learn
whether they be ready to raise him, and waits till he receives a reply.
When this reply has been given, he lights the matches of the squibs or
the ends of the safety fuse, and shouts to be hauled up; or if in any
other situation than a shaft, he retires to a place of safety. Here he
awaits the explosion, and carefully counts the reports as they occur.
After all the shots have exploded, a short time is allowed for the fumes
and the smoke to clear away, and then the workmen return to remove the
dislodged rock. If one of the shots has failed to explode, fifteen or
twenty minutes must be allowed to elapse before returning to the place.
Nine out of ten of the accidents that occur are due to these delayed
shots. Some defect in the fuse, or some injury done to it, may cause it
to smoulder for a long time, and the blaster, thinking the shot has
missed, approaches the fuse to see the effects produced by the shots
that have fired. The defective portion of the fuse having burned
through, the train again starts, and the explosion takes place, probably
with fatal consequences. Thus missed shots are not only a cause of long
delays, but are sources of great danger. Accidents may occur also from
premature explosion. In this case, the fuse is said to “run,” that is,
burn so rapidly that there is not sufficient time for retreat.
[Illustration: FIG. 50.]
When the firing is to take place by means of electricity, the man to
whom the duty is entrusted connects the wires of the fuses in the manner
described in a former chapter, and as shown in Fig. 50. He then connects
the two outer wires to the cables, and retires from the place. Premature
explosion is, in this case, impossible. When he has ascertained that all
are under shelter, he goes to the firing machine, and, having attached
the cables to the terminals, excites and sends off the electric current.
The shots explode simultaneously, so that only one report is heard. But
there is no danger to be feared from a misfire, since there can be no
smouldering in an electric fuse. The face may, therefore, be approached
immediately, so that no delay occurs, and there is no risk of accident.
Moreover, as all the holes can be fired at the moment when all is in
readiness, a considerable saving of time is effected. It is essential to
the success of a blast fired by this means that a sufficient charge of
electricity be generated to allow for a considerable loss by leakage. If
Siemens’ large dynamo-machine be used, the handle should be turned
slowly till a click is heard inside, and then, not before, the cable
wires should be attached to the terminals. To fire, the handle must be
turned as rapidly as possible, a jerky motion being avoided. As
considerable force is required, the machine must be firmly fixed. If a
frictional machine be used, care must be had to give a sufficient number
of turns. As this kind of machine varies greatly, according to the state
of the rubbing surfaces and the degree of moisture in the atmosphere, it
should always be tested for a spark before firing a blast. In this way
only, can the number of turns required be ascertained. It is important
that the discharging knob should be pushed in, or, as the case may be,
the handle turned backward, suddenly. A slow motion may be fatal to the
success of a blast. In testing Bornhardt’s machine, the handle should
always be turned forwards; but in firing, half the number of turns
should be given in one direction and half in the other. The following
table shows the number of turns required for a given number of André’s
fuses with Bornhardt’s machine. The first column, containing the least
number of turns, may be taken also for Julian Smith’s machine as
manufactured by the Silvertown Company with the modifications suggested
by W. B. Brain.
FIRING TABLE FOR FRICTIONAL MACHINE.
-----------------+----------------+----------------+----------------
| When the | When the | When the
| Machine sparks | Machine sparks | Machine sparks
| with 10 Turns. | with 12 Turns. | with 14 Turns.
-----------------+----------------+----------------+----------------
Fuses in Circuit.|Number of Turns.|Number of Turns.|Number of Turns.
-----------------+----------------+----------------+----------------
4 | 12 | 15 | 17
5 | 12 | 15 | 17
6 | 14 | 17 | 20
7 | 16 | 19 | 22
8 | 18 | 22 | 25
9 | 20 | 24 | 28
10 | 22 | 26 | 31
11 | 24 | 28 | 34
12 | 25 | 30 | 35
13 | 26 | 31 | 36
14 | 27 | 33 | 38
15 | 28 | 34 | 39
-----------------+----------------+----------------+----------------
NOTE.--If the machine does not spark with 14 turns, the rubber should
be taken out and brushed.
Places of refuge, called man-holes, are often provided in headings for
the blaster to retire into; these man-holes are small excavations made
in the sides of the heading. Sometimes it is necessary to erect a shield
of timbers in the heading for the protection of the men; such a shield
is frequently needed to protect machine drills from the effects of a
blast. In Belgium, it is a common practice to provide man-holes in the
sides of a shaft as places of retreat for the men; these holes are
called _caponnières_. Instead of caponnières, a hollow iron cylinder is
sometimes used as a protection to the men. This cylinder is suspended in
the shaft at a height of a few yards from the bottom, and is lowered as
the sinking progresses. The men climb into this cylinder to await the
explosion of the shots beneath them.
The workmen, on returning to the working face, remove the dislodged
rock, and break down every block that has been sufficiently loosened.
For this purpose, they use wedges and sledges, picks, and crowbars. And
not until every such block has been removed, do they resume the boring
for the second blast. Sometimes, to facilitate the removal of the rock
dislodged by the shots, iron plates are laid in front of the face in a
heading. The rock falling upon these plates is removed as quickly as
possible, to allow the boring for the succeeding blast to commence. It
is important, in the organization of work of this character, that one
gang of men be not kept waiting for the completion of the labour of
another.
MACHINE BORING.--In machine drilling, the operations necessarily differ
somewhat in their details from those of hand boring, and, in some cases,
other methods of procedure will be adopted more suitable to the
requirements of machine labour. It may even be, and in most cases indeed
is, inexpedient to follow closely the principles which lead to economy
of the explosive substance employed, since the more restricted
conditions under which machine power may be applied may point to more
important gains in other directions. Thus it may be found more conducive
to rapidity of execution to determine the position and the direction of
the shot-holes rather to satisfy the requirements of the machine than
those of the lines of least resistance; or, at least, these requirements
must be allowed to have a modifying influence in determining those
positions and directions. For it is obvious that holes cannot be angled
with the same ease when a machine drill is used, as they can when the
boring is executed by hand.
_Boring the Shot-holes._--It has already been remarked that the
exigencies of machine labour render it impracticable to follow closely
the principles which lead to economy of labour and material in blasting.
In hand boring, economy is gained by reducing to a minimum the number of
holes and the quantity of explosive substance required. But in machine
boring, economy is to be sought mainly in the reduction of the time
needed to accomplish the driving.
Attempts have been made to assimilate the methods of machine boring to
those adopted for hand labour, but the results have not been
satisfactory. On the contrary, the conditions determining the position
and the direction of the holes relatively to the production of the
greatest useful effect have been wholly ignored in favour of those which
determine the most rapid boring. This system has been attended with more
satisfactory results. Another system, partaking of both the preceding,
is widely adopted, and hitherto the best results have been obtained from
this, which may be regarded as a compromise between conflicting
conditions. Thus we have three systems of executing machine boring: one
in which a single machine is used upon a support capable of holding it
in any position, so as to be able to bore at any angle, and in which the
holes are placed according to the lines of least resistance, as in hand
boring. A second, in which several machines are fixed upon a heavy
support, allowing but little lateral or angular motion, and in which the
holes are placed at regular intervals apart, and bored parallel, or
nearly parallel, with the axis of the excavation, irrespective of the
varying nature of the rock, and the lines of least resistance. And a
third, in which it is sought, by the employment of one, two, or at the
most three machines, upon a simple and light support allowing the
position and direction of the machine to be readily changed, to satisfy
in some degree the two sets of conditions determining the two former
systems, by placing the shot-holes as far in accordance with the lines
of resistance as the exigencies of a fairly rapid handling of the
machine will allow.
In the first of these systems, the necessity for extreme lightness in
the machine is unfavourable to its efficient action, and the great
length of time consumed in changing the position of the machine, so as
to comply with the conditions of resistance in the rock, render it
impossible to attain a much higher rate of progress than is reached by a
well-regulated system of hand boring. With such a result, there is
nothing to compensate the first cost of the machinery, or in any way to
justify its adoption. In the second system, the time consumed in
removing and fixing the machines is reduced to a minimum, and the chief
portion of the time during which the machines are at the working face
is, consequently, occupied in actual boring, a circumstance that is
highly favourable to machine labour. Hence the rate of progress attained
by this system is greatly in excess of that accomplished by hand labour;
and this superiority has led to the adoption of the system in several
important cases, and has also led many to regard it as the most
favourable to the exigencies of machine drilling. But as the holes are
bored to suit the requirements of the machine, quite irrespectively of
the resistance of the rock, their positions and directions are very
unfavourable to the action of the explosive. This circumstance
necessitates a much greater number of holes to ensure the fracture of
the rock around each charge, and hence the time saved in shifting the
machines is in part lost in extra boring; besides which, the consumption
of powder is enormously increased. It would, therefore, appear that the
full advantages of machine boring are to be obtained from the
intermediate system, if carried out in accordance with all the
conditions of the case.
