The behaviour of skirted strip footings and circular footings subject to combined vertical, horizontal and moment loading has been studied using finite element and plasticity analysis of equivalent surface foundations. The shape of the yield locus for the two foundation geometries was found to be similar but the pure vertical, moment and horizontal capacities varied with the footing shape and soil strength profile. Design methods to allow for footing shape and soil strength profile have been recommended.
When designing offshore foundations it is necessary to be able to calculate their capacity under monotonic combined total vertical (V), moment (M) and horizontal (H) load as these are often the limiting conditions due to environmental loading Many design methods have been recommended to calculate the vertical capacity of strip or circular footings in uniform strength material but these often rely on empirical shape factors rather than rigorous analysis to allow for the effect of the footing shape on the bearing capacity. Little work has been done to account for the effect of footing shape on the combined vertical, horizontal and moment loading response, especially in non-uniform soils. Recent bearing capacity analysis of offshore footings subject to combinations of V, M and H loading have used the Yield envelope concept (Tan, 1990; Murff, 1994; Butterfield and Gottardi, 1995; Houlsby and Martin, 1992; Dean et al., 1992; Salencon and Pecker, 1995; Ngo-Tran, 1996). For a given footing penetration, a yield envelope in V-M-H space describes the maximum loads that can be applied before failure and corresponding plastic footing penetration occurs. The yield envelope is written as where A is the area of the foundation, D its diameter or breadth and s is a representative soil strength or effective stress. In order to be able to use the theorems of plasticity with the yield locus, displacements are measured at the points at which the loads are taken to act.
