Comparison of failure modes below footings on carbonate and silica sands

To investigate the differences in failure mechanisms beneath vertically loaded shallow foundations on silica and carbonate sands by way of particle image velocimetry analysis, 1g and centrifuge model tests have been performed. A comparison of silica and carbonate sand behaviour at low stresses and low initial soil density was made possible by 1g scaling conditions, while centrifuge conditions focused on high stresses and high soil density. This strategy provided separate insights into the effects of the compressibility and crushability of both sands. Results indicated a distinct difference in failure mode between the two sands. The footing on silica sand exhibited a shallow failure mode with large localised strains close to the soil surface. In contrast, the footing on carbonate sand mobilised strains lower in magnitude over a greater depth. The increased stress level in the centrifuge reduced the surface heave and resulted in deeper failure mechanisms in both sands. The bearing capacity factor for the experimental results could be back-calculated with simple limit analysis solutions, provided that different friction angles were used to account for the variation of stress levels. However, such an approach masks the influence of compressibility on the failure mechanism in the loose and compressible cases by requiring a lower friction angle than is realistically possible.