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Title: Earth Dams, A Study
Author: Bassell, Burr
Language: English
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EARTH DAMS
_A STUDY_
BY
BURR BASSELL, M. Am. Soc. C. E.
_Consulting Engineer_
NEW YORK
THE ENGINEERING NEWS PUBLISHING COMPANY
1904
COPYRIGHT, 1904
BY
THE ENGINEERING NEWS PUBLISHING CO.
ACKNOWLEDGMENTS.
The writer wishes to acknowledge his appreciation of the assistance
given him by Mr. Jas. D. Schuyler, M. Am. Soc. C. E., Consulting
Hydraulic Engineer, in reviewing this paper, and in making suggestions
of value. Appendix II contains a list of authors whose writings have
been freely consulted, and to whom the writer is indebted; the numerous
citations in the body of the paper further indicate the obligations of
the writer.
CONTENTS.
PAGE
CHAPTER I.
Introductory 1
CHAPTER II.
Preliminary Studies and Investigations 3
CHAPTER III.
Outline Study of Soils. Puddle 12
CHAPTER IV.
The Tabeaud Dam, California 17
CHAPTER V.
Different Types of Earth Dams 33
CHAPTER VI.
Conclusions 63
APPENDIX I.
Statistical Descriptions of High Earth Dams 67
APPENDIX II.
Works of Reference 68
ILLUSTRATIONS.
PAGE
Fig. 1. Longitudinal Section of Yarrow Dam Site 10
2. Cross-Section of the Yarrow Dam 10
3. Plan of the Tabeaud Reservoir 17
4. Tabeaud Dam: Plan Showing Bed Rock Drains 18
5. Details of Drains 18
6. View of Drains 19
7. North Trench 20
8. South Trench 21
9. Main Central Drain 21
10. Embankment Work 23
11. Dimension Section 26
12. Cross and Longitudinal Sections 27
13. View of Dam Immediately After Completion 29
14. Cross-Section of Pilarcitos Dam 34
15. San Andres Dam 34
16. Ashti Tank Embankment 35
17. Typical New England Dam 40
18. Two Croton Valley Dams Showing Saturation 41
19. Studies of Board of Experts on the Original Earth
Portion of the New Croton Dam 43
20. Studies of Jerome Park Reservoir Embankment 46
21 to 24. Experimental Dikes and Cylinder Employed
in Studies for the North Dike of the Wachusett
Reservoir 49
25. Cross-Section of Dike of Wachusett Reservoir 49
26. Working Cross-Section of Druid Lake Dam 53
27 to 29. Designs for the Bohio Dam, Panama Canal 55
30. Cross-Section of the Upper Pecos River Rock-Fill Dam 59
31. Developed Section of the San Leandro Dam 59
EARTH DAMS
CHAPTER I.
_Introductory._
The earth dam is probably the oldest type of dam in existence,
antedating the Christian Era many hundreds of years. The literature
upon this subject is voluminous, but much of it is inaccessible and
far from satisfactory. No attempt will here be made to collate this
literature or to give a history of the construction of earth dams,
however interesting such an account might be. The object will rather
be to present such a study as will make clear the application of the
principles underlying the proper design and erection of this class
of structures. In no way, therefore, will it assume the character or
dignity of a technical treatise.
Dams forming storage reservoirs, which are intended to impound large
volumes of water, must necessarily be built of considerable height,
except in a very few instances where favorable sites may exist. Recent
discussions would indicate that a new interest has been awakened in
the construction of high earth dams. As related to the general subject
of storage, it is with the high structure rather than the low that
this study has to do. To the extent that “the greater includes the
less,” the principles here presented are applicable to works of minor
importance.
Many persons who should know better place little importance upon
the skill required for the construction of earthwork embankments,
considering the work to involve no scientific problems. It is far
too common belief that any ordinary laborer, who may be able to use
skillfully a scraper on a country road, is fitted to superintend
the construction of an earth dam. It has been said that the art of
constructing earth dams is purely empirical, that exact science
furnishes no approved method of determining their internal stresses,
and that in regard to their design experience is much more valuable
than theory. When the question of stability is fully taken into
consideration, it certainly requires a large amount of skill
successfully to carry out works of this character.
Extreme care in the selection of the site, sound judgment in the
choice of materials and assiduity in superintending the work while in
progress, are all vitally essential.
Classification of Dams.
Dams may be classified according to their purpose as diverting dams or
weirs and as storage dams. The former may be located upon any portion
of a stream where the conditions are favorable, and the water used for
manifold purposes, being conveyed by means of canals, flumes, tunnels
and pipe-lines to places of intended use. These dams are generally low
and may be either of a temporary or permanent character, depending upon
the uses to which the water is put. Temporary dams are made of brush,
logs, sand bags, gravel and loose rock. The more permanent structures
are built of stone and concrete masonry.
Storage dams may be classified according to the kind of material
entering into their structure, as follows: (1) Earth; (2) Earth and
Timber; (3) Earth and Rock-fill; (4) Rock-fill; (5) Masonry; (6)
Composite Structures.
Low dams forming service reservoirs for domestic water supplies and
for irrigation comprise by far the most numerous class. They are not
designed to impound a large volume of water and therefore may be built
across a small ravine or depression, or even upon the summit of a hill,
by excavating the reservoir basin and using the material excavated to
form the embankment. These reservoirs may be used in connection with
surface or gravity systems, artesian wells, or underground supplies
obtained by pumping. The dams forming these reservoirs being of
moderate size and height may vary greatly in shape and dimensions.
The form may be made to suit the configuration of the dam site. When
the earthwork requires it, they may be lined with various materials
to secure water-tightness. Often such dams are made composite in
character, partly of earth and partly of masonry or some other
material. They are also frequently accompanied by numerous accessories,
such as settling-basins, aerating devices and covers, which present a
diversity in form and appearance. A presentation of the different types
of dams thus employed, with a discussion of the questions pertaining
to utility in design and economy in construction, would be exceedingly
valuable and of general interest. Service reservoirs will receive only
a passing notice, with the hope expressed that some competent authority
will discuss them in the future.
CHAPTER II.
_Preliminary Studies and Investigations._
The preliminary studies and investigations which should be made prior
to the construction of any dam for the storage of water have to do with
(1) the Catchment Area, (2) the Reservoir basin, and (3) the Dam site.
Catchment Area.
It is thought desirable to define a number of terms as we proceed,
for the purpose of correcting erroneous usage and for a clearer
understanding of the subject. The catchment area of a reservoir is that
portion of the country naturally draining into it. The watershed is
the boundary of the catchment area and may be correctly defined as the
divide between adjacent drainage systems. In regard to the catchment
area it is necessary to determine:
1. Its extent and area in square miles.
2. Its topography or the character of its surface.
3. Its hydrography or precipitation and run-off.
4. Its geology, or the character of its soils and subsoils,
and the nature and dip of its rock strata.
5. Its flora, or the extent to which it is clothed with forest
trees or other vegetation.
All of these elements affect the volumes of maximum run-off, which is
the one important factor in the construction of earth dams that must
not be underestimated.
If the proposed dam or reservoir is to be located upon a main drainage
line; that is, upon a river or stream, it is necessary to know both the
flood and low-water discharge of the stream. Frequently no reliable
data on this subject are available, and the engineer must then make
such a study of the whole situation as will enable him to estimate the
minimum and maximum flow with considerable accuracy.
There are numerous factors entering into the solution of this first
problem, such as the shape and length of the catchment area, its
general elevation, the character of its surface, whether mountainous,
hilly or flat, barren or timbered.
Good topographic maps, if available, furnish valuable data on these
subjects and it is to be regretted that only a comparatively small
portion of the United States has been thus mapped in detail.
The results of stream measurements, if any have been made in the
catchment basin, are especially important: These are usually few in
the high areas, on account of their inaccessibility. The year 1902
marked a notable beginning of such measurements in California. In many
parts of the arid region of the United States, the best storage-sites
are situated in the upper or higher portions of the drainage systems.
This is especially true of the streams on the Pacific Slope having
their source in the High Sierras. As regulators of stream-flow and for
power purposes such storage is peculiarly valuable, while storage for
irrigation and domestic uses may be located nearer the valleys and the
centers of population.
Frequently the engineer is required to build his dam where no such
data are available. In such instances he should endeavor to secure
the establishment of rain gauges and make measurements of the flow of
the main stream and its principal tributaries at various places to
obtain the desired information. Even this may not suffice, owing to the
limited time at his disposal, and he must resort to the use of certain
empirical rules or formulas, and make such comparisons and deductions
from known conditions and results as will best answer his purpose.
The engineer should know, approximately at least, the normal yield of
the catchment area, the duration of the minimum and maximum seasonal
flow, and the floods he may have to provide against during the
construction of his dam. These data are absolutely necessary to enable
him to provide ample wasteways for his reservoirs. Many of the failures
of earth dams have been the result of over-topping the embankment,
which would have been averted by an ample wasteway. The most notable
example of this kind in recent years was that of the South Fork Dam, at
Conemaugh, Pennsylvania, in 1889, resulting in what is generally known
as the “Johnstown Disaster.”
There are several empirical rules and formulas for calculating the run
off from catchment areas and for determining the size of spillways
necessary to discharge this flow with safety to the dam. The proper
formula to apply in any given case, with the varying coefficients
of each, involves a thorough knowledge on the part of the designing
engineer of the principles upon which the different factors are based.
It is unwise and often hazardous to make use of any important hydraulic
formula without knowing the history of its derivation. Experiments
are not always properly conducted, and often the deductions therefrom
are unreliable. A presentation and discussion of these formulas would
require more space than can be given in this study, and the technical
reader must therefore consult for himself, as occasion may require, the
various authorities cited. Formulas for the discharge or run-off from
catchment areas, as determined by Messrs. Craig, Dickens, Ryves and
others, are given by most writers on the subject of hydraulics.
Reservoir Basin.
The next subject of inquiry relates to the reservoir basin. It is
necessary that its area and capacity at different depths should be
definitely known, and this information can only be obtained by having
the basin surveyed and contoured. A map should be made showing contours
at intervals of 2 to 10 ft., depending upon the size of the basin and
the use to which the reservoir is to be put. Reservoir basins have been
classified according to their location as follows:
1. Natural lakes.
2. Natural depressions on main drainage lines.
3. Natural depressions on lateral drainage lines.
4. Arbitrary and artificially constructed basins.
Natural lakes may need to be investigated more or less thoroughly to
determine the character of their waters, whether saline, alkaline
or fresh. It may also be necessary to know their normal depth and
capacity, and to make a study of their outlet if they have one. In some
instances the storage capacity of a lake may be enormously increased by
means of a comparatively low and inexpensive embankment.
The area of reservoir basin, mean depth, temperature of the water,
exposure of wind and sunshine, losses by seepage and evaporation, all
have a bearing upon the available water supply and influence the design
of the dam and accessories to the reservoir.
In determining the character and suitability of materials for
constructing a dam it is necessary to make a careful study of the
soil and geological formation. This is best accomplished by digging
numerous test pits over the basin, especially in the vicinity of the
proposed dam site; borings alone should never be relied upon for this
information. For such an investigation the advisability of borrowing
material for dam construction from the reservoir basin is determined.
The porous character of the subsoil strata, or the dip and nature of
the bed rock, may forbid the removal of material from the floor of the
basin, even at a remote distance from the dam site.
The area to be flooded should be cleared and grubbed more or less
thoroughly, depending again upon the use for which the water is
impounded. In no instance should timber be left standing below the
high-water level of the reservoir; and all rubbish liable to float and
obstruct the outlet tunnel and spillway during a time of flood should
be removed.
The accessories to a reservoir, to which reference has been made, may
be enumerated as follows:
1. Outlet pipes or tunnel.
2. Gate tower, screens and controlling devices.
3. Sluiceways for silt or sand.
4. Wasteway channel or weir.
5. Cover, settling basin, aerating devices, etc.
Some of these are necessary and common to all classes of reservoirs,
while others are employed only in special cases, as for domestic water
supplies. All reservoirs formed by earth embankments must have at least
two of these, namely a wasteway, which is its safety valve, and outlet
pipes or outlet tunnel.
It may be stated that the proper location and construction of the
outlet for a reservoir are of vital importance, since either to
improper location or faulty construction may be traced most of the
failures of the past. It is almost impossible to prevent water under
high pressure from following along pipes and culverts when placed in an
earth dam. The pipes and culverts frequently leak, and failure ensues.
Failure may result from one or more of the following causes:
1. By improper design and placement of the puddle around the pipes.
2. By resting the pipes upon piers of masonry without continuous
longitudinal support.
3. By reason of subsidence in the cuts of the embankments and
at the core walls, due to the great weight at these points.
4. Leakage due to inherent defects, frost, deterioration, etc.
Mr. Beardsmore, the eminent English engineer who built the Dale Dyke
embankment at Sheffield which failed in 1864, and who was afterwards
requested to study and report upon the great reservoirs in Yorkshire
and Lancashire, said, after examination and careful study of reservoir
embankment construction, that “in his opinion there were no conditions
requiring that a culvert or pipes should be carried through any portion
of the made bank.” The writer would go even further and say that the
only admissible outlet for a storage reservoir formed by a high earth
dam is some form of tunnel through the natural formation at a safe
distance from the embankment.
Dam Site.
