Centrifuge modelling of jacked piles

Abstract

Displacement piles are usually installed by hammering or vibration, but large pile jacking machines offer a quieter alternative. Pile jacking uses static hydraulic force to install displacement piles, avoiding the noise and ground vibration associated with dynamic pile installation methods. The ‘press-in’ piling machine shown in Figure 1 installs tubular piles of diameter 1000-1200 mm with a jacking force of up to 4 MN. The ‘press-in’ technique uses the negative shaft resistance of previously-installed piles to provide reaction force. Rockhill, et al. (2003) demonstrate that pile jacking reduces ground-borne vibrations by an order of magnitude compared to traditional percussive and vibro-hammer installation techniques. Pile jacking is therefore well suited to the urban environment. This paper describes the modelling of jacked piles in the centrifuge environment. Twelve installations and companion tests are reported and the implementation of load and displacement control is described. The aim of this research is to quantify the behaviour of displacement piles jacked into sand. A control system has been developed to allow the jacking process to be faithfully replicated, and instrumented piles have been used to measure the load-settlement response of the pile base and shaft. Previous research (De Beer 1988, Ghionna, et al. 1993, Lee, et al. 2003 & Deeks, et al. 2005) has identified that dynamically-installed displacement piles are stiffer than non-displacement (bored) piles. Even higher stiffness has been reported for jacked piles (Yetginer, et al. 2003, Deeks, et al. 2005). This relative performance can be attributed to the differing stress histories applied to the soil surrounding the pile by each installation procedure. Pile jacking pre-loads the soil beneath the pile tip, and can trap residual base load. These effects stiffen the base response, and reduce the settlement required to mobilise the full base resistance. There is scope for improved design efficiency if this high axial stiffness can be better predicted and utilised in design. Higher efficiency leads to economies of cost and reduced environmental impact through reduced material use and installation time.