Driven pile behaviour in low-to-medium density chalk: the ALPACA JIP outcomes

Offshore pile design presents multiple challenges in low-to-medium density chalks, whose sensitivity and brit-tleness often leads to very low driving resistances. Also, existing design rules have proved unable to give reli-able predictions for axial and lateral resistances under offshore field loading conditions. This paper summaris-es how the ALPACA and ALPACA Plus Joint Industry Projects (JIPs) addressed the current lack of knowledge through comprehensive field, laboratory and theoretical research. Axial and lateral field tests were conducted, under dynamic, cyclic and monotonic conditions, on 43 piles with diameters between 139mm and 1.8m, made with a range of materials, wall thicknesses and tip conditions, driven to depths between 3 and 18m. Over 100 tests were completed, after ageing for up to 400 days. Most test piles were instrumented with robust fibre-optic strain gauge systems. The experiments were supported by intensive in-situ and laboratory stress-path and cyclic testing. New monotonic and cyclic axial design methods developed for the JIP repre-sent field behaviour far better than existing approaches. Important new observations were also made regarding pile behaviour under cyclic lateral loading, while the monotonic lateral pile tests were interpreted to generate ‘p-y’ load reaction curves and test more advanced 3-D FE approaches, developed through parallel research.