Assuming that we have a machine and a support of such dimensions,
weight, and construction as to be capable of being readily placed in
position, it is evident that we shall still require a much larger number
of holes than would be needed if the boring were performed by hand,
because they are not placed wholly in accordance with the lines of least
resistance. In some parts of the heading, indeed, these lines will have
to be wholly neglected, in order to avoid the loss of time involved in
shifting the supports; for the principle upon which an intermediate
system is based is to fulfil the requirements of the lines of least
resistance, when that can be conveniently done, and to neglect them,
when such fulfilment would involve a considerable expenditure of labour
and time.
In this way, the time both for fixing and removing the machines and of
boring is reduced to a minimum, and thus two conditions favourable to
rapid and economical progress is ensured. It is evident that when this
system is followed, the face will not require the same number of holes
at each blast. Another circumstance operating to increase the number of
shot-holes is the desirability of bringing down the face in fragments
small enough to be lifted without great difficulty. When the rock is
completely broken up, the labour, and, consequently, the time of
removing it after each blast, are lessened in an important degree. Hence
there will be an advantage in placing the shot-holes sufficiently close
together to ensure the fracture of the mass between each. These
circumstances render it necessary to bore a large number of holes when
the work is done by mechanical means. The boring of the additional holes
reduces the superiority of machine over hand labour, and the additional
quantity of the explosive required augments the cost of the work. To
counterbalance these disadvantages, the shot-holes should be bored deep.
It has already been pointed out that when a hole is once started with a
machine, the rate of progress is immensely superior to that attained in
hand boring, and to profit by this advantage, the hole should be
continued to as great a depth as practicable. This is sufficiently
obvious, since it amounts to increasing the proportion of the whole time
consumed that is occupied in actual boring; for as it is in the
rapidity of the operation of boring alone that the superiority of
machine labour exists, it is plain that the longer the proportion of the
time so occupied, the more marked that superiority will be. Thus, by
increasing the depth of the holes to the farthest practicable limit, we
approximate as much as possible to the condition most favourable to
machine boring. The intermediate system, therefore, which takes full
advantage of this means, will lead to the best results. To recapitulate
the main points of such a system; it should follow the lines of least
resistance when that can be conveniently done, and neglect them when the
fulfilment of their requirements would occasion a considerable
expenditure of time; and to counterbalance the disadvantages of machine
boring, it should employ shot-holes of as great a depth as is
practicable.
Supposing such a system in use, it now remains to consider the
operations of boring, and the subsequent operations of charging, firing,
and removing the rock dislodged by the blast. Of the method of executing
the boring, little remains to be said. It may, however, be well to
direct attention to the necessity of keeping the holes clear of the
débris. To ensure this, the bits should be chosen of a form suitable to
the nature and the structure of the rock, and the hole kept well
supplied with water. When the hole becomes deep, it should be cleared
out with a scraper during the time of changing the bit, and in very
argillaceous rock it may become necessary sometimes to withdraw the
tool, and to remove the accumulation with the scraper. When the débris
does not work out freely, its escape may be facilitated by giving a slow
motion to the tool, and then suddenly changing to a rapid motion. When
several machines are employed, the maximum number that can be applied
with advantage is one to the square yard of working face. The absolute
number of holes required in any case, will, of course, depend upon the
tenacity of the rock and the development of the joint planes, and also,
in some degree, by the lines of fracture due to the preceding blast. The
same circumstance will determine the distribution of the holes. Leaving
minor variations out of account, however, the same distribution will be
adhered to throughout the driving.
The manner of distributing the holes over the face of the heading may be
varied according to the judgment of the engineer in charge; that is, the
general features of the distribution to be adopted may be chosen to suit
the requirements of the machines and their supports. Also, it should be
noted that one method of distributing the shot-holes will require a less
number of them than another. Some examples will be found on Plate IX.,
where there are represented the Göschenen end of the St. Gothard tunnel;
the Airolo end of the same tunnel; the face of a stone drift driven at
Marihaye; that of a similar drift at Anzin; and that of a drift of the
same character at Ronchamp; the latter three examples being typical of
the distribution adopted in the French collieries.
The same mode of unkeying the face is adopted with machine as with hand
boring. Generally, two parallel rows of holes, from two to five in a
row, are bored in the middle of the face or fore-breast, the rows being
from 18 inches to 30 inches, according to the strength of the rock,
apart on the surface, and angled so as to be from 9 inches to 15 inches
apart at the bottom. These shots unkey the fore-breast; and it is
greatly conducive to a successful accomplishment of the operation, to
fire these shots simultaneously. Sometimes, when dynamite is used,
another method is adopted. A hole is bored horizontally in the centre;
at about three inches distant, are bored three other holes at an equal
distance apart. These latter are heavily charged with dynamite, the
centre hole being left empty. When these charges are fired, the rock
between them is crushed, and a large hole made. The lines of fracture of
the subsequent shots run into this hole. In this case, it is even more
desirable than in the preceding to fire the central shots
simultaneously.
In shaft sinking, if the strata are horizontal or nearly so, it is usual
to unkey from the centre, as in the heading. But if they be highly
inclined, it will be better to unkey from one side of the excavation.
The water which flows into the workings must be collected into one
place, both for convenience in raising it, and for the purpose of
keeping the surface of the rock clear for the sinkers. The depression
caused by the removal of the key serves to collect the water, and, on
that account, it is called “the sump.” Into this sump, the tub dips, or,
when pumps are used, the suction pipe drops. When the strata are highly
inclined, the water gravitates towards the dip side of the excavation,
and it becomes, therefore, necessary to place the sump in that
situation. The unkeying of the rock from this direction is, moreover,
favourable to the effect of the shots. In putting in the shot-holes, it
is well to avoid, as far as possible, terminating them in, or nearly in,
a bedding plane, because when so terminated, the force of the charge
expends itself along this plane. The position and the direction of the
holes will, however, be determined in some degree by the character of
the support used for the drills, and by other conditions of convenience.
_Charging and Firing._--The operations of charging the holes and firing
the shots demand particular attention when machine labour is employed.
It has been pointed out in the foregoing paragraphs that holes bored by
machine drills cannot be placed or directed strictly in accordance with
the requirements of the lines of least resistance; but that, on the
contrary, these requirements can only be approximately complied with,
and in some cases must be wholly neglected. To compensate in some degree
this defect of machine labour, the strength of the charges should be
varied according to the resistance which they will be required to
overcome. That is, the principles of blasting described in a former
chapter, which cannot be complied with by the borer, should be strictly
followed by the blaster in apportioning his charges. By this means, a
great saving of the explosive compound may be effected, and that without
difficulty or loss of time, if the blaster be intelligent and understand
his work. A glance will be sufficient to show what charge a given hole
of a known depth will require, and as cartridges of different sizes are
ready at hand, no delay is occasioned in making up the charge. The holes
in the centre, which are intended to unkey the face, require, of course,
the heaviest charge, since the conditions are there most unfavourable to
the effects of the explosion. And the more complete is the unkeying
resulting from this first explosion, and the more fractured and jointed
is the rock surrounding the cavity thus formed, the more may the charges
placed behind these unsupported faces be reduced.
As economy of time is, in machine boring, the chief end to be attained,
the tamping should be done with dried clay pellets previously prepared.
This material gives the greatest resistance, and thereby ensures the
maximum of useful effect; and if prepared beforehand, in the manner
described in the preceding chapter, the time consumed in tamping will be
reduced to a minimum. An abundant supply of such pellets should always
be ready at hand. In downward holes, such as are used in shaft sinking,
the plastic clay pellet and sand may be employed. This tamping may be
put in very rapidly, and, in all but very shallow holes, it is very
effective. When it is desired to use sand tamping in horizontal holes,
and holes bored in an ascending direction, the sand should be made up in
paper cartridges. The tamping employed in the St. Gothard tunnel
consisted of sand prepared in this manner. At the Mont Cenis tunnel, an
argillaceous earth was similarly prepared in paper cartridges for
tamping.