The third preliminary study (that relating to the dam site itself) will
be considered under three heads:
1. Location.
2. Physical features, materials, etc.
3. Foundation.
LOCATION.–The location for a dam is generally determined by the use
which is to be made of it, or by the natural advantages for storage
which it may possess. If it be for water power it is very frequently
located upon the main stream at the point of greatest declivity. If for
storage it may be, as we have seen, at the head of a river system, on
one of its tributaries, or in a valley lower down.
The type of dam which should be built at any particular locality
involves a thorough knowledge, not alone of the catchment area and
reservoir basin, but also accurate information regarding the geology of
the dam site itself. It would be very unwise to decide definitely upon
any particular type of dam without first obtaining such information.
Too frequently has this been done, causing great trouble and expense,
if not resulting in a total failure of the dam.
The conditions favorable for an earth dam are usually unfavorable for a
masonry structure, and vice versa. Again, there may be local conditions
requiring some entirely different type.
Dams situated upon the main drainage lines of large catchment areas are
usually built of stone or concrete masonry, and designed with large
sluiceways and spillways for the discharge both of silt and flood
waters. It need scarcely be remarked that, as a rule, such sites are
wholly unsuited to earthwork construction. It is said, however, that
“every rule has at least one exception,” and this may be true of those
relating to dam sites, as will appear later under the head of new
types.
In a general way, the location of high earth dams is governed by the
configuration of the ground forming the storage basin. It may not be
possible, however, to decide upon the best available site without
careful preliminary surveys and examinations of the geological
formation.
All earth dams must be provided with a wasteway, ample to discharge the
maximum flood tributary to the reservoir. Whatever type of wasteway be
adopted, no reliance should ever be put upon the outlet pipes for this
purpose. The outlet should only figure as a factor of safety for the
wasteway, insuring, as it were, the accuracy of the estimated flood
discharge. The safety of the dam demands that ample provision be made
for a volume of water in excess of normal flood discharge. This most
necessary adjunct of earth dams may be an open channel, cut through
the rim of the reservoir basin, discharging into a side ravine which
enters the main drainage way some distance below the dam. It may be
necessary and possible to pierce the rim by means of a tunnel where its
length would not prohibit such a design. Lastly, there may be no other
alternative than the construction of an overfall spillway, at one or
both ends of the embankment. This last method is the least desirable of
any and should be resorted to only when the others are impracticable;
even then, the volume of water, local topography, geology, and
constructive materials at hand must be favorable to such a design. If
they are not favorable it may be asked, “what then?” Simply do not
attempt to build an earth dam at this site.
PHYSICAL FEATURES, MATERIALS, ETC.–An investigation of the location
and the physical features of the dam site should include a careful and
scientific examination of the materials in the vicinity, to determine
their suitability for use in construction. An earth embankment cannot
be built without earth, and an earth dam cannot be built with safety
without the right kind of earth material.
Test pits judiciously distributed and situated at different elevations
will indicate whether there is a sufficient amount of suitable
material within a reasonable distance of the dam. The type of earth
dam best suited for any particular locality, and its estimated cost,
are thus seen to depend upon the data and information obtained by
these preliminary studies. Economical construction requires the use of
improved machinery and modern methods of handling materials, but far
more important even than these are the details of construction.
FOUNDATION.–We may now assume that our preliminary studies relating to
the location and physical features of the dam site are satisfactory.
We must next investigate the foundation upon which the dam is to be
built. This investigation is sometimes wholly neglected or else done in
such a way as to be practically useless. To merely drive down iron rods
feeling for so-called bed rock, or to make only a few bore-holes with
an earth auger should in no instance be considered sufficient. Borings
may be found necessary at considerable depths below the surface and
in certain classes of material, but dug pits or shafts should always
be resorted to for moderate depths and whenever practicable. Only by
such means may the true character of the strata underlying the surface,
and the nature and condition of the bed rock, if it be reached, become
known. If a satisfactory stratum of impermeable material be found it is
necessary also to learn both its thickness and extent. It may prove to
be only a “pocket” of limited volume, or if found to extend entirely
across the depression lengthwise of the dam site it may “pinch out”
on lines transversely above or below. Shafts and borings made in the
reservoir basin and below the dam site will determine its extent in
this direction, knowledge of which is very important.
Fig. 1, showing a longitudinal section of the site of the Yarrow Dam of
the Liverpool Water-Works, England, illustrates the necessity of such
investigation. A bore hole at station 2 + 00 met a large boulder which
at first was erroneously thought to be bed rock. The hole at station 3
+ 50 met a stratum of clay which proved to be only a pocket.
The relative elevation of the different strata and of the bed rock
formation, referred to one common datum, should always be determined.
These elevations will indicate both the dip and strike of the rock
formation and are necessary for estimating the quantities of material
to be excavated and removed, including estimates of cost. They furnish
information of value in determining the rate of percolation or
filtration through the different classes of material and the amount
of probable seepage, as will appear later. The cost of excavating,
draining and preparing the floor or foundation for a dam is often very
great, amounting to 20 or 30% of the total cost.
Fig. 2 is a transverse section of the Yarrow Dam. This particular dam
has been selected as fairly representative of English practice and
of typical design. It is one of the most widely known earth dams in
existence.
[Illustration: FIG. 1.–LONGITUDINAL SECTION OF YARROW DAM SITE.]
[Illustration: FIG. 2.–CROSS-SECTION OF YARROW DAM.]
At the Yarrow dam site it was necessary to go 97 ft. below the original
surface to obtain a satisfactory formation or one that was impermeable.
A central trench was excavated to bed rock, parallel to the axis of the
dam, and filled with clay puddle to form a water-tight connection with
the rock, and prevent the water in the reservoir from passing through
the porous materials under the body of the embankment. This interesting
dam will be more fully described later, when the different types of
earth dams are discussed.
CHAPTER III.
_Outline Study of Soils. Puddle._
The following study of soils is merely suggestive and is here given
to emphasize the importance of the subject, at the risk of being
considered a digression. Soil formations are made in one of three ways:
1. By decomposition of exposed rocks.
2. By transportation or sedimentation of fine and coarse
materials worn from rocks.
3. By transformation into humus of decayed organic matter.
The transforming agencies by which soils succeed rocks in geological
progression have been classified as follows:
1. Changes of temperature.
2. Water.
3. Air.
4. Organic life.
_Heat_ and its counter agent frost are the most powerful forces in
nature, their sensible physical effects being the expansion and
contraction of matter.
_Water_ has two modes of action, physical and chemical. This agent is
the great destroyer of the important forces, cohesion and friction.
_Cohesion_ is a force uniting particles of matter and resists their
separation when the motion attempted is perpendicular to the plane of
contact. _Friction_ is a force resisting the separation of surfaces
when motion is attempted which produces sliding. The hydrostatic
pressure and resultant effect upon submerged surfaces need to be
kept constantly in mind. When the surface is impermeable the line of
pressure is normal to its plane, but when once saturated there are also
horizontal and vertical lines of pressure. Since the strength of an
earth dam depends upon two factors, namely, its weight and frictional
resistance to sliding, the effect of water upon different materials
entering into an earth structure should be most carefully considered.
This will therefore occupy a large place in these pages. An earth
embankment founded upon rock may become saturated by water forced up
into it from below through cracks and fissures, reducing its lower
stratum to a state of muddy sludge, on which the upper part, however
sound in itself, would slide. The best preliminary step to take in such
a case is to intersect the whole site with wide, dry, stone drains,
their depths varying according to the nature of the ground or rock.
_Air_ contains two ingredients ever active in the process of
decomposition, carbonic acid and oxygen.
_Organic Life_ accomplishes its decomposing effect both by physical
and chemical means. The effect of organic matter upon the mineral
ingredients of the soil may be stated as follows:
1. By their hydroscopic properties they keep the soil moist.
2. Their decomposition yields carbonic acid gas.
3. The acids produced disintegrate the mineral constituents,
reducing insoluble matter to soluble plant food.
4. Nitric acid results in _nitrates_, which are the most
valuable form of nutritive nitrogen, while ammonia and the
other salts that are formed are themselves direct food for
plants.
_Vegetable Humus_ is not the end of decomposition of organic matter,
but an intermediate state of transformation. Decay is a process almost
identical with combustion, where the products are the same, and the end
is the formation of water and carbonic acid, with a residue of mineral
ash. The conditions essential to organic decomposition are also those
most favorable to combustion or oxidation, being (1) access of air, (2)
presence of moisture, and (3) application of heat.
Now the coöperation of these chemical and physical forces, which are
ever active, is called “weathering.” Slate rock, for instance, weathers
to clay, being impregnated with particles of mica, quartz, chlorite and
hornblend. Shales also weather to clay, resulting often in a type of
earth which is little more than silicate of aluminum with iron oxide
and sand.
In the vicinity of the Tabeaud Dam, recently built under the personal
supervision of the author, the construction of which will be described
later, there is to be found a species of potash mica, which in
decomposing yields a yellow clay (being ochre-colored from the presence
of iron), mixed with particles of undecomposed mica. This material
is subject to expansion, and by reason of its lack of grit and its
unctuous character it was rejected or used very sparingly. Analysis of
this material gave, Silica, 54.1 to 59.5%; potash, 1.5 to 2.3%; soda,
2.7 to 3.7%.
Soil analysis may be either mechanical or chemical. For purposes of
earthwork, we are most interested in the former, having to deal with
the physical properties of matter. Chemical analysis, however, will
often afford information of great value regarding certain materials
entering into the construction of earth dams. The most important
physical properties are:
(1) Weight and specific gravity.
(2) Coefficient of friction and angle of repose.
(3) Structure and coloring ingredients.
(4) Behavior toward water.
There are two distinct methods of mechanical analysis: (1) Granulating
with sieves, having round holes. (2) Elutriating with water, the
process being known as silt analysis.
It would require a large volume to present the subject of soil analysis
in any way commensurate with its importance. Experiments bearing upon
the subjects of imbibition, permeability, capillarity, absorption and
evaporation, of different earth materials, are equally interesting and
important.[1]
The permeability of soils will be discussed incidentally in connection
with certain infiltration experiments to be given later.
Puddle.
_Puddle_ without qualification may be defined as clayey and gravelly
earth thoroughly wetted and mixed, having a consistency of stiff mud
or mortar. Puddle in which the predominating ingredient of the mixture
is pure clay, is called _clay puddle_. _Gravel puddle_ contains a much
higher percentage of grit and gravel than the last-named and yet is
supposed to have enough clayey material to bind the matrix together and
to fill all the voids in the gravel.
The term _earthen concrete_ may also be applied to this class of
material, especially when only a small quantity of water is used in
the mixture. These different kinds of puddling materials may be found
in natural deposits ready for use, only requiring the addition of the
proper amount of water. It is usually necessary, however, to mix,
artificially, or combine the different ingredients in order to obtain
the right proportions. Some engineers think grinding in a pug-mill
absolutely essential to obtain satisfactory results.
Puddle is handled very much as cement concrete, which is so well
understood that detailed description is hardly necessary. Instead of
tampers, sharp cutting implements are usually employed in putting
puddle into place. Trampling with hoofed animals is frequently resorted
to, both for the purpose of mixing and compacting.
As has been stated, clays come from the decomposition of crystaline
rocks. The purest clay known (kaolin) is composed of alumina, silica
and water. The smaller the proportion of silica the more water it will
absorb and retain. Dry clay will absorb nearly one-third of its weight
of water, and clay in a naturally moist condition 1-6 to 1-8 its weight
of water. The eminent English engineers, Baker and Latham, put the
percentage of absorption by clayey soils as high as 40 to 60%. Pure
clays shrink about 5% in drying, while a mixture by weight of 1 clay
to 2 sand will shrink about 3%. It follows, then, that the larger the
percentage of clay there may be in a mixture the greater will be both
the expansion and the contraction.
Clay materials may be very deceptive in some of their physical
properties, being hard to pick under certain conditions, and yet when
exposed to air and water will rapidly disintegrate. Beds of clay,
marl and very fine sand are liable to slip when saturated, becoming
semi-fluid in their nature, and will run like cream.
The cohesive and frictional resistances of clays becoming thus very
much reduced when charged with water, a too liberal use of this
material is to be deprecated. The ultimate particles forming clays,
viewed under the microscope, are seen to be flat and scale-like, while
those of sands are more cubical and spherical. This is a mechanical
difference which ought to be apparent to even a superficial observer
and yet has escaped recognition by many who have vainly attempted a
definition of _quicksand_.
Mr. Strange recommends filling the puddle trench with material having
three parts soil and two parts sand. After the first layer next to bed
rock foundation, which he kneads and compacts, he would put the layers
in dry, then water and work it by treading, finally covering to avoid
its drying out and cracking.
Prof. Philipp Forchheimer, of Gratz, Austria, one of the highest
authorities and experimentalists, affirms that if a sandy soil contains
clay to such an extent that the clay fills up the interstices between
the grains of sand entirely the compound is practically impervious.
Mr. Herbert M. Wilson, C. E., in his “Manual of Irrigation
Engineering,” recommends the following as an ideal mixture of materials:
Cu. yds.
Coarse gravel 1.00
Fine gravel 0.35
Sand 0.15
Clay 0.20
––
Total 1.70
This mixture, when rolled and compacted, should give 1.25 cu. yds. in
bulk, thus resulting in 26½% compression.
