MPM Simulation of the Installation of an Impact-Driven Pile in Dry Sand and Subsequent Axial Bearing Capacity

Pile installation leads to significant changes in soil state (i.e., void ratio and effective stress) around the pile, which affects stiffness and bearing capacity. Currently, the driveability of piles is analyzed using empirical methods, and the ultimate bearing capacity is estimated without considering the installation effects. This paper presents simulations of the entire installation and subsequent axial bearing capacity of a close-ended pile using a single numerical tool based on the material point method (MPM). A lab-scale experiment is used as a validation case, where the pile is first impact-driven in dry sand, with different initial relative densities (from loose to very dense), and then axially loaded. A state-dependent constitutive model (DeltaSand) is used in the numerical simulations to predict the mechanical behavior of the sand at different relative densities with a single set of input parameters. The paper also illustrates several enhancements needed to obtain more accurate results: (1) an improved contact algorithm that allows gap closure; (2) a rigid-body formulation for the pile body; and (3) a general analytical solution for calculation of energy-consistent impact forces in uncoupled hammer-pile systems.