Effect of footing shape on penetration in sand overlying clay

This paper reports on a series of centrifuge model tests investigating the effect of shape on the penetration resistance of spudcan and conical footings on sand overlying clay. The effect of footing shape and geometry on single-layer soil has been studied intensely. However, there is still limited understanding for conical footings on sand over clay. In the present study, digital images were captured during penetration of various shapes of half-footing held against a transparent window of a strongbox. The images were then analysed using particle image velocimetry techniques. Experimental evidence has shown that, irrespective of the conical angle of the underside within the range of 7-21°, when the footing penetrates through sand into an underlying clay layer: (a) accumulated radial and deviatoric shear strains along the future failure surface counteract each other, resulting in similar peak resistance in the sand layer and (b) a trapped sand plug of constant height is pushed into the underlying clay layer. These observations serve to justify the previously proposed methods for predicting the full penetration resistance profile on sand overlying clay, which is required to predict the potential for, and severity of, punch-through failure.