Mr. Clemens Herschel suggests the following test of “good binding
gravel:” “Mix with water in a pail to the consistency of moist earth;
if on turning the pail upside down the gravel remains in the pail it is
fit for use, otherwise it is to be rejected.” For _puddling material_
he would use such a proportion as will render the water invisible.
CHAPTER IV.
_The Tabeaud Dam, California._
The Tabeaud Dam, in Amador County, Cal., built under the supervision
of the author for the Standard Electric Co., is an example of the
homogeneous earth dam. A somewhat fuller description and discussion
will be given of this dam than of any other, not on account of its
greater importance or interest, but because it exemplifies certain
principles of construction upon which it is desired to put special
emphasis. This dam was described in Engineering News of July 10, 1902,
to which the reader is referred for more complete information than is
given here.
[Illustration: FIG. 3.–PLAN OF TABEAUD RESERVOIR, WITH CONTOURS.]
[Illustration: FIG. 4.–PLAN OF TABEAUD DAM, SHOWING BED ROCK DRAINAGE
SYSTEM.]
[Illustration: FIG. 5.–DETAILS OF BED ROCK DRAINS AT THE TABEAUD DAM.]
Fig. 3 is a contour map of the Tabeaud Reservoir, showing the relative
locations of the dam, wasteway and outlet tunnel. Fig. 4 shows the bed
rock drainage system and the letters upon the drawing will assist in
following the explanation given in the text. The whole up-stream half
of the dam site was stripped to bed rock. As the work of excavation
advanced pockets of loose alluvial soil were encountered, which were
suggestive of a refill, possibly the result of placer mining operations
during the early mining days of California. In addition to this were
found thin strata of sand and gravel deposited in an unconformable
manner. The slate bed rock near the up-stream toe of the dam was badly
fissured and yielded considerable water. A quartz vein from 1 to 2 ft.
in thickness crossed the dam site about 150 ft. above the axis of the
dam. The slate rock above this vein or fault line was quite variable in
hardness and dipped at an angle of 40 degrees toward the reservoir.
[Illustration: FIG. 6.–VIEW OF BED ROCK TRENCHES, TABEAUD DAM.]
The rear drain terminates at a weir box (Z) outside of the down-stream
slope at a distance of 500 ft. from the axis of the dam. This drain
branches at the down-stream side of the central trench, (Y), one branch
being carried up the hillside to high-water level (W) at the North end
of the dam, and the other to the same elevation at the South end (X).
Fig. 5 shows how these drains were constructed. After the removal of
all surface soil and loose rock, a trench 5 to 10 ft. wide was cut into
the solid rock, the depth of cutting varying with the character of the
bed rock. Upon the floor of this trench a small open drain was made by
notching the bed rock and by means of selected stones of suitable size
and hardness. The stringers and cap-stones were carefully selected and
laid, so that no undue settlement or displacement might occur by reason
of the superincumbent weight of the dam. All crevices were carefully
filled with spawls and the whole overlaid 18 ins. in depth with broken
stone 1 to 3 ins. in diameter. Upon this layer of broken stone and
fine gravel was deposited choice clay puddle, thoroughly wetted and
compacted, refilling the trenches.
[Illustration: FIG. 7.–VIEW OF NORTH TRENCH, TABEAUD DAM.]
[Illustration: FIG. 8.–VIEW OF SOUTH TRENCH, TABEAUD DAM.]
[Illustration: FIG. 9.–VIEW OF MAIN CENTRAL DRAIN, TABEAUD DAM.]
These drains served a useful purpose during construction, in drying
off the surface of the dam after rains. The saturation of the outer
slope of the dam by water creeping along the line of contact should
thus be prevented, and the integrity or freedom from saturation of the
down-stream half should be preserved. It is believed that the puddle
overlying these rock drains will effectually prevent any water from
entering the body of the embankment by upward pressure and that the
drains will thus forever act as efficient safeguards.
The main drain was extended, temporarily during construction, from
the central trench (Fig. 4), to the up-stream toe of the dam. This
was cut 5 or 6 ft. deep into solid rock, below the general level of
the stripped surface. Fig. 6 is reproduced from a photograph of this
trench. An iron pipe 2 ins. in diameter was imbedded in Portland cement
mortar and concrete, and laid near the bottom of the trench.
At the point (B) where the quartz vein (already described) intersected
this drain, two branch drains were made, following the fault well into
the hill on both sides. Figs. 7 and 8 are views of the North and South
trenches, respectively. These trenches were necessary to take care of
the springs issuing along the quartz vein. This water led to a point
(N, Fig. 4) near the up-stream toe, by means of the drain shown in Fig.
9.
The lateral drains and that portion of the main central drain extending
from their junction (B) to a point (N) about 230 ft. from the axis
of the dam have pieces of angle iron or wooden Y-fluming laid on the
bottom of the trenches immediately over the 2-in. pipe, as shown in
Figs. 7, 8 and 9. These are covered in turn with Portland cement
mortar, concrete, clay puddle and earth fill. The water will naturally
flow along the line of least resistance, and consequently will follow
along the open space between the angle irons and the outside of the
pipe until it reaches the chamber and opening in the pipe, permitting
the water to enter and be conveyed through the imbedded pipe-line to
the rear drain. This point of entry is a small chamber in a solid
cross-wall of rich cement mortar, and is the only point where water
can enter this pipe-line, the two branches entering the wells and the
stand-pipe at their junction (soon to be described) having been closed.
That portion of the foundation between the axis of the dam and the
quartz vein, a distance of about 160 ft., was very satisfactory,
without fissures or springs of water. In this portion the 2-in. pipe
was imbedded in mortar and concrete without angle irons, and the
continuity of the trench broken by numerous cross-trenches cut into
the rock and filled with concrete and puddle. It is believed that no
seepage water will ever pass through this portion of the dam. If any
should ever find its way under the puddle and through the bed rock
formation, the rear drain, with its hillside branches, will carry it
away and prevent the saturation of the lower or down-stream half of the
dam.
[Illustration: FIG. 10.–VIEW OF TABEAUD DAM WHEN ABOUT HALF COMPLETED.]
At the up-stream toe of the embankment, two wells or sumps (shown at
“S” and “K,” Fig. 4) were cut 10 or 12 ft. deeper than the main trench,
which received the water entering the inner toe puddle trench during
construction. This water was disposed of partly by pumping and partly
by means of the 2-in. branch pipes leading into and from these wells.
At their junction (J) a 2-in. stand-pipe was erected, which was carried
vertically up through the embankment, and finally filled with cement.
The branch pipes from the wells were finally capped and the wells
filled with broken stone, as previously mentioned.
EMBANKMENT.–As has been said, the upper surface of the slate bed rock
was found to be badly fissured, especially near the up-stream toe of
the dam, and as the average depth below the surface of the ground was
not very great, it was thought best to lay bare the bed rock over the
entire upper half of the dam site. Had the depth been much greater,
it would have been more economical and possibly sufficient to have
put reliance in a puddle trench, alone, for securing a water-tight
connection between the foundation and the body of the dam.
At the axis of the dam and near the inner toe, where the puddle walls
abutted against the hillsides, the excavation always extended to bed
rock. Vertical steps and offsets were avoided and the cuts were made
large enough for horses to turn in while tramping, these animals being
used, singly and in groups, to mix and compact the puddle and thus
lessen the labor of tamping by hand. In plan, the hillside contact
of natural and artificial surfaces presents a series of corrugated
lines, (as is clearly shown in Fig. 4.) After all loose and porous
materials had been removed, the stripped surface and the slopes of all
excavations were thoroughly wetted from time to time by means of hose
and nozzle, the water being delivered under pressure. Fig. 10 is a view
of the dam taken when it was about half finished and shows the work in
progress.
The face puddle shown in Fig. 11 was used merely to “make assurance
doubly sure” and was not carried entirely up to the top of the dam.
The earth of which the dam was constructed may be described as a
red gravelly clay, and in the judgment of the author is almost ideal
material for the purpose. Physical tests and experiments made with the
materials at different times during construction gave the following
average results:
Pounds.
Weight of 1 cu. ft. earth, dust dry 84.0
“ “ 1 “ saturated earth 101.8
“ “ 1 “ moist loose earth 76.6
“ “ 1 “ loose material
taken from test pits on the dam 80.0
“ “ 1 “ earth in place
taken from the borrow pits 116.5
“ “ 1 “ earth material
taken from test pits on the dam 133.0
Per cent.
Percentage of moisture in natural earth 19
“ “ voids in natural earth 52
“ “ grit and gravel in natural earth 38
“ “ compression on dam over earth at borrow pit 16
“ “ compression on dam over earth in wagons 43
Degrees.
Angle of repose of natural moist earth 44
Angle of repose of earth, dust dry 36
Angle of repose of saturated earth 23
CONSTRUCTION DETAILS.–The materials forming the bulk of the dam were
hauled by four-horse teams, in dump wagons, holding 3 cu. yds. each.
The wagons loaded weighed about six tons and were provided with two
swinging bottom-doors, which the driver could operate with a lever,
enabling the load to be quickly dropped while the team was in motion.
If the material was quite dry, the load could be dumped in a long row
when so desired.
After plowing the surface of the ground and wasting any objectionable
surface soil, the material was brought to common earth-traps for
loading into wagons, by buck or dragscrapers of the Fresno pattern. In
good material one trap with eight Fresno-scraper teams could fill 25
wagons per hour. The average length of haul for the entire work was
about 1,320 ft.
The original plans and specifications were adhered to throughout, with
the single exception that the central puddle wall was not carried above
elevation 1,160, as shown on Figs. 11 and 12, more attention being
given to the inner face puddle. This modification in the original plans
was made because of the character of the materials available and the
excellent results obtained in securing an homogeneous earthen concrete,
practically impervious.
[Illustration: FIG. 11.–DIMENSION SECTION OF TABEAUD DAM.]
The top of the embankment was maintained basin-shaped during
construction, being lower at the axis than at the outer slopes by 1-10,
to the height below the finished crown. This gave a grade of about 1
in 25 from the edges toward the center, resulting in the following
advantages:
(1) Insuring a more thorough wetting of the central portion of the dam;
any excess of water in this part would be readily taken care of by the
central cross drains.
(2) In wetting the finished surface prior to depositing a new layer
of material, water from the sprinkling wagons would naturally drain
towards the center and insure keeping the surface wet; the layers being
carried, as a rule, progressively outward from the center.
(3) It centralized the maximum earth pressure and enabled the
depositing of material in layers perpendicular to the slopes.
(4) It facilitated rolling and hauling on lines parallel to the axis of
the dam, and discouraged transverse and miscellaneous operations.
(5) It finally insured better compacting by the tramping of teams in
their exertions to overcome the grade.
[Illustration]
[Illustration: FIG. 12.–CROSS AND LONGITUDINAL SECTIONS OF TABEAUD
DAM.]
The specifications stipulated that the body of the dam should be built
up in layers not exceeding 6 ins. in thickness for the first 60 ft.,
and not exceeding 8 ins. above that elevation. The finished layers
after rolling varied slightly in thickness, the daily average per month
being as follows:
April 4 ins.
May 3½ “
June 4 “
July 4½ “
August 5 “
September 6 “
October 7 “
November and December 8 “
During the last few months more than one whole layer constituted the
day’s work, so that a single layer was seldom as thick as the daily
average indicates.
It was stipulated in the specifications that the up-stream half of the
dam was to be made of “selected material” and the lower half of less
choice material, not designated “waste.” “Waste material” was described
as meaning all vegetable humus, light soil, roots, and rock exceeding 5
lbs. in weight, too large to pass through a 4-in. ring.
It may be well to define the expression “selected material,”
so commonly used in specifications for earth dams. In England,
for instance, it is said to refer to materials which insure
_water-tightness_, while in India it refers to those employed to obtain
_stability_. It ought to mean the best material available, selected by
the engineer to suit the requirements of the situation.
The method employed in building the body of the embankment may be
described as follows:
(1) The top surface of every finished layer of material was sprinkled
and harrowed prior to putting on a new layer. The sprinkling wagons
passed over the older finished surface immediately before each
wagon-row was begun. This insured a wetted surface and assisted the
wheels of the loaded wagons, as well as the harrows, to roughen, the
old surface prior to depositing a new layer.
(2) The material was generally deposited in rows parallel to the axis
of the dam. However, along the line of contact, at the margins of the
embankment, the earth was often deposited in rows crosswise of the
dam, permitting a selection of the choicest materials and greatly
facilitating the work of graders and rollers.
(3) Rock pickers with their carts were continually passing along the
rows gathering up all roots, rocks and other waste materials.
[Illustration: FIG. 13.–VIEW OF TABEAUD DAM IMMEDIATELY AFTER
COMPLETION.]
(4) The road-graders drawn by six horses leveled down the tops of
the wagon-loads, and if the material was dry the sprinkling wagons
immediately passed over the rows prior to further grading. When the
material was naturally moist the grader continued the leveling process
until the earth was evenly spread. The depth or thickness of the layer
could be regulated to a nicety by properly spacing the rows and the
individual loads. The grader brought the layer to a smooth surface
and of uniform thickness, and nothing more could be desired for this
operation.
(5) After the graders had finished, the harrows passed over the new
layer to insure the picking out of all roots and rocks, followed
immediately by the sprinkling wagons.
