(2) Seepage through the embankment, foundation, and abutments must be collected and controlled to prevent excessive uplift pressures, piping, sloughing, removal of material by solution, or erosion of material by loss into cracks, joints, and cavities. In addition, the purpose of the project may impose a limitation on the allowable quantity of seepage. The design should consider seepage control measures such as foundation cutoffs, adequate and nonbrittle impervious zones, transition zones, drainage blankets, upstream impervious blankets, and relief wells.
(3) Freeboard must be sufficient to prevent overtopping by waves and include an allowance for the normal settlement of the foundation and embankment as well as for seismic effects where applicable.
(4) Spillway and outlet capacity must be sufficient to prevent overtopping of the embankment.
b. Special attention. Special attention should be given to possible development of pore pressures in foundations, particularly in stratified compressible materials, including varved clays. High pore pressures may be induced in the foundation, beyond the toes of the embankment where the weight of the dam produces little or no vertical loading. Thus, the strengths of foundation soils outside of the embankment may drop below their original in situ shear strengths. When this type of foundation condition exists, instrumentation should be installed during construction .
2-5. Selection of Embankment Type
a. General. Site conditions that may lead to selection of an earth or a rock-fill dam rather than a concrete dam (or roller-compacted concrete dam) include a wide stream valley, lack of firm rock abutments, considerable depths of soil overlying bedrock, poor quality bedrock from a structural point of view, availability of sufficient quantities of suitable soils or rock fill, and existence of a good site for a spillway of sufficient capacity.
b. Topography. Topography, to a large measure, dictates the first choice of type of dam. A narrow
V-shaped valley with sound rock in abutments would favor an arch dam. A relatively narrow valley with high, rocky walls would suggest a rock fill or concrete dam (or roller-compacted concrete). Conversely, a wide valley with deep overburden would suggest an earth dam. Irregular valleys might suggest a composite structure, partly earth and partly concrete. Composite sections might also be used to provide a concrete spillway while the rest of the dam is constructed as an embankment section. The possibility of cracking resulting from arching in narrow valleys and shear cracks in the vicinity of steep abutments must be investigated and may play a role in the selection of the type of dam. At Mud Mountain Dam, arching of the soil core material within a narrow, steep-sided canyon reduced stresses making the soil susceptible to hydraulic fracturing, cracking, and piping. Haul roads into narrow valleys may be prohibited for safety and/or environmental reasons. At Abiquiu and Warm Springs Dams, borrow material was transported by a belt conveyor system. Topography may also influence the selection of appurtenant structures. Natural saddles may provide a spillway location. If the reservoir rim is high and unbroken, a chute or tunnel spillway
may be necessary.
c. Geology and foundation conditions. The geology and foundation conditions at the damsite may dictate the type of dam suitable for that site. Competent rock foundations with relatively high shear strength and resistance to erosion and percolation offer few restrictions as to the type of dam that can be built at the site. Gravel foundations, if well compacted, are suitable for earth or rock-fill dams. Special precautions must be taken to provide adequate seepage control and/or effective water cutoffs or seals. Also, the liquefaction potential of gravel foundations should be investigated. Silt or fine sand foundations can be used for low concrete and earth dams but are not suitable for rock-fill dams. The main problems include settlement, prevention of piping, excessive percolation losses, and protection of the foundation at the downstream embankment toe from erosion. Nondispersive clay foundations may be used for earth dams but require flat embankment slopes because of relatively low foundation shear strength. Because of the requirement for flatter slopes and the tendency for large settlements, clay foundations are generally not suitable for concrete (or roller-compacted concrete) or rock-fill dams .
d. Materials available. The most economical type of dam will often be one for which materials can be found within a reasonable haul distance from the site, including material which must be excavated for the dam foundation, spillway, outlet works, powerhouses, and other appurtenant structures. Materials which may be available near or on the damsite include soils for embankments, rock for embankments and riprap, and concrete aggregate (sand, gravel, and crushed stone). Materials from required excavations may be stockpiled for later use. However, greater savings will result if construction scheduling allows direct use of required excavations. If suitable soils for an earth-fill dam can be found in nearby borrow pits, an earth dam may prove to be more economical. The availability of suitable rock may favor a rock-fill dam. The availability of suitable sand and gravel for concrete at a reasonable cost locally or onsite is favorable to use for a concrete (or roller-compacted concrete) dam.
e. Spillway. The size, type, and restrictions on location of the spillway are often controlling factors in the choice of the type of dam. When a large spillway is to be constructed, it may be desirable to combine the spillway and dam into one structure, indicating a concrete overflow dam. In some cases where required excavation from the spillway channel can be utilized in the dam embankment, an earth or rock-fill dam may be advantageous.
f. Environmental. Recently environmental considerations have become very important in the design of dams and can have a major influence on the type of dam selected. The principal influence of environmental concerns on selection of a specific type of dam is the need to consider protection of the environment, which can affect the type of dam, its dimensions, and location of the spillway and appurtenant facilities .
g. Economic. The final selection of the type of dam should be made only after careful analysis and comparison of possible alternatives, and after thorough economic analyses that include costs of spillway, power and control structures, and foundation
treatment.
2-6. Environmental Considerations
Public Law 91-190, National Environmental Policy Act of 1969, as amended, and the Clean Water Act of 1977 established the national policy for promoting efforts that will prevent or mitigate damage to the Nation’s rivers and to the environment. The goal is to achieve clean and healthy watersheds that support aquatic life, economic development, and human needs. Managing water resources in a river basin has an impact on its natural water cycle. The scale of the impact depends on the actual size and natural condition of the area to be developed and the extent of development. Mitigation measures are essential elements in the planning, design, construction, and operation of a project, including clearing of vegetation in the area to be flooded, multilevel outlet structures to optimize downstream water temperature and quality, provisions for the migration of fish and other aquatic organisms, and operational rules for regulating downstream flows at critical times to protect habitat for reproduction or migratory routes. Appropriate site selection, together with the implementation of these techniques, will result in both new and rehabilitated projects that minimize unacceptable environmental impacts. Environmental conservation includes mitigation and enhancement for new projects, maintaining the existing conditions and restoration where appropriate.