Bearing Capacity and Failure Mechanism of Strip Footings on Anisotropic Sand

Abstract

Sand typically exhibits anisotropic internal structure (or fabric), and the fabric anisotropy has a dramatic influence on the mechanical behavior of sand. Meanwhile, the fabric evolves when sand is subjected to external loading. This eventually makes the response of strip footings on sand dependent on fabric anisotropy and fabric evolution. A numerical investigation on this effect is presented using a critical state sand model accounting for fabric evolution. The model parameters are determined based on plane strain and triaxial compression test data, and the model performance is validated by centrifuge tests for strip footings on dry Toyoura sand. The bearing capacity of strip footings is found to be dependent on the bedding plane orientation of dense sand. However, this effect vanishes as the sand density decreases, though the slope of the force-displacement curve is still lower for vertical bedding. Progressive failure is observed for all the simulations. General shear failure mode occurs in dense and medium dense sand, and the punching shear mode is the main failure mechanism for loose sand. In general shear failure, unsymmetrical slip lines develop for sand with an inclined bedding plane due to the noncoaxial sand behavior caused by fabric anisotropy. For strip footing on sand with horizontal bedding, the bearing capacity and failure mechanism are primarily affected by the sand density. The bearing capacity of a strip footing is higher when the sand fabric is more isotropic for the same soil density. An isotropic model can give significant overestimation on the bearing capacity of strip footings.