At ultimate load per unit area the failure surface in soil will be as shown in the
figure. If the depth H is relatively small compared with the foundation width B, a punching
shear failure will occur in the top soil layer, followed by a general shear failure in the
bottom soil layer. This is shown in Figure 4.8(a). However, if the depth H is relatively
large, then the failure surface will be completely located in the top soil layer, which is the
upper limit for the ultimate bearing capacity. This is shown in Figure 4.8b.
The ultimate bearing capacity for this problem, as shown in Figure 4.8a, can be
given as