The bearing capacity of a granular layer on clay

Granular layers are often placed over weaker clay soils to improve the bearing capacity of working platforms and spread foundations. Their design requires the calculation of a two-layer bearing capacity. The commonly used existing calculation models are quite empirical with imprecise input parameters and can err on the non-conservative side in some situations. Other proposed methods tend to involve multiple design charts and are suited only to either strip or circular foundations. In this paper a new and highly practical design method is proposed whose input parameters are derived directly from the shear strengths of the two layers without the need for empirical-based charts. The output can be obtained either from a single chart or a few equations that are straightforward to implement into a spreadsheet. It can be applied quite generally to both surface and shallow embedded foundations, circular and rectangular and with dry or saturated granular layers, and also to working platform design for tracked plant. The method was validated using three independent numerical studies and centrifuge testing. The results showed that, although errors can increase for foundation shapes towards square or circular and with increasing overburden, the errors are smaller than those of the commonly used existing methods and are also conservative.