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Oriqinal qround surface .
TYPICAL SECTION FOR DAMS FOUNDED ON RIVER BOTTOM MATERIAL
ing dangerous conditions as the reservoir is filled and emptied during the first few years of operation. A record is being kept of the pressure head at some 60 to 80 points in each dam. This will eventually lead to very valuable information on the design and construction of earth dams.
The embankment material should be designed and placed so as to prevent water from reaching the downstream end under any remaining pressure or in dangerous quantity.
Adequate cut-offs should be provided to prevent water from passing through the foundation material with sufficient velocity to move fine particles of foundation material at the downstream toe. In the absence of bedrock, a long percolation distance from the upstream to the downstream toe must be provided. In some cases where a sand and gravel foundation exists this distance has been taken as 10 to 12 times the depth of water in the reservoir.
The existence of fine material of low shear value along
the downstream toe often presents a difficult problem in dam design. This material should be removed to a solid foundation and deep drains provided downstream from the toe.
The designed slopes of the dam may vary widely, depending on the embankment materials available. It is imperative that the designer have the results of all laboratory tests on embankment materials and that he satisfy himself that all borrow pits within economical hauling distance have been thoroughly investigated to determine the nature of each material and the available quantities from each pit.
The upstream slope is designed with sufficient shear value to withstand the pressure exerted by the hydrostatic head in the dam as the water in the reservoir is drawn down. The rate of draw-down, therefore, has much to do with the safe design of upstream slopes.
Where only fine material of low shear values is available, it is necessary to construct a very flat upstream slope. If a free-draining material, such as sand and gravel, is available to hold down the fine material, a great saving in cost can be made. This may also be accomplished where an excess of rock exists in required excavation. However, in this case a filter of sand and gravel, or fines from the rock quarry, is essential between the fine and coarse material to prevent washing out the fines by saturation and wave action. Upstream slopes may vary from 2^:1 to as flat as 8:1 depending on available materials. The percolation or creep distance may be greatly increased by designing a flat upstream toe blanket on slopes of 8:1 or 6:1. These slopes will require no rock riprap near the bottom of the reservoir. Thus the longer percolation distance can be obtained with a large saving in the cost of riprap or concrete facing.
The material to be used for paving on the upstream slope is again dependent on the materials available. Where rock can be obtained from required excavation, quarries, or cobble pits, a facing of dumped rock of about 3 feet for large dams and 2 feet'for small dams is the most economical. A concrete facing is expensive and is generally used only where dumped riprap is not feasible. A long haul on rock can generally be justified in lieu of concrete.
The downstream slope of the dam will depend to some extent on the percolation distance; but is generally established within closer limits than the upstream slope. If rock is to be used both for erosion protection and weight, the outer rock slope is generally designed as 2:1 or 2%'A; and the underlying earth slope, not steeper than 1 %: 1. A rock blanket downstream from the earth section may be constructed on slopes of 4:1 to 8:1, to extend the percolation distance. The designed percolation distance should fall within the limits of the blanket.
The crest widths of small earth dams depend on the steepness of the outer slopes and the desired distance through the embankment at the maximum water surface in the reservoir. The width may vary from 15 feet for small dams to 35 feet for large dams. The width should take into consideration the amount of freeboard and the construction details required in securing the freeboard. It has been found economical to secure 3 feet of freeboard by the use of a concrete parapet wall for all dams over 20 feet high (see fig. 1). In the case of a dam having a 3:1 upstream slope and a 2:1 downstream slope, the parapet would account for 15 feet of crest width if the crest were not increased to take care of this detail. The criterion should therefore be the desired distance through the dam on the water line rather than the elevation where the earth crest is terminated. The freeboard distance varies from 5 feet on small dams to 11 feet on high dams. Each case, however, deserves special attention taking into consideration the fetch length of the reservoir, height of the dam, and the climatic conditions.
Toe drains are of no value in earth dams founded on rock;
but are common practice for dams founded on earth, sand and gravel, and river fill, since they prevent saturation and the resultant lowering of the shear value of the material at the downstream toe. The drains should not be placed far into the dam as this reduces the percolation distance and the seepage water may reach the tile under too great a velocity, thus piping fines from the foundation into the drain tile.
