The Effects of Rotary Installation on the Axial Capacity of Displacement Piles in Sand

Deep foundations may be used in a range of soil types where significant foundation resistance is required but, depending on the pile type, their installation may be associated with environmental disturbance due to noise and vibration e.g. in classic pile driving. Rotary installed non-driven displacement piles are “silent” piling methods which utilise vertical and rotational displacement to install piles with minimal noise and vibration. One such method is the continuous helical displacement (CHD) pile. The CHD pile is a cast-insitu auger displacement pile developed by Roger Bullivant Ltd in the UK. The CHD pile is installed into the soil using a drilling rig and a auger tool known as the CHD bullet. To install the pile, the tool is rotated into the ground to a desired depth, before being retracted and concrete is pumped into the soil to form the CHD pile. The CHD pile has performance characteristics of both displacement and non-displacement piles, although there is limited available knowledge regarding how the installation method effects the axial capacity of the fully formed pile. Numerical simulations using the discrete element method were conducted on a variety of rotary installed piles in an aim to investigate the effect of rotary installation on elements of the axial capacity of the CHD pile in sands of different relative density. Due to the complexity of the CHD pile’s geometry and installation methodology, the installation effects of advancement ratio or rotation rate were first investigated for simpler pile geometries, starting with a straight shafted pile, then moving onto a single helix steel screw pile before using this information to build up an understanding of installation effects on different components of the CHD pile. The simulations highlighted the significant effects of the advancement ratio on both the installation requirements and the axial capacity of all of the pile geometries investigated. A significant finding was the ability of advancement ratio to enhance either the compressive or tensile performance of a flighted pile, with over-flighting increasing the tensile performance and under-flighting the compressive performance. The simulations allowed for updated design parameters to be created for an analytical axial compressive capacity prediction method and the development of a CPT based design procedure for the axial capacity incorporating installation effects. Insights gained through the discrete element method, highlighted the particle displacement, locked in residual soil stresses and general relative density changes that occurred in the soil bed during the installation process. The location and magnitude of these properties could then be used to explain the global and local pile behaviour, allowing for a greater understanding of axial performance of screw piles and the CHD pile and selecting what diameter should be used when computing axial capacity of the helically shaped CHD pile in sands. These insights will allow further efficient use of the CHD pile and future optimisation.