Firing the charges also affords an occasion for the exercise of
knowledge and judgment. A skilful determination of the order in which
the charges are to be fired will in a great measure compensate the ill
effects of badly-placed holes. The firing of a shot leaves the
surrounding rock more or less unsupported on certain sides; and it is
evident that to profit fully by the existence of these unsupported
faces, the succession of explosions must be regulated so that each shall
have the advantage of those formed by the preceding shots. This
condition can be wholly fulfilled only by simultaneous firing; but when
the firing is to take place successively, the condition may be
approximated to by regulating the succession according to the
indications observed on a careful inspection of the rock. Before firing
the charges, the blaster should consider the relative positions of the
holes, the stratification and jointing of the rock, the fissures caused
by the preceding blast, and any other circumstances that may influence
the results. The charges intended to unkey the face will be fired first,
and those in the concentric series will be then fired, in the order
determined upon, by means of different lengths of fuse. The series will
follow each other from the centre outwards. When a large number of shots
regularly placed in series have to be fired, a convenient practical
means of ensuring the successive explosion of the series, in the case of
the whole being lighted simultaneously, consists in bringing the fuses
from all the shot-holes together to one point at the centre. This method
of regulating the length of the fuses was adopted at the St. Gothard
tunnel.
It is obvious that the acceleration of the labour of excavation, which
has been effected in so remarkable a degree by the introduction of
machine drills and strong explosives, may be still further promoted by
the adoption of electricity as the firing agent. The advantages of
firing a number of shots simultaneously, some of which have already been
pointed out, are great and manifest. In the case of a driving, for
example, when all the holes have been bored and charged, and the
machines withdrawn, it is clearly desirable to blast down the face as
quickly and as effectively as possible. If the whole of the shots can
be fired at once, the time is reduced to a minimum, and, consequently,
the maximum of progress in a given time is ensured. Electricity affords,
indeed, the most convenient, the most effective, and the most safe means
of firing blasts. Hofrath Ritter von Pischof, the Austrian Chief
Inspector of Railways, in one of his reports, says:--“A greatly
increased amount of work and a notable saving of cost are effected when
the shots can be so disposed and fired as to mutually aid one another.
These results are obtained by employing electricity as the firing agent.
The experience which has been gained at the Büchenberg cutting, where
electrical firing has been extensively adopted, has shown that, when
properly employed, this means allows, in comparison with the ordinary
methods, twice the amount of work to be performed in a given time. It is
therefore highly desirable to adopt electrical blasting whenever it is a
question of economy of time and money.”
_Removing the dislodged Rock._--As the removal of the rock brought down
by the blast consumes a large proportion of the time saved by machine
boring, it becomes necessary to provide means for reducing this loss to
a minimum. The most important of these means is a suitable provision for
the rapid removal of the machine to a place of safety, and a
conveniently designed and well-laid tramway, upon which the rock may be
quickly run out without confusion and its consequent delay. The number
of wagons required to remove a given cube of rock may be readily
ascertained, and sufficient provision should be made for the transport
of these to “day” in the most rapid succession. The wagons should be of
such dimensions as to be capable of being handled without great
difficulty; the importance of this condition will be understood when the
frequency of derailments is borne in mind. The shovelling up of the
rubbish is greatly facilitated by laying iron plates in front of the
face to be brought down previously to the firing of the blast. This
expedient is often adopted in important drivings. It has also been
remarked that the dislodged rock can be more rapidly removed when it
exists in small blocks. Thus there will be an advantage in placing the
charges and in regulating their strength so as to completely break up
the rock. Another matter of importance in the arrangements for the rapid
removal of the rock brought down by the blast, is the proportioning of
the number of hands employed to the requirements of the case. This
number will increase with the size of the blocks to be lifted, the
distance to be run over, and the want of suitability in the _matériel_
employed.
_Division of Labour._--A proper division of labour is greatly conducive
to rapid and economical progress. The operations may be divided into
three series, namely: boring the shot-holes, charging and firing, and
removing the rock dislodged. Each of these series of operations may be
performed by different sets of men, and in several instances this
division of labour has been adopted. But it does not appear that such a
division leads to the most satisfactory results. The work of boring
occupies a much longer time than either of the other two series of
operations, and hence the distribution of the time is unequal. It has
been found that, generally, where all the arrangements have been well
considered, the labour of charging the shot-holes, firing the blast, and
removing the rock brought down, can be performed in about the same time
as that of boring. Thus it would seem to be more conducive to economy of
time to divide the men employed into only two sets: one set to bore the
holes, the other to perform all the subsequent operations. This division
has been adopted in numerous instances with favourable results.
Sometimes the whole of the operations have been performed by the same
set; but such an arrangement is not to be recommended. The labour of
directing the machines is of too distinct and skilled a character to be
confounded with that of removing the débris, without a strong reason for
such a proceeding, which does not appear to exist. Besides reserving a
set of men specially for this portion of the work, it is desirable to
keep the same men to the same machine, for in such a case each man gets
accustomed to the peculiarities of the machine entrusted to him, and
besides conceives a kind of affection for it that leads to careful
handling and watchful attention. In addition to the men required for the
operations referred to above, smiths will be needed to re-sharpen the
bits and to repair the machines. The amount of this labour will
obviously depend upon the number of machines employed, and the hardness
of the rock to be passed through.
EXAMPLES OF DRIVINGS.
_The St. Gothard Tunnel._--The St. Gothard tunnel is driven in five
sections. First, the “heading” is driven at the roof level 6 feet 6
inches wide, and 7 feet high. The position of the holes is shown in the
drawings on Plate IX. The number of holes at the Göschenen end is 28,
and the depth about 40 inches. The shots are fired by means of safety
fuse, the ends of the fuse being brought together at the centre. This
arrangement causes the shots to explode in the proper order of
succession. At a certain distance back from the face, is the “right
enlargement;” this is a widening of the heading to the limits of the
tunnel in that direction. Farther back is the “left enlargement,” by
which the heading is widened to the full width of the tunnel. Still
farther back is the first “bench cut,” in which one half of the floor is
blasted out to the full depth of the tunnel, and behind this again is
the second bench cut, in which the remaining half is removed. The
boring machines employed are the Dubois-François, the McKean, and the
Ferroux. The explosive agent used is dynamite. The rock is a tough
granite.
_The Hoosac Tunnel._--At the west end of the Hoosac tunnel, the system
adopted was the following. First, a centre cut was made by drilling two
rows of five or six holes each, about 9 feet apart on the face, and
converging to about 3 feet at their lower ends. The depth of these holes
was from 9 to 12 feet, according to the hardness of the rock. These
holes are numbered from 1 to 11 on Plate X. They were charged with
nitro-glycerine, and fired by electricity, Mowbray’s frictional machine
being used. As soon as the rock had been removed, the next series of
fourteen holes, numbered from 12 to 25, were drilled. These holes were
then charged and fired simultaneously like those of the first series.
When the rock dislodged had been removed, the third series of holes,
numbered from 26 to 41, were bored. This series, like the other two,
were charged, and fired by electricity. The effect of these three
blasts, which were fired within twenty-four hours, was to advance the
heading, 9 feet in height by the full width of 24 feet, to the extent of
7 feet 6 inches. The drawings on Plate XI. are: an elevation of the
fore-breast, which shows the positions of the shot-holes; a sectional
plan, which shows the directions of the first series of holes; a similar
plan, showing the directions of the second series of holes, and the
centre cut removed; and a sectional plan of the heading after the second
series have been fired, showing the direction of the third series of
holes.
The operations of taking out the “bench” were carried on at a distance
of about 170 yards back from the fore-breast. This was effected by first
drilling six holes 7 feet deep; two of these were each about 4 feet from
the face of the bench and close to the side of the tunnel, whilst two
others were each 4 feet behind these first holes, and the remaining two
holes were 8 feet from the face, 8 feet from the sides of the tunnel,
and 8 feet from each other. These were fired simultaneously, the result
being to lower the bench about 7 feet throughout the full width of the
tunnel. At a safe distance beyond this first bench cut, the same
operations were carried on by another gang of men, whereby the bench was
lowered to the floor of the tunnel, the full area of 24 feet in width by
22 in height being thus completed. The rock was a moderately tough
granite.
_The Musconetcong Tunnel._--The heading of the tunnel, shown on Plate
XII., like that of the Hoosac, was driven to the full width of the
tunnel. It is clear from theoretical considerations, and experience has
confirmed the conclusions, that the method of taking, with machine
drills, the whole width of the excavation at once conduces to rapidity
of advance, and to economy of explosive. In the example under
consideration, three tram lines were laid up to the face. The carriages
carrying the drills were run upon the two outside lines. These carriages
were simply stout frameworks of oak, each having in front three
horizontal iron bars, on which the drills were clamped in a way that
ensured easy lateral and vertical motion. After the firing of a blast,
all hands were set to shovel the dislodged rock into the middle between
the machine lines for the purpose of clearing the latter as soon as
possible to make way for the machines to be brought up for the next
boring. The lines being thus cleared, drilling was recommenced, and the
broken rock removed in wagons upon the centre line of rails. The heading
being 26 feet wide, there was ample room, and, a convenient system of
switching having been adopted, no delay was occasioned by a want of
wagons.
