Concrete Gravity and Gravity Arch Dams

A concrete gravity dam has a cross-section such that, with a flat bottom, the dam is free-standing; that is, the dam has a center of gravity low enough that the dam will not topple if unsupported at the abutments. Gravity dams require maximum amounts of concrete for their construction as compared with other types of concrete dams, and resist dislocation by the hydrostatic pressure of reservoir water by sheer weight. Concrete gravity dams have been constructed up to 285 m (950 ft)high. Properly constructed gravity dams with adequate foundation probably are among the safest of all dams and least susceptible to failure with time. They withstand the pressure or push of water by their weight. In cross section, they are like a triangle, broad at the base and narrow at the crest. They are built in this shape because water pressure becomes greater with the depth of water. A typical gravity dam is shown in figure 1.3.

Final selection of the site for a gravity or gravity-arch dam is made only after comprehensive investigation of hydrologic, topographic, and, especially,

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Figure 1.3: Cross-section of typical concrete gravity dam subsurface geologic conditions. A favorable site usually is one in a constriction in a valley where sound bedrock is reasonably close to the surface both in the floor and abutments of the dam.

An important consideration in construction of a concrete gravity or gravity-arch dam is the availability, within a reasonable hauling distance, of adequate deposit of aggregate suitable for manufacture of concrete, whether the aggregate is obtained from unconsolidated deposits or is quarried.

The simplest form of a gravity dam is one in which the top or crest is straight. depending on the topographic configuration of a valley and the foundation geology, it may be possible to construct a gravity-arch dam which incorporates the advantages of mass weight and low center of gravity of a gravity dam with those inherent in an arch dam. in gravity-arch dams the requirements for sound rock in abutments are somewhat more stringent than in simple gravity dams.

Figure 1.4: Cross-section of a concrete buttress dam 1.1.4 Concrete Slab and Buttress Dams

In locations where aggregate for concrete or for earthfill is in limited supply and the foundation rocks are moderately to highly competent, buttress dams provide a possible alternative to other kinds of dams. They are up to 130 m (430 ft) high. In cross-section, buttress dams resemble gravity dams, but with flatter upstream slopes. In a buttress dam a slab of reinforced concrete upstream rest on a succession of upright buttress which have thicknesses and a spacing sufficient to support the concrete slab and the load of the water in the reservoir that exert on the slab. A cross-section of a typical buttress dam is shown in figure 1.4

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