Calculation of the ultimate bearing capacity of shallow footing on a two layered
system of soil depends on the pattern of the failure surface that develops below
the footing. For a weak clay layer overlaid by a top dense sand layer, previous
studies assumed that the failure surface is a punching shear failure through the
upper sand layer and Prandtl's failure mode in the bottom weak clay layer. By
adapting this assumption in this study, the ultimate bearing capacity equation
was derived as a function of the properties of soils, the footing width, and the
topsoil thickness. The paper presents a detailed parametric study of the design
parameters including the effect of angle of friction, the ratio of the thickness
of sand layer to the footing width, the ratio of the depth of embedment to the
footing width, and the ratio of the clay soil cohesion to the product of the clay
unit weight by the footing width. Design charts were developed in dimensionless
form for very wide ranges of design parameters. The available method based on
the limit equalibrium analysis was developed in dimensionlised form and for a
limited range of design parametrs. The new charts give another option for
those who believe that the design charts developed based on the upper limit
analysis overestimate the bearing capacity due to the very nature of the upper
bound solution. The new design charts are limited to shallow footings
