Three-dimensional DEM simulations of monotonic jacking in sand

Understanding the soil responses during monotonic jacking can help improve the estimation of axial capacity in pile foundation design. To examine the soil responses in terms of strain and stress distributions, stress paths, the movement of particles and contact force mobilization during monotonic jacking, a three-dimensional simulation of a centrifuge model pile test was performed using the discrete element method. The simulation results show great resemblance to the measurements from the centrifuge model pile test. The distribution of incremental deviatoric strain d epsilon(q) indicates the formation of a shear band in the shoulder of the 'nose cone' that is beneath the pile base. Based on this, the soil mass can be divided into three zones, i.e., the 'nose cone' (zone I), the shear band (zone II) and the surrounding soil beyond the shear band (zone III). The distributions of the radial stress sigma(r)', hoop stress sigma(theta)', vertical stress sigma(v)' and shear stress tau(rv)' are in different modes and high-gradient distributions of mean stress p' and deviatoric stress q are found in zone II. As for the stress path, the surrounding soil experiences four phases in terms of changes in p' and q during monotonic jacking, and the change in the soil stress state decreases with increasing distance from the pile centerline. From a microscopic point of view, the particles in the shear band rotate strongly in a similar direction, fully mobilizing the frictions at the associated contacts. This provides a micromechanical explanation for the soil failure during monotonic jacking.