(6) Finally the rollers thoroughly compacted the layer of earth,
generally passing to and fro over it lengthwise of the dam. Along the
line of contact at the ends, however, they passed crosswise. Then again
they frequently went around a portion of the surface until the whole
was hard and solid.
Two rollers were in use constantly, each drawn by six horses. One
weighed five tons and the other eight tons, giving respectively 166 and
200 lbs. pressure per lin. in. They were not grooved, but the smooth
surface left by the rollers was always harrowed and cut up more or
less by the loaded wagons passing over the surface previously wetted.
The wagons when loaded gave 750 lbs. pressure per lin. in., and the
heavy teams traveling wherever they could do the most effective work
compacted the materials better even than the rollers.
Several test pits which were dug into the dam during construction
showed that there were no distinct lines traceable between the layers
and no loose or dry spots, but that the whole mass was solid and
homogeneous.
A careful record is being kept of the amount of settlement of the
Tabeaud Dam. It will be of interest to record here the fact that just
one year after date of completion the settlement amounted to 0.2 ft.,
with 90 ft. depth of water in the reservoir.
Water was first turned into the reservoir five months after the dam was
finished. The very small amount of settlement here shown emphasizes
more eloquently than words the author’s concluding remarks relating to
the importance of thorough consolidation, by artificial means, of the
embankment. (See p. 64, Secs. 6 to 8.)
OUTLET TUNNEL.–The outlet for the reservoir is a tunnel 2,903 ft. in
length, through a ridge of solid slate rock formation, which was very
hard and refractory. At the north or reservoir end of the tunnel, there
is an open cut 350 ft. long, with a maximum depth of 26 ft.
Near the south portal of the tunnel and in the line of pressure pipes
connecting the “petty reservoir” above with the power-house below,
is placed a receiver, connected with the tunnel by means of a short
pipe-line, 60 ins. in diameter.
A water-tight bulkhead of brick and concrete masonry is placed in the
tunnel, at a point about 175 ft. distant from the receiver. In the line
of 60-in. riveted steel pipe, which connects the reservoir and tunnel
with the receiver, there is placed a cast iron chamber for entrapping
silt or sand, with a branch pipe 16 ins. in diameter leading into a
side ravine through which sand or silt thus collected can be wasted or
washed out. By the design of construction thus described, it will be
seen that all controlling devices, screens, gates, etc., are at the
south end of the tunnel and easily accessible.
WASTEWAY.–The wasteway for the reservoir is an open cut through its
rim, 48 ft. in width and 300 ft. long. The sill of the spillway is 10
ft. below the crown of the dam. The reservoir having less than two
square miles of catchment area, and the feeding canals being under
complete control, the dam can never be over-topped by a flood. Fig.
3 shows the relative location of the dam, outlet tunnel and wasteway
channel.
Almost the whole of the embankment forming the Tabeaud Dam, not
included in the foundation work, was built in less than eight months.
The contractor’s outfit was the best for the purpose the writer has
ever seen. After increasing his force from time to time he finally had
the following equipment:
1 steam shovel (1½ yds. capacity),
37 patent dump wagons,
11 stick-wagons and rock-carts,
39 buck-scrapers (Fresno pattern),
21 wheel scrapers,
3 road-graders,
3 sprinkling wagons,
2 harrows,
2 rollers (5 and 8-ton),
233 men,
416 horses and mules,
8 road and hillside plows.
STATISTICS.–The following data relating to the Tabeaud Dam Reservoir
will conclude this description:
DAM.
Length at crown 636 ft.
Length at base crossing ravine 50 to 100 “
Height to top of crown (El. 1,258.) 120 “
“ at ends above bedrock 117 “
“ at up-stream toe 100 “
“ at down-stream toe 123 “
Effective head 115 “
Width at crown 20 “
Width at base 620 “
Slopes, 2½ on 1 with rock-fill 3 to 1.
Excavation for foundations 40,000 cu. yds.
Refill by company 40,000 “
Embankment built by contractor 330,350 “
Total volume of dam 370,350 “
Total weight 664,778 tons.
Length of wasteway (width) 48 ft.
Depth of spillway sill below crown 10 “
Depth of spillway sill below ends 7 “
Height of stop-planks in wasteway 2 “
Maximum depth of water in reservoir 92 “
Area to be faced with stone 1,933 sq. yds.
RESERVOIR.
Catchment area (approximate) 2 sq. miles.
Area of water surface 36.75 acres.
Silt storage capacity below outlet tunnel 1,091,470 cu. ft.
Available water storage capacity 46,612,405 “
Elevation of outlet tunnel 1,180 ft.
“ “ high-water surface 1,250 “
“ “ crown of dam 1,258 “
Fig. 13 is a view of the finished dam, taken immediately after
completion.
CHAPTER V.
_Different Types of Earth Dams._
There are several types of earth dams, which may be described as
follows:
1. Homogeneous earth dams, either with or without a puddle
trench.
2. Earth dams with a puddle core or puddle face.
3. Earth dams with a core wall of brick, rubble or concrete
masonry.
4. New types, composite structures.
5. Rock-fill dams with earth inner slope.
6. Hydraulic-fill dams of earth and gravel.
The writer proposes to give an example of each type, with such remarks
upon their distinctive features and relative merits as he thinks may be
instructive.
Earth Dams with Puddle Core Wall or Face.
YARROW DAM.–The Yarrow dam of the Liverpool Water-Works is a notable
example of the second type, (a section of which is shown in Fig. 2.)
An excavation 97 ft. in depth was made to bed rock through different
strata of varying thickness, and a trench 24 ft. wide was cut with
side slopes 1 on 1 for the first 10 ft. in depth below the surface.
The trench was then carried down through sand, gravel and boulders
with sides sloping 1 in 12. The upper surface of the shale bed rock
was found to be soft, seamy and water-bearing. Pumps were installed
to keep the water out of the trench while it was being cut 4 or 5 ft.
deeper into the shale. The lower portion was then walled up on either
side with brickwork 14 ins. in thickness, and the trench between the
walls was filled with concrete, made in the proportion of 1 of cement,
1 of sand and 2 of gravel or broken stone. By so doing a dry bed was
secured for the foundation of the puddle wall. Two lines of 6-in. pipes
were laid on the bed rock, outside of the walls, and pipes 9 ins. in
diameter extended vertically above the top of the brickwork some 27
ft. These pipes were filled with concrete, after disconnecting the
pumps. After refilling the trench with puddle to the original surface,
a puddle wall was carried up simultaneous with the embankment, having a
decreasing batter of 1 in 12, which gave a width of 6 ft. at the top.
This form of construction is very common in England and Figs. 14 and 15
show two California dams, the Pilarcitos and San Andres, of the same
general type.
[Illustration: FIG. 14.–CROSS-SECTION OF PILARCITOS DAM.]
[Illustration: FIG. 15.–CROSS-SECTION OF SAN ANDRES DAM.]
ASHTI EMBANKMENT.–This is not a very high embankment, but being typical
of modern dams in British India, where the puddle is generally carried
only to the top of the original surface of the ground, and not up
through the body of the dam, it is thought worthy of mention. Fig. 16
shows a section of this embankment, which is located in the Sholapur
District, India.
The central portion of this dam above the puddle trench is made of
“selected black soil;” then on either side is placed “Brown Soil,”
finishing on the outer slopes with “Murum.” Trap rock decomposes first
into a friable stony material, known in India as “Murum” or “Murham.”
This material further decomposes into various argillaceous earths, the
most common being the “black cotton soil” mentioned above.
[Illustration: FIG. 16.–CROSS-SECTION OF ASHTI TANK EMBANKMENT.]
This particular dam has been adversely criticised on account of the
lack of uniformity in the character of the materials composing the
bank. It is claimed that the materials being of different density and
weight, unequal settlement will result, and lines of separation will
form between the different kinds of materials.
Earth materials do not unite or combine with timber or masonry, but
there are no such distinct lines of transition and separation between
different earth materials themselves as Fig. 16 would seem to indicate.
Puddle Trench.
In the last three dams mentioned (Figs. 14, 15, 16) the puddle
trenches are made with vertical sides or vertical steps and offsets.
A wedge-shaped trench certainly has many advantages over this form.
Puddle being plastic, consolidates as the dam settles, filling the
lowest parts by sliding on its bed. It thus has a tendency to break
away from the portion supported by the step, and a further tendency to
leave the vertical side, thus forming cracks and fissures for water to
enter. The argument advanced by those holding a different view, namely,
that it is difficult to dress the sides of a trench to a steep batter
and to timber it substantially, has in reality little weight when put
to practical test. Mr. F. P. Stearns, in describing the recent work
of excavating the cut-off trench of the North Dike of the Wachusett
reservoir, Boston, said it was found to be both better and cheaper
to excavate a trench with slopes than with vertical sides protected
by sheeting. He favored this shape in case of pile-work and for the
purpose also of wedging materials together.
Mr. Wm. J. McAlpine’s “Specifications for Earth Dams,” representing the
best practice of 25 years ago, which are frequently cited, contain the
following description of how to prepare the up-stream floor of the dam:
Remove the pervious and decaying matter by breaking up the natural
soil and by stepping up the sides of the ravine; also by several
toothed trenches across the bottom and up the sides.
One of Mr. McAlpine’s well known axioms was, “water abhors an
angle.” The “stepping” and “toothed” trenches above specified need
not necessarily be made with vertical planes, but should be made by
means of inclined and horizontal planes. The writer’s experience and
observation leads him to think that all excavations in connection with
earth dams requiring a refill should be made wedge-shaped so that the
pressure of the superincumbent materials in settling will wedge the
material tighter and tighter together and fill every cavity. A paper
by Mr. Wm. L. Strange, C. E., on “Reservoirs with high Earthen Dams
in Western India,” published in the Proceedings of the Institution of
Civil Engineers, Vol. 132, (1898), is one of the best contributions to
the literature on this subject, known to the writer. Mr. Strange states
that
the rate of filtration of a soil depends upon its porosity,
which governs the frictional resistance to flow, and the
slope and length of the filamentary channels along which the
water may be considered to pass. It is evident, therefore,
that the direct rate of infiltration in a homogeneous soil
must decrease from the top to the bottom of the puddle
trench. The best section for a puddle trench is thus a
wedge, such as an open excavation would give. It is true
that the uppermost infiltrating filaments when stopped by
the puddle, will endeavor to get under it, but a depth will
eventually be reached when the frictional resistance along
the natural passages will be greater than that due to the
transverse passage of the puddle trench, and it is when
this occurs that the latter may be stopped without danger,
as the _filtration to it_ will be less than that
_through_ it. This depth requires to be determined in
each case, but in fairly compact Indian soils 30 feet will
be a fair limit.
Puddle Wall vs. Puddle Trench.
There is a diversity of opinion among engineers in regard to the proper
place for the puddle in dam construction. Theoretically, the inner
face would be preferable to the center, for the purpose of preventing
any water from penetrating the embankment. It is well known that all
materials immersed in water lose weight in proportion to the volume of
water they displace. If the upper half of the dam becomes saturated
it must neccesarily lose both weight and stability. Its full cohesive
strength can only be maintained by making it impervious in some way.
The strength of an earth dam depends upon three factors:
1. Weight.
2. Frictional resistance against sliding.
3. Cohesiveness of its materials.
These can be known only so long as no water penetrates the body of
the dam. When once saturated the resultant line of pressure is no
longer normal to the inner slope, for the reason that there is now a
force tending to slide the dam horizontally and another due to the
hydrostatic head tending to lift it vertically. When the water slope
is impervious the horizontal thrust is sustained by the whole dam and
not by the lower half alone. When once a passage is made into the body
of the dam, the infiltration water will escape along the line of least
resistance, and if there be a fissure it may become a cavity and the
cavity a breach.
For practical reasons, mainly on account of the difficulty of
maintaining a puddle face on the inner slope of a dam, which would
require a very flat slope, puddle is generally placed at the center as
a core wall.
It was thought possible at the Tabeaud dam to counteract the tendency
of the face puddle to slough off into the reservoir by use of a broken
stone facing of riprap. This covering will protect the puddle from the
deteriorating effects of air and sun whenever the water is drawn low
and also resists the pressure at the inner toe of the dam.
Percolation and Infiltration.
The earlier authorities on the subject of percolation and infiltration
of water are somewhat conflicting in their statements, if not confused
in their ideas. We are again impressed with the importance of a clearly
defined and definite use of terms. The temptation and tendency to use
language synonymously is very great, but it is unscientific and must
result in confusion of thought. Let it be observed that _filtration_
is the process of mechanically separating and removing the undissolved
particles floating in a liquid. That _infiltration_ is the process
by which water (or other liquid) enters the interstices of porous
material. That _percolation_ is the action of a liquid passing through
small interstices; and, finally, that _seepage_ is the amount of fluid
which has percolated through porous materials.
Many recent authorities are guilty of confusion in thought or
expression, as will appear from the following:
One says, for instance, that a
rock is water-tight when non-absorbent of water, but that a soil
is not water-tight unless it will absorb an enormous quantity of
water.
This would seem to indicate that super-saturation and not pressure is
necessary to increase the water-tightness of earth materials.
Again, in a recent discussion regarding the saturation and percolation
of water through the lower half of a reservoir embankment, it was
remarked, that
the more compact the material of which the bank is built,
the steeper will be the slope of saturation.
