Load Response and Soil Displacement Field for a Vertically Loaded Strip Footing on Sand Underlain by a Stiff Base

Traditional bearing capacity theories assume that a foundation element is supported by a thick, uniform soil deposit. However, natural soil deposits are often stratified, i.e., a soil layer of finite thickness could be underlain by another soil layer with different material properties or shallow bedrock. In this paper, we investigate the effect of the presence of a stiff base on the load–settlement response of a half-strip model footing placed on the surface of Ottawa 20–30 (OTC) sand samples prepared inside a half-cylindrical calibration chamber. The model footing load tests were performed for different values of sand relative density $D_R$ and sand layer thickness-to-footing width ratio $H/B$. A series of high-resolution images collected during model footing loading were analyzed using the digital image correlation (DIC) technique to obtain the displacement and strain fields in the sand domain. The bearing capacity factor $N_{\gamma }$ was back-calculated from the model footing test results as a function of $D_R$ and $H/B$. The magnitude and extent of the cumulative maximum shear strains, horizontal displacements, and vertical displacements in the sand layer depend on the depth at which the stiff base is located below the footing. The results obtained in this study show that (1) the attainment of the peak unit load for the model footing does not coincide with the formation of a fully-developed collapse mechanism, (2) the presence of a stiff base below the sand sample produces a stiffer footing load–settlement response than that obtained in the absence of the stiff base, and (3) the unit bearing capacity of the footing increases when the stiff base lies within the footing’s depth of influence.