The fundamental behaviour of displacement screw piling augers

The use of both displacement and non-displacement cast in-situ screw piling techniques has seen considerable growth over the last 10 years. In this time, significant advances have been made in the equipment used to install screw piles. The torque capacity of screw piling rigs has increased dramatically from as low as 10 k:Nm for continuous flight auger (CF A) rigs in the early 1980s, to in excess of 700 kNm for modern displacement screw piling rigs. The advent of computer drilling instrumentation allowed detailed construction records to be maintained for each pile and concrete technology has also improved to the point where piles can be constructed from concrete instead of grout, thus reducing material costs and improving reliability.

However, investigations into fundamental auger mechanics have been almost exclusively directed towards CFAs. The aim of the current investigation is to study the behaviour of displacement screw piling augers in sands. In particular, the relationship between soil transport, soil displacement, radial stress and installation forces has been investigated. This thesis presents a new theoretical model to predict the soil transport, soil displacement and installation torque characteristics of screw piling augers with variable geometry.

The performance of the model has been verified by comparison with 1:4 scale model tests performed on three different displacement screw piling augers. The physical soil transport and displacement fields have been observed by installing augers into dyed, layered sands and later exhuming the augers. A second series of tests was also performed to compare the installation parameters (torque, penetration rate and rotational speed) and radial stresses acting on the auger during installation.