Interface Ring-Shear Testing to Assist the Effective Stress-Based Design of Driven Piles in Sand and Chalk

Modern design approaches for driven piles that address the axial resistances developed during installation and loading include theICP-05 method for clays and sands, and the ICP-18 approach for chalk ground conditions. Laboratory characterisation of pile shaft interface shearing resistances and dilation characteristics are central to detailed design applying these approaches. However, challenges remain in: (i) adopting appropriate specimen formation procedures and pre-conditioning stages that can capture the mutations that occur in the soil around the pile and on the pile shafts during installation; (ii) applying stress conditions and consolidation and/or ageing durations that are comparable to those experienced in the field; (iii) reproducing any physiochemical interactions between pile material, adjacent soil, ground/pore water and air that may affect the piles’ load-carrying capacity. This paper describes steel-soil interface shearing experiments on samples from piling research tests sites involving low-to-medium density chalk and silica sand. Parallel direct shear and Bishop ring shear interface tests are discussed that illustrate the impact of matching the conditions around driven pile shafts. Important physiochemical interactions involving the steel-soil-air-water system are demonstrated by parallel tests involving different consolidation (ageing) periods and steel types that may be encountered onshore or offshore. Conclusions are drawn regarding how these variables affect interface shear in chalk and silica sand, and so influence the axial behaviour of steel piles driven in such deposits.