Prediction and Evaluation of Size Effects for Surface Foundations on Sand

This paper describes finite element analyses using a generalized effective stress soil model, MIT-S1, to investigate size effects in the load-settlement response of surface foundations on deep homogeneous sand layers. Prior studies have shown that MIT-S1 can describe realistically the compression and shear behavior of sands over a wide range of confining pressures and densities using a unique set of material input parameters calibrated for a given material. The current analyses show that the model is able to represent transitions in the mechanisms of ground deformation from general to punching modes with changes in foundation size (${\gamma }^{\prime }D/p_{at}$) and sand density (initial void ratio, $e_0$). These mechanisms explain differences in the computed load-settlement responses and produce large decrements in the bearing capacity factor $N_{\gamma }$ with increased foundation size (in the range $0.1\le {\gamma }^{\prime }D/p_{at}\le 20$). Small circular foundations (${\gamma }^{\prime }D/p_{at}\approx 0.1$) have a bearing resistance of about 50% that of comparable strip foundations, but shape effects are negligible for very large foundations (${\gamma }^{\prime }D/p_{at}\approx 20$). Comparisons of computed results for two sands (Toyoura and Berlin) of contrasting particle shape and formation void ratio exhibit similar bearing capacity factors at the same relative density levels. Model predictions of the bearing capacity factor $N_{\gamma }$ are in very good agreement with data reported from centrifuge model tests of circular and strip foundations on Toyoura sand.