Exception was taken to this, and the statement made, that
_with compact material_, the sectional area of flow
is larger below a given level with porous material, and as
the bank slope is one determining factor of the line of
saturation, this line tends to approach the slope line;
while with porous material in a down-stream bank, the slope
of saturation is steeper and the area of the flow less.
In reply to this, it was said,
that it is obvious that if the embankment below the core
wall is built of material so compact as to be impervious
to water, no water passing through the wall will enter it,
and the slope of saturation will be vertical. If it be
less compact, water will enter more or less according to
the head or pressure, and according to its compactness or
porosity, producing a slope of saturation whose inclination
is dependent on the frictional resistance encountered by
the water. And the bank will be tight whenever the slope of
saturation remains within the figure of the embankment.
Further,
that it was necessary to distinguish between the slope assumed
by water _retained in_ an embankment and that taken by water
_passing through_ an embankment made of material too porous to
retain it; where the rule is clearly reversed and where the more
porous the material the steeper the slope at which water will run
through it at a given rate.
These citations are sufficient to emphasize the importance of exact
definition of terms and clear statement of principles.
The latest experiments relating to the percolation of water through
earth materials and tests determining the stability of soils are
those made during the investigations at the New Croton Dam and
Jerome Park Reservoir, New York, and those relating to the North
Dike of the Wachusett Reservoir, Boston. These are very interesting
and instructive, and it is here proposed to discuss the results and
conclusions reached in these cases, after some introductory remarks
reciting the order of events.
NEW CROTON DAM.–In June, 1901, the Board of Croton Aqueduct
Commissioners of New York requested a board of expert engineers,
consisting of Messrs. J. J. R. Croes, E. F. Smith and E. Sweet, to
examine the plans for the construction of the earth portion of the New
Croton Dam, and also the core wall and embankment of the Jerome Park
reservoir.
This report was published in full in Engineering News for Nov. 28,
1901. It was followed in subsequent issues of the said journal by
supplemental and individual reports from each member of the board
of experts, and by articles from Messrs. A. Fteley, who originally
designed the works, A. Craven, formerly division engineer on this work,
and W. R. Hill, at that time chief engineer of the Croton Aqueduct
Commission.
After describing the New Croton Dam, the board of experts preface their
remarks on the earth embankment by saying that
it has been abundantly proven that up to a height of 60
or 70 ft. an embankment founded on solid material and
constructed of well-selected earth, properly put in place,
is fully as durable and safe as a masonry wall and far less
costly.
There are, in fact, no less than 22 earth dams in use to-day exceeding
90 ft. in height, and twice that number over 70 ft. in height. Five of
the former are in California, and several of these have been in use
over 25 years. The writer fails to appreciate the reason for limiting
the safe height of earth dams to 60 or 70 ft.
The New Croton Dam was designed as a composite structure of masonry
and earth, crossing the Croton Valley at a point three miles from the
Hudson River. The earth portion was to join the masonry portion at a
point where the latter was 195 ft. high from the bed rock. The Board
thought there was no precedent for such a design and no necessity
for this form of construction. The point to be considered here was
whether a dam like this can be made sufficiently impermeable to water
to prevent the outer slope from becoming saturated and thus liable to
slide and be washed out.
The design of the embankment portion was similar to all the earth dams
of the Croton Valley. In the center is built a wall of rubble masonry,
generally founded upon solid rock, and “intended to prevent the free
seepage of water, but not heavy enough to act alone as a retaining wall
for either water or earth.”
Fig. 17 shows a section which is typical of most New England earth
dams; and Fig. 18, the sections of two of the Croton Valley dams, New
York water supply. These dams all have masonry core walls, illustrating
the third type of dams given on page 33.
[Illustration: FIG. 17.–CROSS-SECTION OF A TYPICAL NEW ENGLAND DAM.]
The board of experts made numerous tests by means of borings into
the Croton Valley dams to determine the slope of saturation. The
hydraulic laboratory of Cornell University also made tests of the
permeability of several samples of materials taken from pits. All the
materials examined were found to be permeable and when exposed to water
to disintegrate and assume a flat slope, the surface of which was
described as “slimy.”
Pipe wells were driven at different places into the dams and the line
of saturation was determined by noting the elevations at which the
water stood in them. In all the dams the entire bank on the water side
of the core wall appeared to be completely saturated. Water was also
found to be standing in the embankment on the down-stream side of the
core wall. The extent of saturation of the outer bank varied greatly,
due to the difference in materials, the care taken in building them,
and their ages. Fig. 19 gives the average slopes of saturation as
determined by these borings.
The experts stated
that the slope of the surface of the saturation in the bank
is determined by the solidity of the embankment: The more
compact the material of which the bank is built, the steeper
will be the slope of saturation.
As a result of their investigations, the experts were of the opinion
that the slope of saturation in the best embankments made of the
material found in the Croton Valley is about 35 ft. per 100 ft., and
that with materials less carefully selected and placed the slope may be
20 ft. per 100 ft.
Further, that taking the loss of head in passing through the core wall,
and the slope assumed by the plane of saturation, the maximum safe
height of an earth dam with its top 20 ft. above water level in the
reservoir and its outside slope 2 on 1, is 63 to 102.5 ft. This is a
remarkable finding in view of the fact that the Titicus Dam, one of the
Croton Valley dams examined, has a maximum height above bed rock of 110
ft. and has been in use seven years. This dam is not a fair example to
cite in proof of their conclusion, because its _effective head_ is only
about 46 ft.[2]
[Illustration: BOG BROOK DAM.]
[Illustration: MIDDLE BRANCH DAM.
FIG. 18.–CROSS-SECTION OF TWO CROTON VALLEY DAMS, SHOWING SATURATION.]
Mr. Fteley gave as a reason for the elevation of the water slope
found in the outer bank of the Croton dams the fact of their being
constructed of fine materials and stated that with comparatively porous
materials they would have shown steeper slopes of saturation.
Mr. Craven argued that all dams will absorb more or less water, and
that porosity is merely a degree of compactness; that slope implies
motion in water, and that there is no absolute retention of water in
the outer bank of a dam having its base below the plane indicated by
the loss of head in passing through the inner bank and then through a
further obstruction of either masonry or puddle; that there is simply a
partial retention, with motion through the bank governed by the degree
of porosity of the material.
Fig. 19 is a graphical interpretation of the conclusion reached by the
board of experts, as already given on page 41. “A” is an ideal profile
of a homogeneous dam with the inner slope 3 on 1 and the outer slope 2
on 1. The top width is made 25 ft. for a dam having 90 ft. effective
head, the high-water surface in the reservoir being 10 ft. below the
crest of the dam. This ideal profile is a fair average of all the earth
dams of the world. Not having a core wall to augment the loss of head,
it fairly represents what might be expected of such a dam built of
Croton Valley material, compacted in the usual way. It should be noted
that the intersection of the plane of saturation with the rear slope of
the dam at such high elevation as shown indicates an excessive seepage
and a dangerously unstable condition.
Preliminary Study of Profile for Dam.
The preliminary calculations for designing a profile for an earth dam
are simple and will here be illustrated by an example. Let us assume
the following values:
a. Central height of dam, 100 ft.
b. Maximum depth of water, 90 ft., with surface 10 ft. below
crest of dam.
c. Effective head, 90 ft.
d. Weight of water, 62.5 lbs. per cu. ft.
e. Weight of material, 125 lbs. per cu. ft.
f. Coefficient of friction, 1.00, or equal to the weight.
g. Factor of safety against sliding, 10.
The width corresponding to the vertical pressure of 1 ft. is,
(62.5 × 10)/125 = 5 ft.
[Illustration]
[Illustration: FIG. 19.–GRAPHICAL INTERPRETATION OF STUDIES OF BOARD OF
EXPERTS ON THE ORIGINAL EARTH PORTION OF THE NEW CROTON DAM.]
The hydrostatic pressure per square foot at 90 ft. depth is, 62.5 × 90
= 5,625 lbs.
The dam, having a factor of safety of 10, must present a resistance of,
5,625 × 10 = 56,250 lbs., or 28 tons per square foot.
The theoretical width of bank corresponding to 90 ft. head and a factor
of 10 is shown by the dotted triangle (A-B-B) to be 450 ft., (B, Fig.
19) with slopes 2½ on 1.
To this must be added the width due to the height of crest above the
water surface in the reservoir and the width of crest.
The former would be, 2 (2½ × 10) = 50 ft., and the latter by
Trautwine’s rule, 2 + 2√100 = 22 ft., giving a total base width of 522
ft.
Let us now assume that the slope of saturation may be 35 ft. per 100
ft. We observe that this intersects the base 40 ft. within the outer
toe of the bank slope. If the plane of saturation was 33 ft. per 100,
it would just reach the outer toe. It would be advisable to enlarge
this section by adding a 10-ft. berm at the 50-ft. level, having a
slope not less than 3 on 1 for the up-stream face, and two 15-ft. berms
on the down-stream face, having slopes 2½ on 1. The additional width
of base due to these modifications in our profile amounts to 65 ft.,
giving a total base width of 587 ft., and increasing the factor of
safety from 10 to 13. It should be remembered that if the bank becomes
saturated this factor of safety may be reduced 50%, the coefficient of
moist clay being 0.50.
The loss of head due to a core wall of masonry, as designed for the
New Croton Dam, was assumed by the board of experts to be 21 ft., or
17% of the depth of water in full reservoir. It has been stated by
several authorities that the primary object of a masonry core wall is
to afford a water-tight cut-off to any water of percolation which may
reach it through the upper half of the embankment. It appears that
absolute water-tightness in the core wall is not obtained, although the
core walls of the Croton dams are said to be “the very best quality of
rubble masonry that can be made.”
Mr. W. W. Follett, who is reported to have had considerable experience
in building earth dams, and who has made some valuable suggestions
thereupon, is emphatic in saying,
that the junction of earth and masonry forms a weak point, that
either a puddle or masonry core in an earthen dam is an element
of weakness rather than strength.
He also thinks the usual manner of segregating and depositing
materials different in density and weight, and thus subject to
different amounts of settlement, as bad a form of construction as could
be devised.
Core walls may prevent “free passage of water” and “excessive seepage,”
but are nevertheless of doubtful expediency.
Earthwork Slips and Drainage.
Mr. John Newman, in his admirable treatise on “Earthwork Slips and
Subsidences upon Public Works,” classifies and enumerates slips as
follows:
Natural causes, 7.
Artificial causes, 31.
Additional causes due to impounded water, 7.
After describing each cause he presents 39 different means used to
prevent such slips and describes methods of making repairs.
Mr. Wm. L. Strange has had such a large and valuable experience and has
set forth so carefully and lucidly both the principles and practice of
earth dam construction, that the writer takes pleasure in again quoting
him on the subject of _drainage_, of which he is an ardent advocate. He
says that,
thorough drainage of the base of a dam is a matter of vital
necessity, for notwithstanding all precautions, some water will
certainly pass through the puddle.
It is at the junction of the dam with the ground that the maximum
amount of leakage may be expected. The percolating water should be
gotten out as quickly as possible. The whole method of dealing with
slips may be summed up in one word–_drainage_.
The proper presentation of these two phases of our subject would in
itself require a volume. The interested reader is therefore referred to
the different authorities and writers cited in Appendix II.
Jerome Park Reservoir Embankments.
The Jerome Park reservoir is an artificial basin involving the
excavation and removal of large quantities of soil, and the erection of
long embankments with masonry core walls, partly founded on rock and
partly on sand. The plan and specifications call for an embankment 20
ft. wide on top, with both slopes 2 on 1, and provide for lining the
inner slope with brick or stone laid in concrete, and for covering the
bottom with concrete laid on good earth compacted by rolling.
[Illustration: Section at Sta. 99.]
[Illustration: Section at Sta. 76+20.
FIG. 20.–GRAPHICAL EXHIBIT OF STUDIES OF JEROME PARK RESERVOIR
EMBANKMENT.]
Wherever bed rock was not considered too deep below the surface the
core walls were built upon it. In other places the foundation was
placed 8 to 10 ft. below the bottom of the reservoir and rested upon
the sand.
It appears that the plans of the Jerome Park embankment were changed
from their original design, prior to the report of the board of
experts, on account of two alleged defects, namely, “cracks in the core
wall” and “foundation of quicksand,” and incidentally on account of the
supposed instability of the inner bank.
In describing the materials on which these embankments rest the experts
remarked
that all these fine sands are unstable when mechanically
agitated in an excess of water, and that they all settle in
a firm and compact mass under the water when the agitation
ceases. That they are quite unlike the true quicksands whose
particles are of impalpable fineness and which are “quick”
or unstable under water.
Fig. 20 is a graphic exhibit of the results of tests made at “Station
76 + 20,” and at “Station 99,” to determine the flow line of water in
the sand strata underlying the embankment and bottom of the Jerome Park
reservoir.
The experts reported that there was no possible danger of sliding or
sloughing of the bank; that the utmost that could be expected would
be the percolation of a small amount of water through the embankment
and the earth; and that this would be carried off by the sewers in the
adjacent avenues; that a large expenditure to prevent such seepage
would not be warranted nor advisable.
In concluding their report, however, they recommended changing the
inner slope of 2 on 1 to 2½ on 1, and doubling the thickness of the
concrete lining at the foot of the slope to preclude all possibility
of the sliding or the slipping of the inner bank in case of the water
being lowered rapidly in the reservoir.
