Bearing capacity of a circular foundation on layered sand-clay media

The bearing capacity of a circular footing lying over fully cohesive strata, with an overlaying sand layer, is computed using the axisymmetric lower bound limit analysis with finite elements and linear optimization. The effects of the thickness and the internal friction angle of the sand are examined for different combinations of cu/(γb) and q, where cu=the undrained shear strength of the cohesive strata, γ=the unit weight of either layer, b=the footing radius, and q=the surcharge pressure. The results are given in the form of a ratio (η) of the bearing capacity with an overlaying sand layer to that for a footing lying directly over clayey strata. An overlaying medium dense to dense sand layer considerably improves the bearing capacity. The improvement continuously increases with decreases in cu/(γb) and increases in ϕ and q/(γb). A certain optimum thickness of the sand layer exists beyond which no further improvement occurs. This optimum thickness increases with an increase in ϕ and q and with a decrease in cu/(γb). Failure patterns are also drawn to examine the inclusion of the sand layer.