4.2 TYPES OF BEARING CAPACITY FAILURE

Figure 4.1a shows a shallow foundation of width B located at a depth D _f and supported by a dense sand [or stiff clayey soil) extending to a great depth (relative to B). If this foundation is subjected to a load Q and the load is gradually increased, the load per unit area, q = Q/ A ( A = area of the foundation), will increase and the foundation will undergo increasing settlement. When q becomes equal to q _u at foundation settlement S = S _u, the soil supporting the foundation will undergo sudden shear failure. The failure surface in the soil is shown in Fig. 4.1a along with the q vs. S plot in Fig. 4.1b, This type of failure is referred to as general shear failure, and q _u is called the ultimate bearing capacity. Note that in this type of failure a peak value q = q _u is clearly noted.

Figure 4.1: General shear failure in soil.

If the same foundation is now supported by a medium sand or clayey soil (Fig. 4.2a), the plot of q vs. S will be as shown in Fig. 4.2b. Note that the magnitude of q increases with settlement up to q = q _u(1), which is usually referred to as the first failure load (Vesic, 1963). At...