The system followed was that of centre cuts, and subsequent squaring up.
It consists in first blasting out an entering wedge or “key,” about 10
feet deep in this case, in the centre, and afterwards squaring up the
sides by several blasts. In the Musconetcong heading, twelve holes were
first drilled, as shown in the drawing, and marked C, A being the floor
of the heading. These holes were drilled with from 1½-inch to 2¾-inch
“bits,” in two rows of six, 9 feet apart on the face, and angled to meet
at the bottom. They were charged with 25 lb. of No. 1 and 50 lb. of No.
2 dynamite, and fired simultaneously by electricity. The No. 1 dynamite
was used in the bottom of these centre holes; in all the subsequent
blasts in squaring up, No. 2 only was used.
As soon as the cut was out, a second round of holes was started for the
first squaring up, as shown in the drawings, where they are numbered 1,
1, 1, 1, &c. In these and in the subsequent rounds, numbered 2, 2, 2, 2,
&c., and 3, 3, 3, 3, &c., the resistance to be overcome is, of course,
not so great as in the cut. In the first and the second squaring-up
rounds, from 50 lb. to 60 lb. of dynamite was used, and, in the third
round, this quantity was increased to 80 lb. or 90 lb., the resistance
becoming greater as the roof arch falls at the sides. In this third
round, there were generally one or two additional roof holes; these are
not shown in the drawing, as their position varied, according to the lay
of the rock. The top holes in the first round are also intended to bring
down any roof not shaken by the cut, and these are therefore angled
sharply towards the centre, and bored from 12 feet to 14 feet deep. In
the plan, Plate XII., the number 3 indicates the cut holes, and 4, 5,
and 6, the squaring-up rounds. The holes of the first squaring round
were always drilled about a foot deeper than the cut holes; when
blasted, these generally brought out an additional foot of shaken rock
at the apex of the cut. The following table shows approximately the
number and the depth of the holes required, and the quantity of dynamite
used for a linear advance of 10 feet.
---------------------+------+---------+------+------+------
| | |Total | |
|No. of| Depth |Depth | |
|Holes.|of Holes.| of |No. 1.|No. 2.
| | |Holes.| |
+------+---------+------+------+------
| | ft. in. | ft. | lb. | lb.
Cut | 12 | 10 6 | 126 | 25 | 50
1st square up | 8 | 12 0 | 96 | .. | 55
2nd „ | 8 | 12 0 | 96 | .. | 55
3rd „ | 6 | 12 0 | 72 | .. | 85
Additional roof holes| 2 | {10 0} | 18 | .. | ..
| | { 8 0} | | |
+------+---------+------+------+------
| 36 | .. | 408 | 25 | 245
---------------------+------+---------+------+------+------
The cut holes being 10 feet 6 inches deep, the blast usually brought out
about 9 feet full, which, as explained above, was increased to 10 feet
in the subsequent rounds. The cross section being about 175 square feet,
in an advance of 10 linear feet, there are about 65 cubic yards of rock
to be broken; this gives on an average 0·4 lb. of No. 1 and 4 lb. of No.
2 dynamite, and a little over 6 feet of drilling per cubic yard.
The “bench” was kept from 150 yards to 200 yards back from the face of
the heading, to avoid interruptions from the heading blasts, and to
allow plenty of room for handling the wagons, and for running back the
machines to a safe distance, previously to firing. The system adopted in
removing the bench is shown on Plate XII. First, six top holes, from 12
feet to 13 feet deep, were drilled and blasted; their relative positions
are shown in the drawings, A being the centre line, B, the sides in the
enlargement, B′, the sides of the heading, C, the face of the bench, and
1, 2, 3, 4, 5, 6, the holes. These six holes lifted the greater portion
of the rock; what was left was broken by several horizontal holes. These
two sets of holes, at the top and at the bottom, gave an average advance
of about 9 feet. The following table shows, for that advance, the number
of feet drilled, and the quantity of dynamite burned.
------------+------+------+---------+---------
| |Depth | Total |
|No. of| of | Depth | No. 2
|Holes.|Holes.|of Holes.|Dynamite.
+------+------+---------+---------
| | ft. | ft. | lb.
Top holes | 6 | 12 | 72 | 62
Bottom holes| 4 | 10 | 40 | 45
+------+------+---------+---------
Totals | 10 | 22 | 112 | 107
------------+------+------+---------+---------
The sectional area of the bench being about 306 square feet, an advance
of 9 linear feet gives about 102 cubic yards of rock to be removed. The
quantity of dynamite used was therefore 1·05 lb., and the depth of
boring 1·1 foot, per cubic yard of rock broken.
Three machines were used at this bench, two on the top and one below.
The holes were commenced with 2¾-inch bits, and terminated by 1½-inch
bits. The rock was a tough syenite.
CHAPTER VI.
SUBAQUEOUS BLASTING.
_Preparation of the Charge._--It is essential to the success of
subaqueous blasting operations, that the explosive substance used should
be suitable to the conditions under which it is to be applied. This is
true of all blasting, but the requirement is frequently overlooked in
some of the operations that have to be performed under water. In
clearing a wreck for salvage purposes, gunpowder will in most cases act
more effectually than either gun-cotton or dynamite. Also, in many
cases, this compound will prove more suitable than the stronger
substances in removing obstructions in water-courses. Examples of this
will be given hereafter. But when a wreck has to be broken up, when
piles, or objects of a similar character, have to be removed, or when
rocks have to be blasted, the more violent compounds will be found to
accomplish the purpose much more effectively. Generally, it may be
stated that when it is required merely to _remove_ objects, gunpowder is
the most suitable explosive agent to employ; and that when it is
required to _break_ objects, the nitro-cotton and the nitro-glycerine
compounds are the agents whose application is likely to be attended
with the greatest degree of success.
When gunpowder is used, means must be adopted to protect it from the
water, since a small proportion of moisture is sufficient to lessen, in
a very important degree, the force developed, while a large proportion
of moisture will destroy altogether its explosive properties. It is no
easy matter, under the most favourable circumstances, to keep the water
from the charge; but when the depth of water is considerable, it becomes
very difficult to attain that object. The pressure of a considerable
“head” will force the water through substances that, without a pressure,
are sufficiently impervious. At ordinary depths, metal canisters are
usually employed to contain gunpowder. Old oil-cans are as good as
anything for this purpose. The fuse, whether safety or electric, is
passed through the cork, and the latter is luted with some waterproofing
composition. The best consists of:
Tallow 1 part.
Rosin 3 parts.
Guttapercha 4 parts.
Swedish pitch 12 parts.
Instead of metal canisters, indiarubber bags are sometimes used. These
are, however, more expensive than the oil-cans, and, in many cases, they
are scarcely more efficient or suitable. Small charges of gunpowder may
be put into short lengths of indiarubber tubing, so as to form a kind
of cartridge. But care must be taken to close the ends securely. The
best way is to insert a cork, or if that cannot be obtained, a
cylindrical piece of wood, and to tie the tubing to this very tightly
with twine. The ends should then be dipped into the luting composition
described above. Tubing suitable for this purpose is sold under the
designation of “blasting tubes.” For large blasts, wooden casks are the
most suitable receptacle for the charge. The casks should be well
tarred, or, if the depth of water be great, laid over with pitch applied
very hot. Great care must be taken to protect the aperture through which
the safety fuse, or the wire of the electric fuse, passes.
In blasting under water with gunpowder, only the best and strongest
qualities of that compound should be used. The extra strong mining
powder of the Messrs. Curtis’s and Harvey’s, commercially known as the
E.S.M. powder, is, of all, the most suitable. It is also highly
conducive to success to detonate the charge. If the charge be not
detonated, the enclosing vessel is ruptured when only a small proportion
of the number of grains have been ignited, and, consequently, a large
proportion of the charge is blown away into the water unburned. Were
gunpowder in blasting charges always fired by a detonation, it would
compare in its effects far more favourably with the nitro-cotton and the
nitro-glycerine compounds than it does under the circumstances
attending the common method of firing it.
