Ch. 8] APPRAISAL OF FOUNDATION 149 



ically rendered relatively impermeable. All foundations must be in- 

 soluble during the lifetime of the project. These requirements apply- 

 ing to the rock mass upon which the structure is founded or abutted 

 are stated in greater detail by Houk and Keener (1941, p. 1116). 



Consider a dam. The foundation rock is under compressive forces 

 occasioned by the mass of the structure. Shearing stresses are im- 

 posed upon the foundation by the hydrostatic head acting horizontally 

 on the upstream side of the dam. This horizontal stress may be con- 

 sidered to produce a tendency to slide either on the surface of the 

 foundation rock or on a plane of weakness at some depth within the 

 foundation rock. A combination of shearing and sliding is usually 

 considered in the design of gravity dams. The impermeability of the 

 foundation is a necessity if the dam is to hold water, and yet the en- 

 tire foundation from the upstream side of the dam to the toe need not 

 be impermeable. Ideally it would be desirable if the foundation at 

 the heel were essentially impermeable and downstream therefrom were 

 permeable. This relationship would reduce to a minimum the area 

 of the base of the dam on which full reservoir head could be exerted 

 and correspondingly increase the base area subjected only to tail- 

 water uplift. This type of foundation would permit a theoretically 

 thinner section with consequent reduction in concrete and cost. The 

 conditions of loading of a dam vary with the variations in the eleva- 

 tion of the reservoir, and the foundation rock must be sufficiently 

 elastic to withstand these changes. 



Consider a bridge pier or column footing. The foundation rock is 

 under compressive stress, but with the flow of traffic and wind load 

 on the bridge shearing stresses are developed in the foundation. How- 

 ever, it need not be impermeable, but it should be homogeneous and 

 must be competent to support the load. 



Consider a retaining wall. The foundation is subjected to the same 

 stresses as the foundation for a dam, although the variation in stresses 

 is greatly reduced. As in the case of a bridge pier, it need not be 

 impermeable but it must be competent to support the load, and it is 

 better if homogeneous. 



Appraisal of Foundation 



There are three methods of approaching the problem of determin- 

 ing safe values in bearing, shearing, sliding friction, elasticity, and 

 permeability of a foundation: (1) experience and general practice; 

 (2) assumption of an idealized homogeneous mass, the properties of 

 which can be determined from extrapolation of the properties of rep- 

 resentative samples; (3) recognition of the heterogeneity of the mass. 