Mr. W. R. Hill, then chief engineer of the Croton Aqueduct Commission,
favored extending the core walls to solid rock. He took exception to
the manner of obtaining samples of sand by means of pipe and force-jet
of water, claiming that only the coarsest sand was obtained for
examination. He did not consider fine sand through which three men
could run a ¾-in. rod 19 and 20 ft. to rock without use of a hammer,
very stable material upon which to build a wall.
North Dike of the Wachusett Reservoir, Boston.
The North Dike of the Wachusett Reservoir is another large public work
in progress at the present time. It is of somewhat unusual design
and the preliminary investigations and experiments which led to its
adoption are interesting in the extreme.[3]
The area to be explored in determining the best location for the dike
was great, and the preliminary investigations conducted by means of
wash drill borings, very extensive. A total of 1,131 borings were made
to an average depth of 83 ft., the maximum depth being 286 ft. The
materials were classified largely by the appearance of the samples,
though chemical and filtration tests were also made. The plane of the
ground water was from 35 to 50 ft. below the surface, and the action of
the water-jet indicated in a measure the degree of permeability of the
strata.
In addition to these tests experimental dikes of different materials,
and deposited in different ways, were made in a wooden tank 6 ft. wide,
8 ft. high and 60 ft. long. The stability of soils when in contact with
water was experimented with, as shown in Fig. 21, in the following
manner:
An embankment (Fig. 21) was constructed in the tank of the material to
be experimented with, 2 ft. wide on top, 6 ft. high, with slopes 2 on
1, and water admitted on both sides to a depth of 5 ft. The top was
covered with 4-in. planks 2 ft. long and pressure applied by means of
two jack screws resting upon a cross beam on top of the planks.
With a pressure of three tons per square foot, the 4-in. planks were
forced down into the embankment a little more than 6 ins., resulting
in a very slight bulging of the slopes a little below the water level.
Immediately under the planks the soil became hard and compact. A man’s
weight pushed a sharp steel rod, ¾-in. in diameter, only 6 to 8 ins.
into the embankment where the pressure was applied, while outside of
this area the rod was easily pushed to the bottom of the tank.
These results corroborate in a general way the practical experience of
the author, both in compressed embankments, where he found it necessary
to use a pick vigorously to loosen the material of which they were
composed, and in embankments made by merely dumping the material from
a track, in which case the earth is so slightly compressed that an
excavation is easily made with a shovel.
[Illustration: Fig. 21.]
[Illustration: Fig. 22.]
[Illustration: Fig. 23.–CAN FOR DETERMINING FRICTIONAL RESISTANCE]
[Illustration: Fig. 24.]
[Illustration: Fig. 25.
FIGS. 21 TO 24.–EXPERIMENTAL DIKES AND CYLINDER EMPLOYED IN STUDIES FOR
THE NORTH DIKE OF THE WACHUSETT RESERVOIR; AND (FIG. 25) CROSS-SECTION
OF THE DIKE.]
The difference in the coefficient of friction of the same material
when dry and when wet greatly modifies the form of slope. The harder
and looser the particles, the _straighter_ will be the slope line in
excavation and slips. The greater the cohesion of the earth, the _more
curved_ will be the slope, assuming a parabolic curve near the top–the
true form of equilibrium.
RATE OF FILTRATION.–The rate of filtration through different soils was
experimented with by forming a dike in the tank previously mentioned,
as shown in Fig. 22.
The dike was made full 8 ft. high, 7 ft. wide on top, with a slope on
the up-stream side of 2 on 1, and on the down-stream side 4 on 1. This
gave a base width of 55 ft. Immediately over the top of the dike there
was placed 3 ft. of soil to slightly consolidate the top of the bank
and permit the filling of the tank to the top without overflowing the
dike. The water pressure in different parts of the dike was determined
by placing horizontal pipes through the soil crosswise of the tank.
These pipes were perforated and covered with wire gauze, being
connected to vertical glass tubes at their ends. The end of the slope
on the down-stream side terminated in a box having perforated sides and
filled with gravel, thus enabling the water to percolate and filter out
of the bank without carrying the soil with it.
When the soil was shoveled loosely into the tank, without consolidation
of any kind, it settled on becoming saturated and became quite compact.
It took five days for the water to appear in the sixth gauge pipe near
the lower end of the tank. After the pressure, which was maintained
constant, had been on for several weeks, the seepage amounted to one
gallon in 22 minutes. When the soil was deposited by shoveling into the
water, the seepage amounted to one gallon in 34 minutes.
The relative filtering capacities of soils and sands were thought to
be better determined by the use of galvanized iron cylinders of known
areas.
Fig. 23 shows one of the cylinders. These latter experiments confirmed
those previously made at Lawrence, by Mr. Allen Hazen, for the
Massachusetts State Board of Health. They showed that the loss of head
was directly proportioned to the quantity of water filtered and that
the quantity filtered will vary as the square of the diameter of the
_effective size_ of the grains of the filtering material.[4]
The material classed as “permeable” at the North Dike of the Wachusett
Reservoir has an effective diameter of about 0.20 mm. A few results are
given in the following table:
Amount of Filtration in Gallons per Day, Through an Area of 10,000 Sq.
Ft., With a Loss of Head or Slope of 1 ft. in 10 ft.
Material. Unit ratios. U. S. gallons.
(1) Soil 1 510
(2) Very fine sand 14 7,200
(3) Fine sand 176 90,000
(4) Medium sand 784 400,000
(5) Coarse sand 4,353 2,200,000
To be sure that the accumulation of air in the small interstices of
the _soil_ was not the cause of the greatly reduced filtration through
it, another series of experiments was conducted in the wooden tank, as
shown in Fig. 24.
A pair of screens was placed near each end of the tank, filled with
porous material, sand and gravel, and the 50-ft. space between filled
with soil. The soil was rammed in 3-in. layers, and special care taken
to prevent water from following along the sides and bottom of the tank.
One end was filled with water to near the top, while the other end gave
a free outlet.
After this experiment had been continued for more than a month, the
amount of seepage averaged 1.7 gallons per 24 hours, or about 32 drops
per minute.
Filtration tests were also made through soil under 150 ft. head, or 5
lbs. per sq. in., with results not materially different, it is stated,
from those already given. The soil used in all these tests contained
from 4 to 8% by weight of organic matter. This was burned and similar
tests made with the incinerated soil, resulting in an increase of about
20% more seepage water.
PERMANENCE OF SOILS.–This last material experimented with suggests the
subject of _permanence_ of soils. This was reported upon separately and
independently by Mr. Allen Hazen and Prof. W. O. Crosby. These experts
agreed in their conclusion, stating
that the process of oxidation below the line of saturation would be
extremely slow, requiring many thousands of years for the complete
removal of all the organic matter, and that the tightness of the
bank would not be materially affected by any changes which are
likely to occur.
It has been remarked,
that of all the materials used in the construction of
dams, _earth_ is physically the least destructible of
any. The other materials are all subject to more or less
disintegration, or change in one form or another, and in
earth they reach their ultimate and most lasting form.
In speaking of the North Dike of the Wachusett Reservoir, Mr. Stearns
remarked that,
it was evident by the application of Mr. Hazen’s formula for
the flow of water through sands and gravels, that the very
fine sands found at a considerable depth below the surface
would not permit enough water to pass through them if a dike
of great width were constructed, to cause a serious loss of
water, and it was also found that the soil, which contained
not only the fine particles or organic matter, but also a
very considerable amount of fine comminuted particles, which
the geologist has termed “rock flour,” would be sufficiently
impermeable to be used as a substitute for clay puddle.
Fig. 25 shows the maximum section of the North Dike with its cut-off
trench. The quantities and estimated cost of the completed structure
are given in the table herewith:
|––– Cost –––|
Per cent.
Work. Quantities. Unit Actual. total.
(cu. yds.) Price.
Soil 5,250,000 $0.05 $262,500 34.7
Cut-off trench 542,000 .20 108,400 19.3
Borrowed
earth and gravel 200,000 .20 40,000
Slope paving 50,000 2.20 110,000 14.6
Sheet-piling,
pumping, etc. 117,000 15.5
Engineering and
preliminary investigations 120,000 15.9
––––––– –––––
Total cost $757,900 100.0
Druid Lake Dam, Baltimore, Md.
Another very interesting and instructive example of high earth dam
construction is that of the Druid Lake Reservoir embankment, Baltimore,
Md.
This dam was built under the supervision of Mr. Robt. K. Martin.
Construction was begun in 1864, and the dam was finished in 1870. Mr.
Alfred M. Quick, present chief engineer of the water-works of the
City of Baltimore has given a very lucid description of this work in
Engineering News of Feb. 20, 1902.
Fig. 26 is a cross-section of this dam, showing the method of
construction so clearly as to scarcely need further description. The
banks D-D on either side of the central puddle wall were carried up in
6-in. layers with horses and carts, and kept about 2 ft. higher than
the puddle trench, which always contained water. The banks E-E were
made of dumped material, after which the basins F-F were first filled
with water and finally filled by dumping material into the water from
tracks being moved in toward the center.
[Illustration: FIG. 26–WORKING CROSS-SECTION OF DRUID LAKE DAM.]
After reaching the top of this fill, banks B-B-B were built up in
layers similar to D-D. The second set of basins C-C were then filled
in a manner similar to F-F. The remaining portion A-A was constructed
in layers like D-D and B-B, with the addition of compacting each layer
with a heavy roller.
Finally the inner face slope was carried up in 3-in. layers and
thoroughly rolled, after which 2 ft. of “good puddle” was put upon the
inner slope the latter was rip-rapped, the crown covered with gravel
and the rear slope sodded.
Some years after completion, a driveway was built along the outer
slope, as shown, which had a tendency to strengthen the dam, though not
designed expressly for that purpose.
It is of interest to know that the influent, effluent and drain pipes
were originally constructed through or under the embankment. These
pipes were laid upon solid earth, and where they passed through the
puddle wall were supported upon stone piers 6 ft. apart. As might be
expected, they soon cracked badly and were finally abandoned, new ones
being placed in the original ground at the south side of the lake. Mr.
Quick states that so far as is known there has never been any evidence
of a leak through the embankment during these 32 years of service.
New Types of Dams; Bohio, Panama Canal.
A brief description will now be given of three different dams designed
for Bohio, on the proposed Panama Canal. Mr. George S. Morison’s paper
before the American Society of Civil Engineers, on “The Bohio Dam,”
and the discussion thereon, especially that by Mr. F. P. Stearns, were
quite fully reported in Engineering News for March 13 and May 8, 1902.
In constructing the Panama Canal it will be necessary to impound the
waters of the Chagres River, near Bohio, to maintain the summit level
of this canal and supply water for lockage.
THE FRENCH DESIGN.–Fig. 27 is an enlarged section of the original
design of the new French Co. This design has no core wall, but at
the up-stream toe a concrete wall was to be built across the river
between the two lines of sheet-piling. At the down-stream toe a large
amount of riprap was to be placed to prevent destruction of the dam
during construction. In this case it would be necessary to construct
a temporary dam above and also to use the excavation for the locks
as a flood spillway. This method would involve considerable risk to
the work, on account of the large volume of flood waters it might be
necessary to take care of during construction.
ISTHMIAN CANAL COMMISSION.–The dam proposed by the Isthmian Canal
Commission is shown by Fig. 28. This was designed to be an absolutely
water-tight closure of the geological valley, by using a masonry core
wall carried down to bed rock. The maximum depth being 129 ft., it was
planned to rest the concrete wall on a series of pneumatic caissons
reaching to rock. The spaces between the caissons would be closed and
made water-tight. Both slopes of the earth embankment were to have
horizontal benches and be revetted with loose rock.
MR. MORISON’S DESIGN.–To appreciate fully the object and aim of the
third design, Fig. 29, which may be called a new type, although similar
in many respects to the North Dike of the Wachusett reservoir already
illustrated and described, it should be stated that the equalized flow
of the Chagres River is put at 1,000 cu. ft. per sec. Of this quantity
it is estimated that 500 cu. ft. would be needed for lockage and 200
cu. ft. for evaporation. This leaves 300 cu. ft. per sec. available for
seepage and other losses or to be wasted.
[Illustration]
[Illustration]
[Illustration: FIGS. 27 TO 29.–DESIGNS FOR THE BOHIO DAM, PANAMA
CANAL.]
It will thus be seen that a scarcity of water is not in this instance
a condition demanding an absolutely water-tight dam. The amount of
seepage permissible without endangering the stability of the structure
is the real point now to be discussed.
The third design, which was proposed by Mr. Morison, is shown by Fig.
29. The topography and configuration of this dam site is not unlike
that of the San Leandro Dam, California, soon to be described, while
the general design is similar, as has been remarked, to the North Dike
of the Wachusett Reservoir.
This third design contemplates a compound structure, formed by two
rock-fill dams situated about 2,120 ft. apart, with the intervening
space filled with loose rock, earth and other available material.
Immediately below the upper and higher rock-fill dam, it is proposed
to place across the canyon a puddle wall 50 ft. in width, resting over
two lines of sheet-piling 30 ft. apart. This piling would probably not
reach farther than 50 ft. below tidewater, the solid rock floor being
about 100 ft. deeper.