When gun-cotton is used, the difficulty of waterproofing is much
lessened, but not wholly removed. Inasmuch as this compound may be
detonated in the wet state, it is not required to take those precautions
which are necessary in the case of gunpowder. But, as we have pointed
out in a former chapter, the detonation of wet gun-cotton is effected by
means of that of a small quantity of the dry substance. This quantity,
which is generally employed in the form of a cylinder, and is called the
“priming,” must be thoroughly protected from the water. For this
purpose, indiarubber tubing may be used, or, if the primer be large,
indiarubber bags. When the pressure of the water is not great, a very
efficient protective covering is obtained by dipping the primer into
melted paraffine. Care should be taken to avoid raising the temperature
of the paraffine above the degree required to melt it completely. The
primer should be placed in contact with the charge, and it is desirable
that the latter, when it can be conveniently made to do so, should
surround the former.
Charges of gun-cotton for subaqueous blasts are usually made up of discs
of a large diameter, or of slabs of a rectangular form. When, however,
the charge has to be put into a bore-hole in rock, the common cartridge
is employed.
Tonite, or cotton powder, is largely used in subaqueous blasting
operations. This substance is always applied in a dry state, and
requires, therefore, to be protected from the water. This protection it
is however, not difficult to give. Being prepared for use in a very
highly compressed state, it does not readily absorb moisture. In this
state, it is enclosed in cartridges, which are subsequently dipped into
melted paraffine. This is the form and preparation adopted for ordinary
use. For application under water, especially when the depth is
considerable, additional protection is given. For wreckage purposes,
tonite may be obtained in convenient charges, made up in suitable forms,
and sufficiently protected.
When dynamite is used, the conditions are similar to those prevailing in
the case of gun-cotton. Since nitro-glycerine is unaffected by water, no
necessity exists for protecting it from moisture. But when a charge of
dynamite is immersed in water, and not contained in a bore-hole, the
nitro-glycerine rapidly exudes. The writer once made several ineffectual
attempts to explode a charge of dynamite at a depth of 70 fathoms
beneath the surface. The cause of failure was found to be this
exudation; for subsequent experiments showed that, though the dynamite
was in the form of the ordinary parchment paper cartridges, and was
contained in a stout canvas bag, the kieselguhr retained hardly a trace
of nitro-glycerine when the charge reached the surface from that depth,
after being rapidly lowered and raised. Hence it becomes necessary to
enclose dynamite within some fairly impervious substance, to prevent the
exudation of the nitro-glycerine. Waxed linen, or fine canvas overlaid
with the composition already described, may be used as a protective
covering; for blasts in deep water, indiarubber bags and tubing are
employed. When the charge is contained in a bore-hole in rock, exudation
can hardly occur, and therefore in such cases waterproofing is
unnecessary.
For firing subaqueous blasts with safety fuse, only the guttapercha
covered kinds are suitable. Great care must be taken to render the
junction of the fuse and the detonator water-tight. A stronger detonator
is required under water than in dry ground. Electric fuses offer not
only a cheaper, but a far more certain and suitable means of firing in
water. This means is now very generally employed. When tension currents
are used, the insulation must be very good. In all cases, ample power
should be possessed by the firing machine or battery.
The shattering class of explosives are very suitable for subaqueous-rock
blasting. In many cases, their employment renders the boring of
shot-holes unnecessary, an advantage of obviously great importance. When
detached or projecting masses of rock have to be broken up, it is
sufficient to place the charges upon them. Of course, when so applied,
larger quantities of the explosive are required; but though the method
is wasteful of explosive, it is very economical of labour and time.
Even when large undetached masses of rock have to be removed, the same
method may often be successfully followed. Suppose a level surface of
rock, for example. A few heavy charges judiciously distributed over this
surface will blow out craters of a considerable radius, and more or less
fracture the rock in their immediate neighbourhood. A few other blasts
then fired between these shattered points will break up the intervening
solid portions. Sometimes the rock will be disintegrated to a
considerable depth, and so broken up generally that it may be removed by
dredging. By proceeding in this way, the whole of the rock may often be
removed without any labour of boring.
But when the rock is too tough to be removed in this way, recourse must
be had to boring, though even when boring is necessary, an occasional
“loose” shot may be found to be very efficacious.
_Boring under Water._--The percussive drills, one of which, the
Darlington, was described in a former chapter, may be used effectively
under water. Compressed air is used as the motor fluid. The tripod
stand, having its legs weighted to give it stability, is generally the
most suitable support. These drills need the immediate attention of a
diver. Sometimes the boring is carried on by hand from the deck of a
vessel or from a raft provided for the purpose. The following
description will give a general notion of the operations involved in
subaqueous boring:--
The working vessel having been moored over the rock by means of
mooring-lines attached to buoys placed about 50 yards from each quarter
of the vessel, the diver descends and selects the most suitable position
for the blast; he then signals, by a certain number of pulls upon his
signal line, to have the drill and stand lowered to him. This being
quickly done by means of a steam derrick, he guides the drill-stand to
its place, and finally fixes it in position by means of its adjustable
legs. This being done, he signals for air to commence drilling.
It has been found that the drill can be worked in a rapid current as
well as in slack water. This allows the operations of drilling and
blasting, by a proper division of time and labour, to be conducted in an
extremely rapid tidal current, so that the principal work of the diver,
in inserting charges for blasting and slinging stone, may be done near
the periods of slack water, while the drilling may be advantageously
continued during the period of rapid flow. In a rapid current, the
stoppage of the drill for the purpose of “spooning out” the hole becomes
unnecessary, as the motion of the drill works up the débris to the mouth
of the hole, whence it is sucked out and carried off by the current in a
dark stream, like the smoke from the funnel of a locomotive. In a
sluggish current, or during slack water, the hose of the air-pump is
sometimes introduced, and air forced into the bore-hole to create a
current of water, by which means the hole is cleared more thoroughly
than by the most careful “spooning out.”
As soon as the hole is drilled to the required depth, the drill is
stopped; the diver then fastens the derrick chain, which is lowered to
him for the purpose, to the drill-stand, and signals to hoist away,
whereupon the machine is quickly hoisted on deck.
After having examined the hole and cleared away any débris remaining at
the bottom, the diver comes to the surface, and taking in his hand the
charge contained in a water-tight cartridge, and provided with its
electric fuse to which a sufficient length of insulated wire is
attached, returns with it, and inserts it into the drill hole, carefully
pressing it to the bottom with a rod. The tamping, if any is used, is
then inserted above the cartridge, and the diver comes up.
The working vessel having been quickly hauled by the mooring-lines to a
safe distance by means of capstans worked, whenever practicable, by the
steam-engine, the wires are attached to the machine, and at the signal
“all ready” the charge is fired.
The working vessel is then hauled back to her position, and as soon as
the water becomes sufficiently cleared of the dark muddy matter stirred
up by the blast, to enable the diver to see in it, he descends and
examines the result.
If the blast has been effective, he signals for the stone chains to be
lowered to him; which being done, he proceeds to sling the large pieces
of broken rock, one after another, as they are hoisted up and deposited
on deck. All the pieces large enough to sling having been thus removed,
he signals for the tub and shovel, and upon their being lowered to him,
proceeds to shovel into the tub the small fragments, and to have them
hoisted up and piled on deck, until the surface of the rock is
sufficiently cleared to place the drill for a new blast.
_Submarine Rocks._--The following brief account of the removal of the
“Tower” and the “Corwin” Rocks from the Narrows, at the entrance of
Boston Harbour, U.S., from the pen of J. G. Foster, is instructive as
illustrating the method of procedure in submarine blasting, and as
showing the unfitness, for work of that character, of the slow-burning
explosives:--
“Tower Rock,” being the smaller of the two, was selected as the one to
be first removed. Its horizontal dimensions being only 50 by 26 feet, it
was estimated that one large central charge surrounded by five or six
others, all in large and deep drill-holes, would be able to rend the
rock into pieces.
The working vessel, the sloop “Hamilton,” of 70 tons, was moored over
this rock on the 30th of July, 1867, and the new submarine drilling
machine, designed for this work, by Mr. Townsend, the contractor, was
placed in position and tried.
Several imperfections were found at the first trial, which prevented its
efficient working. While these were being remedied, a trial was made of
surface blasts, placed in and around the rock in the positions most
favourable to their action. These proved to be entirely without effect.
No seams or breaks were made by them in the smooth surface of the rock.
As soon as the submarine drilling machine was perfected, it was put in
operation, and successfully worked. The central and the surrounding
holes were drilled to depths varying from 2 to 8 feet, each hole being
3½ inches in diameter. These were well charged with black blasting
powder, and tamped with sand. In some holes, the charges produced no
visible effect, the tamping being blown out like the charge from a
cannon. In others, a crater was formed, but with a radius only about
one-half the line of least resistance. The holes that were intact were
then deepened, and new ones drilled; these were charged with Dupont’s
sporting powder. The result was much better, but not what was desired.