Field laboratories for testing foundation and embankment materials and concrete aggregates should be established on all important jobs. If a field laboratory cannot be justified, the materials should be sent to a central or commercial laboratory for investigation. Tests of materials for use in embankments should include mechanical analyses, percolation, solubility, shear values, consolidation and settlement, and determinations of optimum moisture and density at standard laboratory compaction.
Much can be accomplished in economical and safe earth dam design by having a thorough knowledge of all available materials based on complete laboratory tests. Borrow pit material of a variable nature can be selected, mixed, and distributed into the dam, so as to provide the desired qualities of watertightness and weight against subsidence. The matter of prospecting and thoroughly testing the materials to be used, in advance of design, cannot be overstressed. The extent to which this should be carried varies with the complication of natural conditions at the dam site and the size and importance of the structure.
At some localities the materials available for embankment may offer little choice in their selection, in which case only a homogeneous section can be designed. In other instances the borrow pits vary widely in character between the various pits and by stratification in the same pit. The latter condition calls for a large number of tests of composite samples to determine the properties of the various mixes and the depth to which shovel cuts are to be made. In the case of clay, silty clay, or sandy clay, overlying sand and gravel, the top layer can be mixed in certain proportions of the under layer to form an impervious, semi-impervious, or porous embankment.
Under a scheme of zoning the dam into three or five sections as shown on the accompanying drawing, the pit should be carefully tested at rather short-distance coordinates. The travel of the shovel in the pit can then be directed to supply the required quantities of the different grades of material as the fill progresses. The impervious section of dam formed by the central core is composed of about 80 percent overlying clay and 20 percent gravel, the semi-impervious section about 60 percent overlying clay and 40 percent gravel, and the outer porous sections of a small amount of overlying clay and the balance gravel. This forms a watertight and stable dam with proper use of a variety of different materials.
By careful design of the dam section to approximate the various grades of material available in the borrow pits, a satisfactory embankment can be constructed with very little hardship on the construction force. The fill material should be placed in layers not exceeding 6 inches in thickness and should be compacted by rollers. The rollers should be of an approved sheepsfoot type, equipped with suitable cleaners, and should have approximately 1 foot or knob for each square foot of developed area as determined by the bearing surfaces of the knobs. The face area of each foot or knob should be not less than 6 square inches; the length of each foot or knob should be not less than 7 inches nor more than 8J4 inches; and the total weight of each drum divided by the total area of the maximum number of feet or knobs in one row parallel to the axis of the drum should be about 340 pounds per square inch. Not less than four knobs should be used in computing the unit pressure on the knobs. A roller of this type can be purchased through several different manufacturers. In ordinary fill material, 12 passes of the roller will give a compaction comparable to standard laboratory density with negligible final settlement in the fill, even for dams 100 feet high.
After the placement of about every fourth layer, the entire surface of the fill should be leveled with either a road blade or a carry-all scraper. This is sometimes accomplished by running the bulldozer in a backward direction, dragging the dozer blade; but this does not do as well as the road machine blade. Around concrete structures and along rock abutments where the sheepsfoot roller cannot gain access, a compressed air or a gasoline-powered tamper should be operated. These will give the required densities provided thinner layers are deposited.
The application of the optimum amount of water on the fill is often difficult to accomplish, especially where the soils have a high clay content. The most satisfactory method, where feasible, is to irrigate the material in the borrow pit by pumping into small areas surrounded by low dikes. After the existing moisture content is determined in the laboratory, the supply of any deficiency can be accurately controlled by the use of a water meter. This should be done sufficiently ahead of the excavation to allow the moisture to soak in, which may require from 5 days to 4 weeks, depending upon the grading of the material. Water can also be added to the borrow pit material by large sprinklers called "rain makers." Borrow pits located on side hills or knolls are hard to pre-wet and about the only alternative is to add the moisture on the fill. This is generally unsatisfactory and difficult to accomplish. When sprinkling on the embankment is necessary, a more uniform distribution of the moisture can be obtained by initially sprinkling the cut face of the excavation.
