Pile behaviour in low-medium density chalk: preliminary results from the ALPACA project

Chalk is encountered under large areas offshore NW Europe as a low-density, porous, weak carbonate rock. Current European practice for driven pile design assumes ultimate shaft resistances that appear remarkably low in comparison with the chalk’s in-situ strength. Noting that little guidance is available on driveability, set-up or lateral resistance, or on how piles driven in chalk can sustain axial or lateral cyclic loading, the ALPACA and ALPACA Plus Joint Industry Projects have been undertaken to improve design guidance. The research JIPs, which are led by an Academic Work Group from Imperial College London and Oxford University, aim to aid offshore foundation engineering in chalk, particularly for wind-turbine projects, by developing new design guidance through field testing at a low-to-medium density test site in Kent, onshore UK. This paper describes progress on ALPACA to date and provides some new preliminary outcomes from the experiments undertaken on 36 piles driven and tested between 2017 and 2019, sixteen of which were instrumented with Fibre Bragg Grating (FBG) sensors during installation and load testing. The pile testing was supported by parallel site characterization, high-quality sampling, in situ testing and advanced laboratory testing.