The pressure of the water, from 23 to 33 feet in depth, seemed to
diminish largely the ordinary explosive effect of gunpowder upon rock,
as seen in blasts in the open air.
Trial was then made of the patent safety blasting powder, manufactured
by the Oriental Company of Boston, the proportions of the ingredients
having been modified to increase its strength for this especial use.
This produced the desired effect. The rock was rent in pieces; and by
drilling additional holes and continuing large charges of the powder,
the rock was finally reduced to the required depth.
To smooth off its upper surface and break down the sharp projecting
points, large surface charges of sporting powder were employed. These
accomplished the result to a limited extent, but not completely. A large
15-inch shell was then placed in a crevice near the centre of the rock
and fired. Its explosion swept the rock completely, breaking down and
levelling the projecting points.
The work upon this rock occupied eight weeks. In that time, 80 tons of
stone had been blasted out, hoisted up, and deposited on shore,
attaining the required depth of 23 feet at mean low water. About 70 tons
of small fragments were suffered to remain on the bottom around the
rock, where they had been thrown by the blasts, and where they could do
no harm.
The cost per ton of the quantity hoisted up and deposited on shore was
64·93 dollars, no account being taken of the quantity blown, in small
fragments, into deep water.
“Tower Rock” having been entirely removed to the required depth, the
moorings of the working vessel were at once removed to “Corwin Rock,”
and work commenced upon it on the 1st of October, 1867. This rock was
found to be much more difficult to blast, on account of its extremely
tortuous lamination, its great toughness, and the presence of a great
number of iron pyrites.
Surface blasts were also tried upon this rock at the outset, in hopes
that, by being placed in the most favourable positions between the sharp
ridges of the rock, they might break them down. These, however, like
those upon Tower Rock, entirely failed to produce any noticeable effect,
even when they contained four and five hundred pounds of the best
sporting powder. The drilling machine was therefore called into
requisition as before, and used continuously till the completion of the
work.
On account of the extent of this rock, a different plan of operations
for its removal was adopted. One side of the rock most favourable for
blasting was selected, and a row of holes drilled parallel to the edge,
and at a distance from it equal to the depth of the holes, which was
taken to extend 1 foot below the required level, 23 feet at mean low
tide. After blasting out these holes, a new line of holes was drilled
parallel to the former line, or to the “face” left by the blasts, and
these also were blasted out; then a third line, and so on, progressing
regularly across the rock, continually blasting it off in parallel
blocks, extending downward a little below the depth required.
The advantages of this mode of operation were that it enabled the blasts
to act laterally, in which direction they were the most powerful; and
the rock was left, after each series of blasts, with a nearly vertical
side, or “face,” in which the presence of seams could be more readily
detected, and the character of the strata observed, so that the most
favourable positions could be selected for the next blasts.
Sometimes the craters, following the strata, ran under, or left an
overhanging “face,” in which case a large charge placed under its
projecting edge, usually had the effect of throwing off the overhanging
portion, and sometimes of dislodging large masses.
After the rock had been in this way blasted entirely across, and to the
general depth required, a careful survey was made, the soundings being
taken in lines from 5 to 10 feet apart, and at right angles to each
other, the lower end of the sounding pole being placed by the diver
alternately upon the highest and the lowest points.
This survey showed that although more than the required depth had been
generally attained, yet many points projected above this level by
distances varying from 2 to 14 inches.
To remove these, large surface charges were again tried, but with the
same ineffective result. Their only effect was to pile up the sand and
small fragments of stone into irregular windrows on the surface of the
rock. Small holes had, therefore, to be drilled at each of these points
to blast them off. This occupied much more time than could reasonably
have been expected; so that it was not until two months’ labour had been
expended, that all the points were finally reduced to the required
level.
_Obstructions in Water-courses._--The removal of obstructions from
water-courses often leads to much subaqueous blasting. Trees that have
fallen into the stream are most effectively broken up by charges of
gunpowder fired by a detonation. The success of the operation will,
however, be greatly dependent upon the judicious placing of the charges.
Brickwork may also be very effectively dealt with by charges of
gunpowder. But stone masonry and blocks of rock may be more effectively
broken up by gun-cotton, tonite, or dynamite. For work of this
character, electrical firing offers great advantages, for, besides its
convenience, it allows of several charges being exploded simultaneously,
a condition that is always favourable, and in many cases essential, to
success.
The following highly interesting and instructive account of the removal
by blasting of some obstructions in certain rivers in India is given by
Lieut. A. O. Green, R.E.
[Illustration: FIG. 51.]
[Illustration: FIG. 52.]
He, in company with some assistants, left Calcutta for Maldah on the 8th
of April, 1874, where they commenced work on the following day upon the
wreck of a large county boat, which lay on the top of a tree in
mid-stream, as shown in Fig. 51. Soundings were taken over and around
this tree, which was found to be about 3 feet 6 inches in diameter at
its base. The gunpowder intended to be used in these operations not
having arrived, three 5-lb. charges of gun-cotton were made up; it was
thought that under the 15-feet head of water, these would have been
sufficient to break the tree in half. The gun-cotton was in the form of
compressed discs, 2½ inches in diameter, and 2 inches thick, each disc
weighing about 5 ounces. These discs were filled into a tin cylinder to
within about 4 inches of the top. An electric fuse, with wires attached,
having been securely pushed into the hole in the centre of the top disc,
the empty space above was filled up, with first a layer of sawdust, and
then a layer of plastic clay, well rammed. The whole was then painted
over, and the upper end tied up in a covering of waxed cloth, the holes
through which the fuse wires passed being carefully luted. The charge,
thus made up, is shown in section in Fig. 52. It was fired by means of
a dynamo-electric machine.
The first charge produced but little effect; a second failed from the
case not being water-tight; a third charge was more effective, as it
lifted the tree and the boat partially out of the water. The positions
of these gun-cotton charges are indicated by circles on the figure. The
next day, two charges of gunpowder, of about 70 lb. each, were placed
under the boat, these charges being lashed on to the snag by the divers.
These charges consisted simply of common oil-tins, carefully cleaned and
painted over with red-lead paint. The bunghole was closed by a wooden
plug, bored through to allow the fuse wires to pass. This plug, after
being inserted, was coated over with a waterproofing compound. The
effect of the two charges was to completely demolish the boat. Another
charge of 50 lb. removed the tree underneath. The positions of these
gunpowder charges are indicated by squares in Fig. 51.
The next obstruction met with was a sand bank caused by a boat which had
broken in half and then sunk. The sand nearly covered the boat, so that
there was little else to operate upon. A charge of 80 lb. (one large
charge being considered preferable to two or three small ones in getting
rid of the sand), placed close to the part of the boat that was visible,
made a considerable crater, and a second charge of 80 lb. was placed in
a much more favourable position, as nearly all the boat was removed
except portions of the bow and stern, which required two separate
charges of 50 lb. each before they disappeared. In half an hour, the
whole of the sand bank had been washed away by the stream, and there was
from 3 to 4 feet of water over the spot where before the sand was high
and dry out of the water. The removal of this obstruction was dangerous,
owing to the nearness of the boat to the surface, the consequent small
resistance offered to the projection of its pieces through the air, and
the largeness of the charges used. Had, however, small charges been
used, it is more than probable that the small craters made by them would
have become too quickly filled up again to have been of any good in
facilitating the placing in position of subsequent ones.
The following day, a large mango tree, about 4 feet 6 inches in
diameter, was destroyed by two 50-lb. charges, which broke it up into
three pieces, easily removable ashore.
A few days later, a large trunk of a tree, about 3 feet in diameter, was
removed with two 50-lb. charges; but the depth of water over it was so
small that a large portion of the trunk was thrown a considerable
distance on shore. The next day a large tree which had formed a sand
bank was very successfully removed by a charge of 50 lb. placed among
the roots, it being considered that a smaller charge than 50 lb. would
not have effected the purpose. Opposite to Kásimpore, a boat was removed
with a charge of 50 lb. placed in the centre up-stream, which entirely
demolished it, the pieces being all dragged ashore. At Mootyá, a large
cotton tree, the wood of which is extremely tough, was found with many
large branches projecting out of the water. A charge of 70 lb. tied
under the tree at the springing of the branches effectually broke it up,
and the pieces were all hauled to land. Three miles farther down the
river, an attempt was made to destroy another large cotton tree with a
similar charge, but it only broke it into three pieces, and two more
charges of 50 lb. each were necessary to clear it away effectually. This
tree was, if anything, slightly larger than the last, i. e. from 3 to 4
feet in diameter, and there was less water over it.