Many ingenious devices have been made to accomplish wetting on the fill with varying degrees of success. Low densities can often be attributed to the nonuniform distribution of moisture in the fill.
Specifications requiring 6-inch fill layers generally stipulate that all rock greater than 5 inches in thickness shall be removed from the fill. If a large percent of coarse rock exists, the roller and knobs will contact the rock or cobbles and jump, leaving a space uncompacted. Densities are thus difficult to secure. When coarse materials predominate in the borrow pit, it will be found economical to install a revolving trommel to screen out all material above 2% inches. This will produce a well-mixed material under 2}i inches for the impervious embankment area and produce more rock for the downstream slope which is always more expensive to produce and place than the embankment material. This procedure will often make a saving on rock that will pay for the process of screening.
Porous sections containing gravel, sand, and cobbles are generally impossible to compact by the sheepsfoot roller and greater densities may be secured in such sections by sluicing with a stream of water.
The first requisite of spillway design is the determination of the maximum possible flood for which provision must be made. This maximum flood flow should be determined from rainfall and flood-flow records when available. If such records are not available, adequate assumptions must be made in lieu thereof. More earth-dam failures are caused by overtopping because of inadequate spillway capacity than are caused by any other deficiency in design. The cost of spillways for earth dams generally represents a large proportion of the total cost of the structure.
Spillways through the rim of the reservoir some distance from the dam are preferable where local conditions favor such construction; but since such sites are rare the usual location is at one abutment of the dam.
The proper design will depend upon the required capacity, and the topographic and geologic conditions at the abutment. The most common spillway for small dams with small flood flows is an overflow crest leading into a concrete-lined channel. With higher dams and greater flood flows, radial gates are installed to bypass the greater discharges. The side channel overflow type is adaptable to some locations where favorable topography exists. This type of spillway is safer as there are no gates to operate; but requires additional freeboard on the dam to permit the increased elevation of the reservoir water surface required at the lip of the spillway intake. This constitutes an expense affecting the whole structure.
Spillway openings on the abutment generally create a weak point in the dam by reducing the designed percolation distance. Deep cut-offs should be provided and the possibility of percolating water entering around the structure should be carefully studied.
Spillway channels on natural soil should be carefully drained under the floor and side walls. Channels in rock should have one or more grout curtains at the intake and the balance of the wasteway should be drained by installing vitrified tile pipe in gravel-filled trenches.
Adequate concrete-lined stilling basins should be provided at the outlets of all open channel spillways, whether founded on earth or rock to dissipate the energy of flow at the foot of the incline. The design should insure that the hydraulic jump will occur within the stilling basin, and that the outflow therefrom will be at a relatively low velocity. The stilling basin can often be designed to serve both the spillway and outlet conduit.
Irrigation outlets for most small dams are designed as concrete boxes, with cut-off collars located through the earth embankments. Such outlets should be located where the foundation material offers the best possible support. Metal pipes, unless encased in concrete, should not be used. The
backfill around the conduits should be carefully placed by power tampers, producing the same density as in the surrounding earth embankment. The conduit barrel should be designed sufficiently strong to prevent cracking and the resultant entrance of water into the fill.
Control gates for the regulation of outflow through conduits should be placed as near the upstream toe of the dam as possible, to avoid having the conduits under pressure. For larger dams the outlet works should be located through the abutment in a concrete-lined tunnel, preferably in solid rock. It is generally permissible in the latter case to install the operating gates in a gate chamber about midway of the length of the tunnel, allowing the upper end to be under hydrostatic pressure. This construction requires that the rock surrounding the upper section of the tunnel be carefully pressure-grouted to prohibit any leakage from reaching and entering the embankment.
The installation of slide gates, in tandem, which discharge directly into the lower section of the tunnel, is ordinarily permissible under heads up to 90 feet. For heads in excess of 90 feet the normal installation consists of a steel outlet pipe or pipes placed in the lower section of the tunnel provided with a single slide gate at the gate chamber end, and a needle valve at the outlet end.
DAMS CONSTRUCTED BY
Upper Snake River
Sun River..- --
Truckee River Storage
See footnotes at enil of table.