Effect of Footing Geometry and Embedment on the Bearing Capacity and Collapse Mechanism of Shallow Foundations in Sand

The bearing capacity of shallow foundations is often calculated using the classical three-term bearing capacity equation, containing bearing capacity factors and correction factors for embedment depth and footing shape. The available solutions for the determination of these factors were obtained using primarily limit analysis and the method of characteristics, with the assumption that soil is a perfectly plastic material following an associated flow rule. However, in reality, sands are neither perfectly plastic nor do they follow an associated flow rule. The unit bearing capacity of footings in sand depends on (1) the representative friction angle mobilized within the zone encompassing the footing's collapse mechanism, and (2) the size, shape, and embedment depth of the footing. This paper presents the results of load tests on model strip and square footings of different sizes installed in clean silica sand samples pluviated to produce different densities inside a calibration chamber. The test results for the footings placed on the sand surface show that the bearing capacity factors depend on the relative density of the sand sample and the geometry of the footing. For the embedded footings, the unit bearing capacity of the footing is shown to depend on its embedment depth. Insights into the footing collapse mechanism are provided by plotting the maximum shear strain contours for different stages of footing loading; the contours were obtained using the digital image correlation technique. Finally, we determined the shape and depth factors by analyzing the results of the load tests performed for surface and embedded strip and square footings in dense sand using procedures that account for stress level, loading path, and flow rule nonassociativity.