A failure surface for circular footings on cohesive soils

The results of three-dimensional finite element analyses of circular footings under combinations of vertical load, horizontal load and moment are presented in this paper. The soil is assumed to be uniform and homogeneous and deforms under undrained conditions. These studies include the performance of the foundations under large overturning moments and large horizontal loads. The contact between the soil and the rigid footing is unable to sustain tension and therefore allowance is made for breakaway under large overturning moments. The results of the analyses are presented in the form of a failure locus in the vertical, horizontal and moment loading space. A comparison is made between the results of the current analysis and a simple method commonly used in calculation of the bearing capacity of foundations. It is shown that, while the hypothesis of an effective footing width provides a good approximation to the effects of load eccentricity, the conventional method overestimates the effects of load inclination for load combinations with a large horizontal shear component. A new factor is presented in this paper that can be used to describe the effects of load inclination more accurately when used with the simple method to calculate bearing capacity.