MASONRY DAMS Dams are employed to hold back water for municipal supply, for power, and for irrigation; and are made of earth, wood, steel, loose rock, or masonry. Only masonry dams will be considered here; and the discussion will be limited to questions that relate to the stability of the structure, without including gate chambers, waste weir or spillway, roll-ways, scouring sluices, regulators, fish ladders, etc., which are discussed in treatises on water supply, power development, and irrigation. The fundamental principles of stability that are to be considered apply also to retaining walls, bridge abut ments, bridge piers, and arches. The discussions of this chapter are applicable to masonry dams, reservoir walls, or to any wall which counteracts the pressure of water mainly by its weight.
There are two ways in which a masonry dam may resist the thrust of the water, viz.: (1) by the inertia of its masonry, and (2) as an arch. 1. The horizontal thrust of the water may be held
in equilibrium by the resistance of the masonry to sliding forward or to overturning. A dam which acts in this way is called a gravity dam. 2. The thrust of the water may be resisted by being trans mitted laterally to the side-hills (abutments) by the arch-like action of the masonry. A dam which acts in this way is called an arched dam.
Masonry dams of the gravity type are quite common, but only three dams of the pure arch type have ever been built. The almost exclusive use of the gravity type is due to the uncertainty of our knowledge concerning the laws governing the stability of masonry arches. This chapter will be devoted mainly to gravity dams, those of the arch type being considered only incidentally. Arches will be discussed in Chapters