Tensile response of screw piles with varying installation approaches in layered sand

Single or multiple helix screw piles are being considered as fast and economical foundation systems for onshore renewable energy units. These foundations may encounter distinct soil layering and need to perform under a variety of load cases. For this application, the installation and monotonic tensile load response of multiple helix screw piles in uniform and layered sands were investigated by centrifuge modelling. Standard pitch-matched and pile self-weight approaches to installation were considered. The latter is advantageous as it requires no additional vertical force during installation (minimising plant requirements), alongside improved tensile capacity and reduced installation torque. Tensile capacity could also be increased by the inclusion of additional helices, but soil properties and layering arrangement must be considered or tensile capacity may be reduced. The performance of existing tensile capacity design methods was investigated and compared with the test results. A cylindrical shearing mechanism appears appropriate for capacity prediction in the majority of layering arrangements explored. Where individual plate-bearing mechanisms dominate, a modification to an existing approach reasonably predicted tensile capacity for shallow mechanisms, while further work is required to develop reliable prediction methods for deep mechanism behaviour in layered soils.