a. Earth-fill materials.
(2) If a fine-grained soil can be brought readily within the range of water contents suitable for compaction and for operation of construction equipment, it can be used for embankment construction. Some slow-drying impervious soils may be unusable as embankment fill because of excessive moisture, and the reduction of moisture content would be impracticable in some climatic areas because of anticipated rainfall during construction. In other cases, soils may require additional water to approach optimum water content for compaction. Even ponding or sprinkling in borrow areas may be necessary. The use of fine-grained soils having high water contents may cause high porewater pressures to develop in the embankment under its own weight. Moisture penetration into dry hard borrow material can be aided by ripping or plowing prior to sprinkling or ponding operations.
(3) As it is generally difficult to reduce substantially the water content of impervious soils, borrow areas containing impervious soils more than about 2 to 5 percent wet of optimum water content (depending upon their plasticity characteristics) may be difficult to use in an embankment. However, this depends upon local climatic conditions and the size and layout of the work, and must be assessed for each project on an individual basis. The cost of using drier material requiring a longer haul should be compared with the cost of using wetter materials and flatter slopes. Other factors being equal, and if a choice is possible, soils having a wide range of grain sizes (well-graded) are preferable to soils having relatively uniform particle sizes, since the former usually are stronger, less susceptible to piping, erosion, and liquefaction, and less compressible. Cobbles and boulders in soils may add to the cost of construction since stone with maximum dimensions greater than the thickness of the compacted layer must be removed to permit proper compaction. Embankment soils that undergo considerable shrinkage upon drying should be protected by adequate thicknesses of non-shrinking fine-grained soils to reduce evaporation. Clay soils should not be used as backfill in contact with concrete or masonry structures, except in the impervious zone of an embankment.
(4) Most earth materials suitable for the impervious zone of an earth dam are also suitable for the impervious zone of a rock-fill dam. When water loss must be kept to a minimum (i.e., when the reservoir is used for long-term storage), and fine-grained material is in short supply, resulting in a thin zone, the material used in the core should have a low permeability. Where seepage loss is less important, as in some flood control dams not used for storage, less impervious material may be used in the impervious zone