Performance of CFA Piles in Weak Rock of the Tile Hill Mudstone Formation, University of Warwick, Coventry, UK

Rock socket pile design has historically been based on empirical relationship calculations relating rock strength to shaft friction and bearing resistance. These relationships generally incorporate conversion factors back-calculated from static load tests or basically instrumented piles. CIRIA Report 181 presents a series of recommendations for advancing design and construction of piles in weak rock. This paper presents a summary of recent piling experience within the Carboniferous Tile Hill Mudstone Group at the site of the new Faculty of Arts Building at the University of Warwick in Coventry, UK. The experiences demonstrate that conventional Continuous Flight Auger (CFA) piling rigs with augers 450 to 750mm can penetrate sandstone up to 4m thick with a minimum uniaxial compressive strength of 16MN/m². The assessment of pile test data identified that pile head displacement at design verification load is typically 0.5% of pile diameter and that in these ground conditions, operational unit shaft friction was ~65kN/m². This project demonstrates the benefit of staged ground investigation and the involvement of the piling contractor during supplementary stages of ground investigation where the piles are Contractor designed. Trial bores and preliminary pile load test were successfully used to demonstrate constructability and provide confidence in pile performance in difficult ground conditions.