The type and size of dam constructed depends on the need for and the amount of water available, the topography and geology of the site, and the construction materials that are readily obtainable. Dams can be divided into two major categories according to the type of material with which they are constructed, namely, concrete dams and earth dams. The former category can be subdivided into gravity, arch and buttress dams, whereas rolled fill and rockfill embankments comprise the other. As far as dam construction is concerned, safety must be the primary concern, this coming before cost. Safety requires that the foundations and abutments be adequate for the type of dam selected.

A gravity dam is a rigid monolithic structure that is usually straight in plan, although sometimes it may be slightly curved. Its cross section is roughly trapezoidal. Generally, gravity dams can tolerate only the smallest differential movements, and their resistance to dislocation by the hydrostatic pressure of the reservoir water is due to their own weight. A favourable site is usually one in a constricted area of a valley where sound bedrock is reasonably close to the surface, both in the floor and abutments.

An arch dam consists of a concrete wall, of high-strength concrete, curved in plan, with its convex face pointing upstream (Fig. 9.19). Arch dams are relatively thin walled and lighter in weight than gravity dams. They stand up to large deflections in the foundation rock, provided...