The document discusses several important considerations for designing and constructing earth dams, including: 1) Thoroughly investigating the foundation through testing to identify any weak or unstable soils that need removal. 2) Designing appropriate cut-offs and drainage systems depending on the foundation material and reservoir depth, such as trenches, concrete walls, or central clay cores. 3) Carefully designing the upstream and downstream slopes based on available construction materials and the needed stability, drainage, and erosion protection.

1. Test pits. These should show the exact locations of the various materials and the presence and locations of cobbles and boulders. Mechanical analyses and percolation tests should be made for foundation and abutment materials to determine their adequacy or the necessity for their removal.<br /> Preliminary test, consisting of mechanical analysis, percolation, and density at standard compaction, should be run on all available borrow pit materials to determine their suitability and to permit intelligent selection of the most desirable material from the various short-haul areas.<br />The investigations on important dams should include an examination by a competent geologist whose interpretations of the testing result and collaboration in deciding on the adequacy of the foundation are essential.<br />FOUNDATION<br />The adequacy of the foundation materials to provide against shearing, settlement, and excessive percolation which may cause piping at the downstream toe, is of first importance. the weak points in earth-dam construction are generally found in the foundation and at the contacts of the natural ground surface an the placed embankment many of former difficulties with the construction of the embankment proper have been removed by proper laboratory analysis of the embankment material and by improved methods of compaction and moisture control. preliminary testing give some idea of the amount of undesirable material to strip from the foundation area; but this operation should be carefully watched during construction to assure the removal of all material containing vegetable matter and of all material that will be rendered unstable by saturation. Test pits for further explorations should be put down during construction if any doubt exists about the presence of unstable or otherwise unsuitable material.<br />the character of materials in the foundation will dictate the design for the foundation cut-offs. A concrete diaphragm extending from to the crest of the dam is not recommended. Where bedrock is present, concrete walls, bonded 2 to 3 feet into rock and projecting 5 to 10 feet into the fill, will suffice. The number of walls thus constructed, will vary from one to three, depending on geologic conditions and the maximum depth of the water. Where bedrock is not within economical reach, open cut-off trenches should be used. These will vary in bottom width and depth, depending on the reservoir water depth and porosity of the material. Ordinarily, trenches about onethird of the water depth are adequate in most of the western river bed alluviums. Where earth dams are founded on bedrock with comparatively thin layers of the gravel overlying, at least three-fourths of the area should be stripped to bedrock to prevent a concentrated flow of the seepage water at high velocity through the gravel. Such dams should have.<br />a considerable length of impervious material bonded carefully to clean bedrock.<br />Dams founded on rather porous and deep alluvium should have a central clay core brought the dam from the cut-off trench ; also a long percolation distance through the base with an adequate rock-fill blanket extending downstream from the toe of the earth-fill section. Where cut-off trenches are in sand and gravel, with voids filled with fine sand and silt, it is generally preferable not to unwater the trench but to puddle a good grave of watertight clay back into the trench. Unwatering tends to wash out all the fines and render the foundation more porous.<br />Presence of river silt, sandy clay, or fine sand demands careful consideration in foundation design. Such materials should ordinarily stripped to gravel. This is generally expensive and may greatly affect the economic feasibility of the dam.This item very often is grossly underestimated in preparing the preliminary estimates. a decision to leave such material in place can only be made after careful settlement, consolidation, tests have been conducted. If it is feasible entirely to drain the foundation material before and during the placement of embankment, about 60 percent of the final settlement can be attained by the time the embankment has been raised to a height of about 20 feet. Special attention. must be given to the abutment contact and the existing overburden and the composition of the underlying rock should be carefully studied. Porous and unstable material must be removed, sometimes to considerable depth. When the cut-off trenches can be extended to rock, a concrete cut-off should be constructed, bonded into sound bedrock and the bedrock grouted along the line of the cut-off to form a continuous grout curtain. The depth of grout holes will depend upon the nature of the bedrock formation and the head of water to be brought against the abutment.<br />Abutments in earth material, such as cut banks, should be sloped not steeper than 1 1/2: 1, in order that effective bonding may be accomplished by the roller. All rock overhangs must be removed and the resultants slope brought to 1/4:1 if feasible. Seepage along rock abutments can be prevented by one or more well-founded concrete cut-off walls. Dowel bars of large diameter well bonded into the rock are often used to tie the wall to the abutment rock.<br />DESIGN<br />Under the latest methods of testing and earth placing control, a fair prediction can be made of the saturation slope through the embankment. Checks on the assumptions may be made by the electrical analogy and model test methods. The saturation slope should be designed so as to fall within the downstream toe with only a moderate amount of seepage.<br />Hydrostatic pressure cells are now being installed in all bureau of reclamation earth dams to indicate any approaching<br />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.<br />the embankment material should be designed and placed so as to prevent water reaching the downstream end under any remaining pressure or in dangerous quantity.<br />Adequate cut-offs should be provided to prevent water from passing through the foundation material with sufficient velocity to move fine particles of foundations 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.<br />The existence of fine material of low shear value along <br />the downstream toe often present difficult problem in dam design. This material should be removed to a solid foundation and deep drains provided downstream from the toe.<br />The designed slopes of the dam many 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.<br />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.<br />Where only fine materials of low shear values is available,<br />it is necessary to construct a very flat upstream slope. If a free-draining material, such as sand and gravel, is available to hold dows 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 1/2: 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.<br />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.<br />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 to be used both for erosion protection and weight, the outer rock slope is generally designed as 2:1 or 2 1/2 :1; and the underlying earth slope, not steeper than 1 1/2:1. A rock blanket downstream from the earth section may be constructed on slopes of 4:1 to 8:1, to extend the percolation distance. These designed percolation distance should fall within the limits of the blanket.<br />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 widht may vary from 15 feet for small dams to 35 feet for largo dams. The width should take into consideration the amount of freeboard and the construction details requiered 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 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 the high dam. each case, however, deserves special attention taking into consideration the fetch length of the reservoir, height of the dam, and the climatic conditions.<br />toe drains are of no value in earth dams founded on rock;<br />