Bearing capacity predictions of sand overlying clay based on limit equilibrium methods

A dense sand layer is usually assumed to be a bearing stratum in foundation design. It is, however, difficult for engineers to judge if a sand layer with limited thickness can be used as a bearing stratum when it is underlain by a thick soft clay deposit. Many important parameters must be considered in this problem, for example, the thickness of the sand, strength of clay and width, shape and embedment of footing. Several methods have been proposed to calculate the bearing capacities of the sand overlying clay, in which limit equilibrium of forces acting on an imaginary sand block between the base of the footing and the sand/clay interface was considered. The bearing capacities calculated based on existing methods are not the same for different assumptions adopted in each method. The shape of the sand block and the forces acting on the surface of the block are the determining factors in the bearing capacity calculation. In this paper, these factors as well as calculated bearing capacities are compared with the results of well-conditioned centrifuge tests (Okamura et al., 1997) to verify the validity of the methods. It has been confirmed that reasonable assumptions in which the variation of the shape of the sand block and the forces related to the parameters are taken into consideration are important to obtain a reasonable prediction. An alternative method is proposed and the bearing capacity charts are presented for easy reference.