Mr. Morison made use of Mr. Hazen’s filtration formula for estimating
the rate and quantity of seepage through the permeable strata below the
dam. This formula is:
h t + 10°
V = cd² –– ––––––
l 60
where
V = rate of flow in meters per day through the whole section.
c = constant varying from 450 to 1,200,
according to cleanness of the sand.
d = “effective size” of sand in mm.
h = head in feet.
l = length or distance water must pass.
t = temperature of the water (Fahr.)
This formula should be used only when the _effective sizes_ of sands
are from 0.10 to 3.0 mm. and with _uniformity coefficients_ below
5.0[5].
Mr. Morison used the following values: c = 1,000; d = 1.0 mm.; h = 90
ft.; l = 2,500 ft.; t = 90°; for the solution of this problem, and
obtained a velocity of 0.002 ft. per sec. The bed of sand and gravel
was assumed to have a sectional area of 20,000 sq. ft. for 2,500 ft. in
length. This gives a seepage of 40 cu. ft. per sec.
It is believed that the above rate of 0.002 ft. per sec., equivalent to
1⅜ ins. per minute, or 7 ft. per hour, is not sufficient to move any of
the material. The velocity of water percolating through sand is found
to vary directly as the head and inversely as the distance.
The value of “c” in the formula is larger for sands of filters
favorable for flow, and smaller for compacted materials and dams.
Mr. Morison thought it might be nearer the actual conditions to assume
d = 0.50 mm.; c = 500; and l = 5,000 ft.; in which case the seepage
would only amount to 2.5 ft. per sec. In this last assumption the
“effective size” of sand grains is 2½ times that classed as “permeable
material” at the North Dike of the Wachusett Reservoir.
Prof. Philipp Forchheimer, of Gratz, Austria, recommends the use of the
formula,
h
––– = a√ + b√²
l
for the percolation through soils between loam and loamy sand.
Sellheim, Masoni, Smreker, Kröber and other authorities on filtration
use still other formulas, to which the reader and student is referred
for further research.
The writer, having had occasion in his professional practice to study
quite carefully the subject of ground waters, and their percolation
or flow through different classes of materials and under varying
conditions, is of the opinion that rarely does the cross-section of
a stream-channel, filled with sand, gravel and debris, present, even
approximately, a homogeneous or uniform mass; and that there are,
almost without exception, strata of material much coarser and more
porous than the general average. In other words, that it is extremely
difficult to arrive at a uniformity coefficient. It is unwise to place
much reliance upon an estimated flow where this is the case. The
formula may be used with confidence where the layers are artificially
made, and where there is no uncertainty regarding the uniform character
of the material. In most natural channels there are distinct lines
of flow, and under considerable hydrostatic head or pressure these
lines of flow would surely enlarge. There is a wide difference between
permissible and dangerously excessive percolation through an earth
embankment. The local features, economical considerations and magnitude
of the risks, all bear upon this question and must be considered for
each particular case.
It is of interest to compare the estimated cost of the three designs
proposed for the Bohio Dam, based upon the same unit prices, as follows:
French Engineers’ design $3,500,000
Isthmian Canal Commissioners’ design 8,000,000
Mr. Morison’s design 2,500,000
No comments will be made upon these figures, further than to remark
that the successful building of a stable dam, accomplished by the use
of an excessive quantity of materials and at a cost beyond reasonable
requirements, is mainly instructive as illustrating “how not to do it.”
It is creditable to execute substantial works at a reasonable cost,
but it reflects no credit upon any one to construct them regardless of
expense.
Combined Rock-fill and Earth Dam.
Fig. 30 shows a section of the Upper Pecos River Dam near Eddy, N. M.
This dam is quite fully described by Mr. Jas. D. Schuyler, in his
recent book on “Reservoirs for Irrigation, Water-Power and Domestic
Water-Supply,” and need not be mentioned in this paper, further than
to call attention to the combination of rock-fill and earth which
constitutes its particular type of construction. This type of dam is
believed to be for many localities a very good one, but up to the
present time has only been adopted for dams of moderate height, under
60 ft.
The San Leandro Dam, California.
A section of the San Leandro Dam, near Oakland, Cal., is shown by
Fig. 31. This section was supplied by Mr. W. F. Boardman, hydraulic
engineer, who superintended the construction of the dam, from his own
private notes and data. It differs materially from sections heretofore
published, and is 5 ft. higher, thus making it rank as the highest
earth dam in the world of which we have an authentic record.
The dam was commenced in 1874, and brought up to a height of 115 ft.
above the bed of the creek in 1898. At the present time it is 500 ft.
in length on the crest and 28 ft. wide. The original width of the
ravine at the base of the dam was 66 ft. The present width of base
from toe to toe of slopes is 1,700 ft. The height of embankment above
the original surface is 125 ft., with a puddle trench extending 30 ft.
below.
[Illustration: FIG. 30.–CROSS-SECTION OF UPPER PECOS RIVER DAM;
COMBINED ROCK FILL AND EARTH.]
[Illustration: FIG. 31.–DEVELOPED SECTION OF SAN LEANDRO DAM.]
All that portion of the dam within a slope of 2½ on 1 at the rear and
3 on 1 at the face is built of choice material, carefully selected and
put in with great care. The portion outside of the 2½ on 1 slope line
at the down-stream side of the dam, was _sluice in_ from the adjacent
hills regardless of its character, and is composed of ordinary soil
containing more or less rock.
This process of sluicing was carried on during the rainy season, when
there was an abundance of water, and it was intended to be continued
until the canyon below the dam had been filled to an average slope of
6.7 on 1 at the rear of the dam. It was thought that the location was
particularly favorable for this kind of construction, the original
intention being to raise the dam from time to time, not only to
increase the storage as the demand for water increased, but to meet
the annual loss in capacity caused by the silting up of the reservoir
basin. The latter has amounted to about 1 ft. in depth per annum.
METHOD OF CONSTRUCTION.–Under the main body of the dam, the surface was
stripped of all sediment, sand, gravel and vegetable matter. Choice
material, carefully selected, was then brought by carts and wagons and
evenly distributed over the surface in layers about 1 ft. or less in
thickness. This was sprinkled with just enough water to make it pack
well, not enough to make it like mud. During construction a band of
horses was led by a boy on horseback over the entire work, to compact
the materials and assist in making the dam one homogeneous mass. No
rollers were used on this dam.
The central trench was cut 30 ft. below the original bed of the creek.
In the bottom of this trench three secondary trenches, 3 ft. wide by 3
ft. deep, were made and filled with concrete. These concrete walls were
carried up 2 ft. above the general floor of the trench, to break the
continuity of its surface.
The original wasteway, constructed at the north end of the dam, has
been practically abandoned, having been substituted by a tunnel of
larger capacity. The original wasteway was excavated in the bed rock
of the natural hillside, and although lined with masonry, is not in
the best condition. The author considers its location an objectionable
feature, as menacing the safety of the dam, and thinks it should be
permanently closed.
A wasteway tunnel, 1,487 ft. in length, was constructed in 1888,
through a ridge extending north of the dam. This has a sectional area
of about 10×10 ft., lined with brick masonry throughout, having a grade
of 2½%.
The criticism might be made of the tunnel that it is faulty in design
at the entry or reservoir end, where the water must first fall over
a high spillway wall, aerating the water before entering the tunnel
proper. The water even then has not easy access to the tunnel, and no
adequate arrangements have been made for ventilation, so as to insure
the utilization of its maximum capacity. The maximum depth of water in
the reservoir is about 85 ft., and the full capacity 689,000,000 cu.
ft. of water. The catchment area is 43 square miles, and the surface
of the reservoir when full 436 acres. The outlet pipes are placed in
two tunnels at different elevations through the ridge north of the dam.
There are no culverts or pipes extending through the body of the dam
itself.
Hydraulic-fill Dams.
No discussion of earth dams would be complete without some reference
being made to the novel type of construction developed in western
America in recent years, by which railroad embankments and water-tight
dams are built up by the sole agency of water. The water for this
purpose is usually delivered under high pressure, as it is generally
convenient to make it first perform the work of loosening the earth
and rock in the borrow pit, as well as subsequently to transport them
to the embankment, and there to sort and deposit them and finally part
company with them after compacting them solidly in place, even more
firmly than if compressed by heavy rollers. Sometimes, however, water
is delivered to the borrow pit without pressure, in which event the
materials must be loosened by the plow or by pick and shovel by the
process called ground sluicing in placer mining parlance.
An abundance of water delivered by gravity under high pressure is
usually regarded as one of the essential factors in hydraulic-fill dam
building, but it is not essential that there be a large continuous
flow. The Lake Frances Dam, recently constructed for the Bay Counties
Co., of California, by J. D. Schuyler, is 75 ft. high, 1,340 ft. long
on top, and contains 280,000 cu. yds. The dam was built up by materials
sluiced by water that was forced by a centrifugal pump through a 12-in.
pipe and 3-in. nozzle, against a high bank, whence the materials were
torn and conveyed by the water through flumes and pipes to the dam.
About 6 cu. ft. per sec. of water was thus used, and at one stage of
the work the supply stream was reduced to less than 0.1 ft. per sec.,
the water being gathered in a pond and pumped over and over again.
The chapter on hydraulic-fill dams in Mr. Schuyler’s book on
“Reservoirs for Irrigation” will be found to contain matter on the
subject interesting to those who desire to pursue it further, and the
reader is again referred to that work.
An Impervious Diaphragm in Earth Dams.
As a result of the recent extended discussion concerning the design
of the New Croton Dam and the Jerome Park Reservoir embankments,
the Engineering News of Feb. 20, 1902, contained a very suggestive
editorial entitled, “Concerning the Design of Earth Dams and Reservoir
Embankments.” The opinion is given that no type of structure that man
builds to confine water can compare in permanence with earth dams,
after which the following pertinent questions are asked:
1. How shall an earth dam be made water-tight?
2. What is the office and purpose of the masonry core wall?
3. Would not a water-proof diaphragm of some kind be better
than a core wall of either masonry or puddle?
The article then suggests a number of designs of diaphragm construction,
with a special view of obtaining absolute water-tightness, by use of
asphaltum, cement mortar, steel plates, etc. Special emphasis was put
upon the _principle_ of constructing a water-proof diaphragm. The
matter of relative cost is advanced as an argument in favor of the
diaphragm principle as against the usual orthodox method. The saving in
cost is to be accomplished by the use of inferior materials and less
care in the handling of them, or by both. It is suggested that almost
any kind of material available, rock, sand or gravel, will answer
every purpose where good earth is not to be found. Further, that this
material may be dumped from the carts, cars or cableways, or be placed
by the hydraulic-fill method.
The writer believes the diaphragm method of construction may have some
merits, but that it is attended by the very great risk of neglecting
principles most vitally important to the successful construction of
high earth dams, which will now be formulated and advanced, as follows:
CHAPTER VI.
_Conclusions._
The writer in concluding this study wishes to emphasize certain
principles and apparently minor details of construction, which from
observation and personal experience, seem to him of vital importance.
He believes firmly in the truth contained in the following remarks by
Mr. Desmond FitzGerald, of Boston, germane to this subject:
An engineer must be guided by local conditions and the resources at
his command in building reservoir embankments. His design must be
largely affected by the nature of the materials. There are certain
_general principles_, however, which must be observed and which
will be applied by an engineer of skill, judgment and experience to
whatever design he may adopt. It is in the application of these
principles that the services of the professional man becomes
valuable, and it is from a lack of them, that there have been so
many failures.
The details and principles of construction, relating to high earth
dams, may be summarized or stated in order of their application, as
follows:
(1) Select a firm, dry, impermeable foundation, or make it so by
excavation and drainage. All alluvial soil containing organic matter
and all porous materials should be excavated and removed from the
dam site when practicable; that is, where the depth to a suitable
impermeable foundation is not prohibitive by reason of excessive cost.
Wherever springs of water appear, they must be carried outside the
lines of the embankment by means of bed rock drains, or a system of
pipes so laid and embedded as to be permanent and effective.
The drainage system must be so designed as to prevent the infiltration
of water upward and into the lower half of the embankment, and at the
same time insure free and speedy outlet for any seepage water passing
the upper half. All drains should be placed upon bed rock or in the
natural formation selected for the foundation of the superstructure.
They should be constructed in such a manner as to prevent the flow
of water outside the channel provided for it, and also prevent any
enlargement of the channel itself. To this end, cement, mortar, broken
stone, and good gravel puddle are the materials best suited for this
purpose.
(2) Unite the body of the embankment to the natural foundation by means
of an impervious material, durable and yet sufficiently elastic to bond
the two together. When the depth to a suitable foundation is great,
a central trench excavated with sloping sides, extending to bed rock
or other impervious formation, refilled with good puddling material,
properly compacted, will suffice.
When clayey earth is scarce and expensive to obtain, a small amount of
clay puddle confined between walls of brick, stone or concrete masonry,
and extending well into the body of the embankment and so built as
to avoid settlement, will prevent excessive seepage. This form of
construction is not to be carried much above the original surface of
the ground.
(3) The continuity of surfaces should always be broken, at the same
time avoiding the formation of cavities and lines of cleavage. No
excavation to be refilled should have vertical sides, and long
continuous horizontal planes should be intercepted by wedge-shaped
offsets, enabling the dovetailing of materials together.
All loose and seamy rock or other porous material should be removed,
and where the refill is not the best for the purpose, mix the good and
bad ingredients thoroughly, after which deposit in very thin layers.
