Numerical Study of the Effect of Foundation Size for a Wide Range of Sands

This paper presents a numerical investigation of the effect of foundation size on the response of shallow circular foundations on siliceous and calcareous sands. The study is based on the predictive capabilities of the MIT-S1 soil model for simulating both the compression and shear behaviors of natural sands over a wide range of densities, $K_0$ values and confining pressures. The paper highlights the variations in the deformation mechanisms for the siliceous and calcareous sands cases. The assessment of the bearing capacity factor, $N_{\gamma }$ , is examined, showing a dramatic decrease in the values with increasing foundation size for the case of footings on calcareous sands, eventually converging to a terminal $N_{\gamma }$ value. At this stage the sand resistance is insensitive to variations in initial density and foundation size because the sand tends to loose its initial characteristics due to grain crushing, leading the material rapidly toward ultimate conditions. In the silicious sand case, it is found that, eventually, for extremely large footing diameters, the deformation mechanism progresses toward a punching shear mechanism, rather than the classical rapture pattern accompanied by surface heave as employed in current bearing capacity equations. A dimensional transition between the failure mechanisms can clearly be defined, referred to as a “critical size” in the $N_{\gamma }–D$ relationship.