Finite element simulation of the pressure dip in sandpiles

Many industrial bulk solids are commonly stored in open stockpiles that are progressively formed by depositing from above. A classic phenomenon concerning such simple piles is the observation of a significant pressure dip in the vertical pressure on the base underneath the apex which is counter-intuitive as this is the location where a maximum pressure might be expected. Numerous experimental, analytical and numerical studies have been conducted to investigate this problem over the last few decades, but a comprehensive understanding of the problem remains elusive. Mechanical anisotropy developed during pile formation process has recently been suggested to be the main cause of the pressure dip. However, more recent finite element method (FEM) studies have predicted a pressure dip beneath the apex using isotropic material models. The review of the literature shows a lack of understanding of the underlying mechanism and the roles of various factors on the prediction of the pressure dip, such as the progressive mesh activation, stress dependency of modulus and plastic failure parameters. The aim of this paper is to investigate the effects of these factors by modelling a set of conical sandpile experiments formed on a rigid base by concentrated deposition. The results show that significant pressure dip can be predicted without considering material anisotropy. However, within the realm of elasto-plastic models investigated, it appears that the prediction of pressure dip requires a model considering the construction process and the associated plastic deformation. Incorporating stress-hardening elasticity enhances significantly the extent of the dip. The results demonstrate that a greater mobilisation of the base shear traction is an important mechanism in enhancing the arching effect that leads to a significant central pressure dip.