The Field Behaviour of Drilled and Grouted Piles in Rock

This paper describes a pile load testing campaign undertaken to inform the design of drilled and grouted piles in rock supporting steel jacket substructures for an offshore windfarm. The large diameter piles will be subjected to high compressive and tensile irregular cyclic loading resulting from wind, waves and currents. Experience in the design, installation and performance of drilled and grouted piles in this context is limited. The programme comprised of static and cyclic tests performed on 14 piles grouted into rock sockets with diameters ranging from 330mm to 1300mm, drilled in sandstone at a well-characterised nearshore site in Northern Brittany, France.

The field tests demonstrate that unit shaft capacity varies inversely with pile diameter. This is consistent with models that treat shaft resistance as developing by constrained dilation at the grout-to-rock interface, which is governed by the drilled shaft’s roughness and the rock mass stiffness. Most historical “α-UCS” shaft capacity design methods do not consider this strong diameter effect and may therefore lead to unsafe estimates of shaft capacity for large diameter piles. The field tests also identified the loading conditions under which stable or metastable responses were developed by the piles under one-way axial cyclic loading.