4.11 FOUNDATIONS SUPPORTED BY A SOIL WITH A RIGID ROUGH BASE AT A LIMITED DEPTH

The problem of bearing capacity of shallow rough foundations discussed in the preceding sections assume that the soil is homogeneous and extends to a great depth. However, it is possible that a rigid rough base is located at a shallow depth below the bottom of the foundation. In that case, the ultimate bearing capacity of the foundation will be altered to some extent. The problem can be explained more clearly by referring to Fig. 4.46. Figure 4.46a shows a shallow rough continuous foundation supported by a soil which extends to a great depth. The ultimate bearing capacity of this foundation can be expressed (neglecting the depth factors) as

Figure 4.46: Failure surface under a rough continuous foundation: (a) Homogeneous soil extending to a great depth; (b) With a rough rigid base located at a shallow depth.

The procedure for determination of the bearing capacity factors N _c, N _q, and N _? in homogeneous and isotropic soils has been outlined in Sections 4.3, 4.5, and 4.7. The extent of the failure zone in soil at ultimate load q _u is equal to D ?. The magnitude of D ? as obtained during the evaluation of the bearing capacity factors N _c (Prandtl, 1921) and N _q (Reissner, 1924) is given in a non-dimensional form in Fig. 4.47. Similarly the magnitude of D ? obtained during the...