SOLID VS. HOLLOW DAMS. The preceding discussion relates solely to a solid dam, in which for economy the cross section is as narrow as consistent with stability, and the batter of the up-stream face is relatively quite small. In this form of dam, the horizontal thrust of the water is re sisted chiefly or wholly by the weight of the masonry, i.e., the vertical component If the water pressure is an unimportant factor of the stability. Such a dam might with propriety be called an upright dam.
Since the invention of reinforced concrete, a new type of dam has been in troduced, viz.: a hollow dam having a very broad base and a relatively great batter on the up-stream face. In this form, the vertical component of the pressure of the water on the back of the dam is an important factor of the stabil ity. Such a dam could with propriety be called an inclined dam, and is with less propriety often called a pressure dam.
The inclined dam consists of a series of vertical abutments spaced 10 to 15 feet apart, covered on the up-stream side with a continuous deck. If the dam is submerged, there may or may not
be a rollway on the down-stream side. The abutments may be braced by horizontal struts between them; and sometimes inter mediate floors are laid between the buttresses, electric machinery, etc., being placed in the compartments thus formed. Fig. 106 shows the cross section of a hollow reinforced-concrete dam. The advantages of this form, which is patented, are: (1) being hollow it requires only about 40 per cent as much concrete as a solid dam; (2) being light it gives a less pressure upon the foundation; and (3) on account of the great slope of the up-stream face, it gives a nearly uniform pressure on the foundation.
For the complete analysis of the stresses in such a dam, see Engineering News, Vol. Lux, pages 452-53.