(4) Make the dimensions and profile of dam with a factor of safety
against sliding of not less than ten. The preliminary calculations for
designing such a profile have been given on p. 42.
(5) Aim at as nearly a homogeneous mass in the body of the embankment
as possible, thus avoiding unequal settlement and deformation. This
manner of manipulating materials will eliminate many uncertain or
unknown factors, but it means rigid inspection of the work and
intelligent segregation of materials, no matter what method of
transporting them may be adopted. The smaller the unit loads may
be, the more easily a homogeneous distribution of materials will be
obtained.
(6) Select earthy materials in preference to organic soils, with a
view of such combination or proportion of different materials as will
readily consolidate. _Consolidation is the most important process
connected with the building of an earth dam._ The judicious use of
soil containing a small percentage of organic matter may be permitted,
however, when there is a lack of clayey material for mixing with sandy
and porous earth materials. Such a mixture, properly distributed and
wetted, will consolidate well under heavy pressure and prove quite
satisfactory.
(7) Consolidation being the most important process and the only
safeguard against permeability and instability of form, use only the
amount of water necessary to attain this. Too much or too little are
equally bad and to be avoided. It is believed that only by experiment
and experience is it possible to determine just the proper quantity of
water to use with the different classes of materials and their varying
conditions. In rolling and consolidating the bank, all portions that
have a tendency to quake must be removed at once and replaced with
material that will consolidate; it _must not_ be covered up, no matter
how small the area.
(8) In an artificial embankment for impounding water it is
impracticable to place reliance upon time for consolidation; it _must_
be effected by mechanical means. Again we repeat, that consolidation is
the most vitally important operation connected with the building of an
earth dam. When this is satisfactorily attained it is proof that the
materials are suitable and that the other necessary details have been
in a large measure complied with. Light rollers are worse than useless,
being a positive harm, resulting in a smoothing or “ironing process,”
deceptive in appearance and detrimental in many ways.
The matter of supreme importance in the construction of earth dams is
that the greatest consolidation possible be specified and effected.
To this end it is necessary that heavy rollers be employed, and that
such materials be selected as respond best to the treatment. There
are certain kinds of earth materials which no amount of wetting and
rolling will compact. These must be rejected as unfit for use in any
portion of an earth dam. Let the design of the structure be ever so
true to correct engineering principles, it is still necessary to give
untiring attention to the work of consolidation. It is therefore
according to the design of a thoroughly compacted homogeneous mass,
rather than to the suggested _diaphragm type_, to which modern practice
should conform. This is in harmony with Nature’s own methods, and in
conformity to correct principles.
(9) Avoid placing pipes or culverts through any portion of the
embankment. The writer considers it bad practice ever to place the
outlet pipes through a high earth dam, and fails to see any necessity
for so doing.
(10) The surface of the dam, both front and rear, must be suitably
protected against the deteriorating effects of the elements. This may
include pitching the up-stream face, the riprap work at the toe of the
inner slope, the roadway and covering of the crown, the sodding or
other protection of the rear slope, and the construction of surface
drains for the berms.
(11) Ample provision for automatic wasteways should be made for
every dam, so that the embankment can never under any circumstances
be over-topped by the impounded water. Earthquakes and seismic
disturbances will produce no disastrous effects upon an earth dam.
Its elasticity will resist the shock of water lashing backwards and
forwards in the reservoir.
(12) Finally, provide for intelligent and honest supervision during
construction, and insist upon proper care and maintenance ever
afterwards.
APPENDIX I.
High Earth Dams.
–Embankment– ––– Slopes ––– Available
Name of Dam Max. Top depths,
or Reservoir. Location. height, width, Water. Bear. ft.
ft. ft.
San Leandro California 125 28
Tabeaud California 123 20 3 on 1 2½ on 1 70
Druid Hill Maryland 119 60 4 on 1 2 on 1 82
Dodder Ireland 115 22 3½ on 1 3 on 1
Titicus Dam New York 110 30 2 on 1 2½ on 1
Mudduk Tank India 108 3 on 1 2½ on 1
Cummum Tank India 102 3 on 1 1 on 1 90
Dale Dike England 102 12 2½ on 1 2½ on 1
Marengo Algeria 101
Torside England 100 84
Yarrow England 100 24 3 on 1 2 on 1
Honey Lake California 96 20 3 on 1 2 on 1
Pilarcitos California 95 25 2¾ on 1 2½ on 1
San Andres California 95 25 3½ on 1 3 on 1
Temescal California 95 12 3 on 1 2 on 1
Waghad India 95 6 3 on 1 2 on 1 81
Bradfield England 95 12 2½ on 1 2½ on 1
Oued Menrad Algeria 95
St. Andrews Ireland 93 25
Edgelaw Scotland 93 3 on 1 2½ on 1
Woodhead England 90 72
Tordoff Scotland 85 10 3 on 1 2½ on 1
Naggar India 84
Vahar India 84 24 3 on 1 2½ on 1
Rosebery Scotland 84
Atlanta Georgia 82 40
Roddlesworth England 80 16 3 on 1 2½ on 1 68
Gladhouse Scotland 79 12 3 on 1 2½ on 1 68½
Rake England 78 3 on 1 2 on 1
Silsden England 78 3 on 1 2 on 1
Glencourse Scotland 77 3 on 1 58
Leeshaw England 77
Wayoh England 76 22 3 on 1 2½ on 1
Ekruk Tank India 76 20 3 on 1 2 on 1 65
Nehr India 74 8
Middle Branch New York 73
Leeming Ireland 73 10 3 on 1 2 on 1 50
South Fork Penna. 72 20 2 on 1 1½ on 1 50
Anasagur India 70 20 4 on 1
Pangran India 68 8 42
Harlaw Scotland 67 64
Lough Vartry Ireland 66 28 3 on 1 2½ on 1 60
La Mesa California 66 20 1½ on 1 1½ on 1 60
Amsterdam New York 65
Mukti India 65 10 3 on 1 2 on 1 41
Snake River California 64 12 2 on 1 1½ on 1
Stubken Ireland 63 24 3 on 1 2 on 1
Den of Ogil Scotland 60 50
Loganlea Scotland 59 10 3 on 1 2½ on 1 55
Ashti India 58 6 3 on 1 2 on 1 42
Cedar Grove New Jersey 55 18 3 on 1 2 on 1 50
APPENDIX--II.
Works of Reference.
Author. Title. Date.
Baker, Benj. The Actual Lateral Pressure of Earthwork 1881
Baker, Ira O. Treatise on Masonry Construction 1899
Bell, Thos. J. History of the Water Supply of the World 1882
Beloe, Chas. H. Beloe on Reservoirs 1872
Bowie, Aug. J., Jr. A Practical Treatise on Hydraulic Mining 1898
Brant, Wm. J. Scientific Examination of Soils 1892
Brightmore, A. M. The Principles of Water-Works Engineering 1893
Buckley, Robt. B. Irrigation Works in India and Egypt 1893
Cain, Wm. Retaining Walls 1888
Chittenden, H. M. Report and Examination of Reservoir Sites
in Wyoming and Colorado 1898
Courtney, C. F. Masonry Dams 1897
Fanning, J. T. Water-Supply Engineering 1889
Flynn, P. J. Irrigation Canals and Other Irrigation
Works 1892
Frizell, Jos. P. Water Power 1891
Gordon, H. A. Mining and Mining Engineering 1894
Gould, E. S. The Elements of Water-Supply Engineering 1899
Hall, Wm. Ham. Irrigation in California 1888
Hazen, Allen The Filtration of Public Water Supplies 1895
Howe, M. A. Retaining Walls for Earth 1891
Hughes, Saml. Treatise on Water-Works 1856
Jackson, L. D. A. Statistics of Hydraulic Works 1885
Kirkwood, J. P. Filtration of River Waters 1869
Merriman, M. Treatise on Hydraulics, Masonry Dams
and Retaining Walls 1892
Newell, F. H. Irrigation in the United States 1902
Newman, John Earthwork Slips and Subsidences Upon
Public Works 1890
Potter, Thomas Concrete 1894
Schuyler, J. D. Reservoirs for Irrigation, Water Power
and Domestic Water Supply 1901
Slagg, Chas. Water Engineering 1888
Stearns, F. P. Metropolitan Water-Works Reports 1897
Stockbridge, H. E. Rocks and Soils 1888
Trautwine, J. C. Earthwork; and Engineer’s Pocket-Book 1890
Turner, J. H. T. The Principles of Water-Works Engineering 1893
Wilson, J. M. Manual of Irrigation Engineering 1893
Annual Reports.
Massachusetts State Board of Health.
Geological Survey of New Jersey.
Metropolitan Water-Works, Boston and vicinity.
U. S. Geological Survey.
Transactions American Society of Civil Engineers.
Vols. 3, 15, 24, 32, 34 and 35.
Proceedings of the Institution of Civil Engineers.
Vols. 59, 62, 65, 66, 71, 73, 74, 76, 80, 115 and 132.
Engineering News. Vols. 19 to 46.
Engineering Record.
Vols. 23 to 46.
Journal of the Association Engineering Societies.
Vol. 13.
INDEX.
Analyses, soil, Tabeaud Dam, 25
Analyses of soils, 14
Tabeaud Dam, 25
Borings, wash drill, Wachusett Dam, 48
Catchment area, 3
Clay for puddle, 15
Contractors’ outfit, Tabeaud Dam, 31
Core wall, impervious diaphragm as substitute for, 62
necessity for, 44
(See puddle.)
Dam,
Ashti, India, 35
Bog Brook, 41
Bohio, Panama Canal, 54
Croton Valley, slope of saturation in, 40
different types of earth, 33
Druid Lake, Baltimore, 52
high earth, statistical table of, 67
hydraulic-fill, 61
hydraulic-fill, San Leandro, 60
ideal profile of, 42
Isthmian Canal Commission, 54
Lake Frances hydraulic-fill, 61
New Croton, 39
graphical study of original earth portion of, 43
New England, typical section of, 40
new types of, 54
North Dike, Wachusett Reservoir, 48
rock-fill and earth combined, upper Pecos River, 58
safe height of, 39
San Leandro, 58
site location, 7
Tabeaud, 13, 17
Titicus, 41
Upper Pecos River rock-fill and earth, 58
with puddle core wall or face, 33
Yarrow, Liverpool water-works, 9, 33
Diaphragms impervious for earth dams, 62, 65
Dike, north of Wachusett Reservoir (see Dam; also reservoir)
Drainage and slips of earthwork, 45
of dam sites, 63
Drains, bed rock, Tabeaud Dam, 19
Earthwork slips and drainage, 45
Embankment, Ashti, India, 35
Embankments, Jerome Park Reservoir, 45, 46
Factor of safety for dams, 64
Filtration, experiments on nitration through soils
at Wachusett Reservoir, 50
formula, Hazen’s, 56
Foundations, 9, 63
Gravel for puddle, 15
Infiltration and percolation, 38
Isthmian Canal Commission, designs of dams for, 54
Outlet pipes and tunnels, 6
Percolation, 38, 57
Profile, ideal for dams, 42
Puddle, 14
core wall, Ashti Dam, 35
or face, 33
trench, 37
wall, Druid Lake Dam, 53
for Yarrow Dam, 34
vs. puddle face, 37
Reservoir basin, 37
outlets, 6
Wachusett, 48
Rollers for dams, 30, 65
Sands and gravels, flow of water through, 52
(Also see percolation.)
Slips and drainage of earthwork, 45
Soil analyses, Tabeaud Dam, 25
analysis, 14
Soils, experiments on filtration through at Wachusett Reservoir, 50
outline study of, 12
permanence of, 51
selection of, for dams, 64
studies, Wachusett Reservoir, 50
Spillway or wasteway, 8
Tabeaud Dam, 31
Subsidences, earthwork, 45
Test pits, 5, 8, 9
Tunnel, outlet, Tabeaud Dam, 30
Tunnels as outlets to reservoirs, 6
Wasteway or spillway, 8, 66
Tabeaud Dam, 31
FOOTNOTES:
[1] The writer had intended to present a table of physical properties
of different materials, giving their specific gravity, weight,
coefficient of friction, angle of repose, percentage of imbibition,
percentage of voids, etc., but found it impossible to harmonize the
various classifications of materials given by different authorities.
[2] The effective head at any point of an earth dam has been defined
as the difference in the elevation of the high-water surface in the
reservoir and that of the intersection of the down-stream slope with
the natural or restored surface of the ground below the dam.
[3] This work is very fully described in the Annual Reports of
the Metropolitan Water Board of Boston; and by Mr. F. P. Stearns,
Chief Engineer of the Metropolitan Water and Sewerage Board, in the
Proceedings of the American Society of Civil Engineers for April, 1902.
The latter description was reprinted, with the omission of some of the
illustrations, in Engineering News for May 8, 1902.
[4] By effective size of sand grains is meant such size of grain that
10% by weight of the particles are smaller, and 90% larger than itself;
or, to express it a little differently, the effective size is equal
to a sphere the volume of which is greater than ¹/₁₀ that forming the
weight and is less than ⁹/₁₀ that forming the weight.
[5] The term “uniformity coefficient” is used to designate the ratio
of the size of the grain which has 60% of the sample finer than itself
to the size which has 10% finer than itself. The method of determining
the size of sand grains and their uniformity coefficients, is fully
explained in Appendix 3 of Mr. Hazen’s book on “The Filtration of
Public Water Supplies.”
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