7.1 INTRODUCTION

Limit analysis is applied in this Chapter to obtain upper and lower bounds for the average bearing capacity pressure q ₀ of square, rectangular and circular footings on a purely cohesive soil ( ? = 0). The individual problems connected with this application will be taken up in the following sequence. First we consider the bearing capacity of surface footings on a semi-infinite mass of soil (sections 7.2 and 7.3). Then we solve in succession the following two problems: (1) computation of the bearing capacity of footings on a finite block (sections 7.4 and 7.5); (2) computation of the bearing capacity of surface footings on a semi-infinite layer of soil resting on a rough rigid base (sections 7.6 and 7.7). When dealing with these problems the contribution of soil weight on the bearing capacity of footings will be disregarded in most cases. The influence of soil weight and the case ? > 0 on the bearing capacity of circular footing will be discussed in section 7.8.

In the following work, we are considering the bearing capacity of a perfectly plastic soil loaded normally and centrally by a flat-ended rigid footing. Except in section 7.8, the soil is assumed to be an ideally cohesive medium for which ? = 0 (Tresca yield condition). This condition approximates to the behavior of a friction-less cohesive soil such as clay. It is further assumed that the force on the footing is increased until penetration occurs as a...