Farther on, the party came across a collection of three or four trees,
with their branches interlaced, lying on a sand bank near Alumpore
Dáldah; these were sufficiently broken up by a 70-lb. charge to make
them easy of removal by coolie labour. Opposite to the village, another
awkward snag, in the shape of a large tree sticking up in 30 feet of
water, was destroyed by tying a 70-lb. charge at its base. A charge of
50 lb. of powder under this head of water, or even a smaller amount,
might have been sufficient, but as the work had to be done quickly, not
much account was taken of a few pounds of powder more or less, provided
the object was attained. At Gomashtapore, a large tree, branches and
all, was found in 25 feet of water, lying in the channel under the bank.
The current here was considerable, and some difficulty was experienced
in placing the charges. One charge of 70 lb. broke the tree in half;
another of 50 lb. at the springing of the branches broke them up; and
another of the same weight got rid of the roots. Below Gomashtapore, a
large mango tree was demolished with 60 lb. of powder. A short distance
farther on, a bad bamboo snag was met with. These bamboo snags, which
were merely the roots of the bamboos with perhaps a dozen or so whole
ones left, gave much trouble. Fig. 53 gives an idea of what these snags
are like. It was found impossible to place a charge underneath this one,
so an opening was prized between the bamboos, and a charge of 70 lb.
rammed down pretty well into the middle. This cleared the whole of it
away and opened the channel.
[Illustration: FIG. 53.]
At Chandpore, at a re-entering angle of the river and in a place
peculiarly dangerous to navigation during the rains, was an enormous
banyan tree (_Ficus Indica_), the main trunk of which, to judge from the
branches, must have been at least from 12 to 15 feet in diameter. An
approximate measurement was made with a pole, but any such measurement
can only have been a very rough one.
[Illustration: FIG. 54.]
[Illustration: FIG. 55.]
The trunk was lying in deep water, but the branches, more like an
accumulation of large trees, were lying stretched out for a considerable
distance over the bank, covering an area of more than 80 square feet. A
charge of 200 lb. of powder was made up in an indiarubber bag, and
placed by the divers in about 28 feet of water, well under the trunk of
the tree. The effect of this was to split the trunk up into several
pieces, each of which subsequently required separate removal. A 70-lb.
charge was next fired under two of the largest pieces in 18 feet of
water, and this broke them up completely. Having now run out of all the
cases for powder, three charges of gun-cotton, similar to the first,
were made up, and fired separately, each placed under a good thick
branch, about 8 feet in girth. The effect of all three was prodigious;
seemingly greater than that of the 70 lb. of gunpowder. As there were no
more cases left, and time was precious, some common earthenware ghurrahs
were obtained from the village as a makeshift. These held about 20 lb.
of powder; the fuse was placed in the centre in a disc of gun-cotton,
and the neck was closed up with damp earth, white-lead paint, &c., just
in the same way as the gun-cotton charges had been. A rope for lashing
them to the obstacle was securely fastened round the neck, and the fuse
wires were tied under, this lashing, leaving a small loop towards the
fuse free, so as to avoid any chance of a strain being brought on the
fuse in lowering the charge. Figs. 54 and 55 show the arrangement of
this charge. The first one tried had but little effect when placed
under a branch of the tree in deep water, and it was accordingly
determined to wait for cases from Calcutta; but after waiting five days
without their appearing, three more of these charges were tried, and
this time with very excellent results. They were indeed so satisfactory,
that the same evening four more were made up and fired. The first under
a mango tree a little farther down the river. This broke it in half,
throwing one part high and dry on shore, and the other into deep water.
The other three were fired under the remaining branches of the banyan
tree with very good effect, cutting them away.
It is more than probable, observes Lieut. Green, that the good results
obtained with all these ghurrah charges were entirely due to the
gun-cotton disc inside causing the gunpowder itself to detonate, so that
the thinness of the envelope was of little moment in determining the
force of the explosion.
The tin cases having arrived, the rest of the powder was made up into
five charges of 48 lb. and three ghurrah charges of 20 lb each. About
four miles farther down the river, there was an old peepul tree lying in
mid-channel, with several of the branches above water. Two tins, one
placed under the springing of the branches and the other under the
roots, blew away the lower branches on which the tree was resting, and
it sank slightly in the water. A ghurrah was next fired under the trunk
with splendid results, the tree disappearing entirely except one branch,
which required another small charge to remove it. The trunk of this tree
was nearly 8 feet in diameter, but of soft stringy wood.
On returning to camp, a small charge of 2 lb. of gun-cotton was made up
in a section of bamboo, and used against the banyan tree with very good
effect, and a ghurrah charge demolished the last branch but one. The
next day 1½ lb. of gun-cotton in a piece of bamboo finished the last of
this enormous tree.
After clearing away several more trees, the foundations of an old
factory, which had slipped into the stream, were removed by introducing
two charges of 1½ lb. of gun-cotton, in the ends of two bamboos, well
into the crevices of the masonry under the water.
Another obstruction consisted of a row of old piles, about 15 inches
square, stretching across the river below the surface of the water. Six
of the most dangerous of these were removed from the dry season channel
with ghurrah charges tied to the foot of the piles.
An old well that had fallen bodily into the water was afterwards met
with. The position of this well is shown in Fig. 56. A charge of 4 lb.
of gun-cotton completely destroyed it.
Near Azimgunge, the trunk of a very large peepul tree was found sunk in
deep water. It was so large that it was thought necessary to place a
100-lb. charge underneath it; this charge broke it up completely, but
two small charges of 20 lb. each were subsequently required to remove
the pieces.
[Illustration: FIG. 56.]
Later on, a well, similar to the one previously destroyed, was met with.
The brickwork was remarkably good and about 3 feet thick, and the mortar
was excellent. One charge of 4 lb. of gun-cotton broke it up into large
pieces; but it took another similar charge, and two charges of 20 lb. of
gunpowder to destroy it completely. On the same day, two trees were
removed with ghurrah charges, which had been used throughout, for small
charges, with unvarying success.
At a place called Farrashdangah, there was a very bad obstruction in the
river, caused by the remains of an old bathing ghat and bridge having
been cut out from the bank by the water getting underneath the masonry.
Both were projecting about 3 feet above the water, and in the rainy
season they formed the centre of a very nasty and dangerous whirlpool,
in which many boats had, according to the Executive Engineer of the
Nuddea Rivers Division, been lost. There was an immense mass of masonry,
but no means of getting a charge placed underneath it; so a charge of
100 lb. of powder was placed close alongside it in about 15 feet of
water. This shunted the mass bodily over and underneath the water. Two
50-lb. charges were next placed underneath the mass, and these shattered
it all up, except one piece, which was got rid of with a fourth charge
of 20 lb. placed well underneath it. The Executive Engineer wishing that
the wing-wall of the bridge, which was on dry land during the dry
season, might be removed as well, a small hole was made at the foot of
the visible portion of the brickwork, and a charge of 2 lb. of
gun-cotton was introduced into this, and fired with only a tolerable
effect, the brickwork being cracked for a distance of 3 or 4 feet from
the centre of the charge. A hole was next dug down about 5 feet at one
side of the wing-wall, and a charge of 4 lb. of gun-cotton well tamped
was fired. The tamping was blown out, and the wall foundations cracked
a good deal. The excavation was now deepened to 6 feet, and a hole made
under the brickwork big enough to contain a 100-lb. charge. It was then
well tamped up and fired. Its effect was excellent. All the brickwork of
the wing-wall was got rid of, and a crater about 30 feet in width at the
top blown out in the point of the bank that was required to be removed,
and which was one of the chief causes of the whirlpool, so that the next
rise of the river was sure to carry it all away. The following day an
old pucka ghat opposite to Berhampore was entirely broken up with three
20-lb. charges, and an enormous quantity of old bricks were thrown into
the river.
The last operation undertaken consisted in the blowing up of a very
large ghat opposite to the Nawáb of Moorshedabád’s palaces. The river
during successive rains had cut into and underneath the steps of the
ghat, bringing down large masses of it into the river, where they formed
most dangerous obstacles to navigation. The work was necessarily carried
out in a very rough way, for want of the proper tools. Deep excavations
were made under the three largest masses of masonry, at about 25 feet
apart, and into these were introduced three 50-lb. and one 20-lb.
charges of powder. These charges were well tamped, connected up in
divided circuit, and fired simultaneously. All the masonry was broken up
completely, so as to be easily removable afterwards by coolie labour,
which was all that was required.
