An investigation into the strength and stiffness of jacked piles in sand

Andrew Deeks, Cambridge University
Geotechnical Engineering Group


The method by which a pile is installed influences the strength and stiffness (the ‘performance’) of the resulting foundation. High-capacity jacking machines have recently been developed, which offer the opportunity for the foundations of large buildings or heavy structures to be installed without the noise and vibration associated with conventional methods of displacement piling. The purpose of the work reported in this dissertation is to quantify how jacking a displacement pile in to the ground affects the stress history of the soil and the subsequent performance of the foundation system and to develop simple models that can predict this performance and exploit the environmental benefits of pile jacking technology.

Two methods of jacking are investigated. (i) ‘Axial-jacking’; in which purely axial force is used to install pre-formed displacement piles by ‘pushing’ them in to the ground. (ii) ‘Rotary-jacking’; in which simultaneously application of torque and axial force is used to install tubular displacement piles by rotating them into the ground.

The scope of this investigation was limited to investigating the performance of jacked piles in sand (i.e. drained conditions). Two series of tests were conducted using a geotechnical centrifuge. In the first test series 24 pile installations were conducted that simulated axial-jacking. A further series of 35 pile installations simulated the rotary-jacked installation of a pile and the response of a pile subjected to combinations of axial and torsional load.

Field test data was used to validate the behaviour observed in the centrifuge tests. Theoretical models were developed to aid data interpretation and extend the models developed for the behaviour of closed-ended piles to that of open-ended piles.

The key outcomes of this dissertation are:

(a). The axial load-displacement response of an axial-jacked displacement pile installed in sand can be predicted from routinely measured in situ testing parameters.

(b). The key influence of dilation at the soil-pile interface on both the strength and stiffness of a pile shaft is shown using an analytical model and test data. This model links strength and stiffness values measured in centrifuge tests to field scale test results.

(c). Methods to predict the axial and torsional installation loads required to install closed and open ended rotary-jacked piles in sand are developed.

(d). The unit shaft resistance was consistently observed to increase with increasing rotation of the pile in the centrifuge model tests. Mechanisms are proposed that link this increase in strength is to the observation of increasing pile capacity with time, ‘set-up’, in the field.

Keywords: pile, sand, foundation, jacking, set-up, combined loading