_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.