The conclusions to be drawn from the foregoing notes are, that large
trees lying in shallow water require charges of 50 lb. of gunpowder and
upwards for their effectual removal; but that where there is plenty of
water, and the trees are not very large, 20 lb. is sufficient.
For these small charges, it has been seen that the common earthenware
ghurrah answers admirably, and under similar circumstances it would
undoubtedly be advantageous to use them, as they are inexpensive, and
obtainable in nearly every Indian village.
The charges used might, in many cases, at first have been no doubt made
smaller with advantage, both for safety and economy; but as speed was
the great object, these were not so much thought of.
For the removal of masonry under water, it is not necessary to place the
charge underneath the mass, which is often impossible; a large charge
alongside it being generally quite sufficient to break it up pretty
effectually where there is sufficient head of water. Smaller charges can
of course be easily used afterwards, whenever required, and for these
small charges, gun-cotton is very effective, as it can be easily
introduced, in the end of a bamboo, into holes and crevices where it
would be impossible to get any but the smallest charges of gunpowder.
INDEX.
Appliances for firing blasting charges, 42
Auxiliary tools, 17
Batteries, firing, 62
Beche, 21
Bichromate firing battery, 62
Bits, borer, 31
Blasting gear, sets of, 22
---- sticks, 51
---- subaqueous, 164
Borer-bits, 31
Boring under water, 170
---- the shot-holes, 128, 142
Bornhardt’s firing machine, 57
Brain’s powder, 105
Bull, 20
Cables, 53
Cellulose dynamite, 105
Charging and firing, 150
---- the shot-holes, 132
Chemical compounds, 81
Claying iron, 20
Conditions of disruption, 110
Connecting wires, 52
Cotton powder, 103
Darlington drill, 26
Detonation, 95
Detonators, 54
Dislodged rock, removal of, 154
Disruption, conditions of, 110
---- force required to cause, 107
Division of labour, 155
Drills, dimension of, 6
---- form of, 4
---- hand, 1
---- hardening and tempering, 10
---- making and sharpening, 7
---- sets of, 13
Drivings, examples of, 157
Dubois-François carriage, 39
Dynamite, 100
---- composition of, 87
Electrical firing, advantage of, 153
Electric fuses, 47
---- tension fuse, 50
Example of a heading, 115
Examples of drivings, 157
Explosion, force developed by, 72
---- heat liberated by, 67
---- gases generated by, 69
---- nature of, 64
Explosive agents, nature of, 76
Firing batteries, 62
---- blasting charges, appliances for, 42
---- by electricity, 138
---- machines, 55
---- machine, Bornhardt’s, 57
---- ---- induction coil, 61
---- ---- Mowbray’s, 59
---- ---- Siemens’, 60
---- ---- Smith’s, 58
---- points of the common explosive agents, 102
---- table for frictional electric machine, 140
---- the charges, 137
Force developed by an explosion, 72
---- developed by gunpowder, 88
---- required to cause disruption, 107
Fuse, safety, 45
Fuses, electric, 47
Gases generated by an explosion, 69
Gun-cotton, 99
---- constitution of, 81
Gunpowder, 97
---- composition of, 80
---- force developed by, 88
Hammers, 14
---- patterns of, 15
Hand boring, 128
Heading, example of, 115
Heat, action of, in firing, 92
---- liberated by an explosion, 67
---- measure of, 66
---- specific, 66
Hoosac Tunnel, 158
Induction firing coils, 61
Joint and bedding planes, 118
Jumper, 3
Labour, division of, 155
Line of least resistance, 106
Lithofracteur, 104
Machine boring, 142
---- rock-drills, 23
Machines, firing, 55
Means of firing the common explosive agents, 92
Measure of heat, 66
Mechanical mixture, 76
Mowbray’s firing machine, 59
Musconetcong Tunnel, 159
Nature of an explosion, 64
Nitrated gun-cotton, 103
Nitro-glycerine, constitution of, 86
Obstructions in water-courses, 178
Operations of rock blasting, 128
Preparation of subaqueous charges, 164
Principles of blasting, 106
Rammer, 20
Relative strength of gunpowder, gun-cotton, and dynamite, 91
---- ---- of the common explosive agents, 88
Removing dislodged rock, 154
Rock-drill supports, 34
Safety Fuse, 45
St. Gothard Tunnel, 157
Schultze’s powder, 104
Scraper, 18
Shot-holes, boring, 128, 142
---- charging, 132
Siemens’ firing machine, 60
Silvertown firing battery, 62
Sledges, 14
---- North of England, 17
---- North Wales, 17
---- South Wales, 16
Smith’s firing machine, 58
Some properties of the common explosive agents, 97
Some varieties of the nitro-cellulose and the nitro-glycerine
compounds, 103
Squibs, 44
Steel, hardening and tempering, 9
Stemmer, 20
Sticks, blasting, 51
Stretcher bar, 37
Subaqueous blasting, 164
---- charges, preparation of, 164
Submarine rocks, removal of, 173
Swab-stick, 19
Tamping, 121
Tonite, 103
Water, boring under, 170
Water-courses, obstructions in, 178
Waterproofing composition, 165
Weight of explosive in bore-hole, table of, 109
Wires, connecting, 52
LONDON: PRINTED BY WM. CLOWES AND SONS, STAMFORD STREET AND CHARING
CROSS.
[Illustration: Plate I.
SET OF COAL BLASTING GEAR.
_Fig. 1._
_Fig. 2._
_Fig. 4._
_Fig. 5._
_Fig. 3._
E & F. N. Spon. London & New York.]
[Illustration: Plate II.
SET OF SINGLE HAND STONE BLASTING GEAR.
_Fig. 6._
_Fig. 7._
_Fig. 8._
_Fig. 9._
_Fig. 10._
_Fig. 11._
E & F. N. Spon. London & New York.]
[Illustration: Plate III.
SET OF DOUBLE HAND STONE BREAKING GEAR.
_Fig. 12._
_Fig. 15._
_Fig. 13._
_Fig. 14._
_Fig. 16._
_Fig. 17._
_Fig. 18._
_Fig. 19._
E & F. N. Spon. London & New York.]
[Illustration: Plate IV.
THE DARLINGTON MACHINE DRILL.
_Fig. 20._
_Fig. 22._
_Fig. 21._
_Fig. 23._
_Fig. 24._
_Fig. 25._
E & F. N. Spon. London & New York.]
[Illustration: Plate V.
STRETCHER BAR.
_Fig. 26._
E & F. N. Spon. London & New York.]
[Illustration: Plate VI.
MACHINE DRILL SUPPORTS.
_Fig. 27._
_Fig. 28._
E & F. N. Spon. London & New York.]
[Illustration: Plate VII.
MACHINE DRILL SUPPORT.
_Fig. 29._
E & F. N. Spon. London & New York.]
[Illustration: Plate VIII.
DUBOIS-FRANÇOIS MACHINE DRILL CARRIAGE.
_Fig. 30. Elevation._
_Fig. 31. Plan._
_Fig. 32._
_Raising and lowering Screw._
_Fig. 33._
_Back support for the Drill._
E & F. N. Spon. London & New York.]
[Illustration: Plate IX.
S^{T}. GOTHARD TUNNEL.
_Göschenen End._
_Airolo End._
E & F. N. Spon. London & New York.]
[Illustration: Plate X.
EXAMPLES OF HEADINGS.
_Heading, Marihaye._
_Heading, Anzin._
_Heading, Ronchamp._
HOOSAC TUNNEL--_West End._
E & F. N. Spon. London & New York.]
[Illustration: Plate XI.
HOOSAC TUNNEL.
_Centre Cut._
_Side Cuts._
_Squaring the Heading._
E & F. N. Spon. London & New York.]
[Illustration: Plate XII.
MUSCONETCONG TUNNEL.
_The Heading; Plan._
_The Bench._
_Sectional Elevation._
_The Bench, Plan._
_The Heading,_
_Elevation._
E & F. N. Spon. London & New York.]
Transcriber’ Notes
Inconsistent and unusual spelling, hyphenation, use of accents and
lay-out have been retained, except as mentioned below.
There are two sets of numbered illustrations: numbered illustrations
in the text, and numbered illustrations in the plates in the back of
the book.
Calculations and tables have been copied verbatim although some
contain calculation errors.
Some reference letters in the text are missing from the illustrations.
Changes made:
Tables and illustrations have been moved to between paragraphs.
Some minor typographic errors have been corrected and some missing
punctuation has been added silently.
Page 40: plate ~V~ changed to plate V.
Page 109: Curtiss’ and Harvey’s changed to Curtis’s and Harvey’s.
Page 166: Curtis’s and Harvey changed to Curtis’s and Harvey’s.
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