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.
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.FOUNDATION
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 theembankment 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 ofall 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.
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, dependingon 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 velocitythrough the gravel. Such dams should have.
a considerable length of impervious material bonded carefully to clean bedrock.
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. Wherecut-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.
Presence of river silt, sandy clay, or fine sand demands careful consideration in foundation design. Suchmaterials 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...