Numerical analysis of the set-up around the shaft of a closed-ended pile driven in clay

Abstract

When a pile is driven into the ground, soil standing in the path of the pile is heavily distorted as it has to cede for the penetrating pile. At the end of installation, this soil is left in a distressed state, which progressively evolves to an equilibrium with time. As a result, for lightly overconsolidated clays, soil resistance generally decreases during driving but increases after installation; the latter phenomenon is referred to as set-up. Correct assessment of pile set-up is one of the most important considerations of displacement pile design, especially offshore. Indeed, underestimation of set-up leads to unnecessary expense and overestimation of set-up leads to a precarious overestimation of the pile capacity. This paper presents a numerical model, the aim of which is to predict set-up around the shaft of a driven pile in clay. The model was conceived after a careful literature review of the relevant field data, in order to support its assumptions. It is used to predict the stress and pore pressure distributions around the shaft of a driven pile during installation and subsequent equalisation. Numerical results are then compared to the literature results. Although the model underestimates the excess pore pressure created around the pile, it captures a key feature of displacement pile installation in clay: at the end of installation, the radial distribution of excess pore pressure presents a peak located a few radii away from the pile wall. This implies that, during equalisation, the radial effective stress at the pile wall decreases to a short-term minimum, leading to a short-term minimum in pile capacity, before eventually increasing.