Pile Behavior in Low-Medium Density Chalk: Preliminary Results from the ALPACA Project-Paper

Paper

Pdf Summary

The ALPACA project investigates the behavior of driven piles in low-to-medium density chalk, a common offshore North European seabed material for which design guidance on capacity, installation, ageing, lateral response, and cyclic loading remains limited. Existing recommendations for axial shaft resistance are low and based on sparse, inconsistent testing.<br /><br />At a well-characterised site in Kent, UK, researchers drove and tested piles of different sizes, materials, and geometries. The programme included static and cyclic axial and lateral loading, supported by extensive site investigations, laboratory testing, and instrumentation. Fibre Bragg Grating sensors on 16 piles recorded strain during installation and load testing, enabling detailed assessment of stress-wave propagation and pile load transfer.<br /><br />Preliminary results show that pile behaviour changes significantly with time after driving: piles were tested at ages ranging from days to nearly a year, and ageing had a considerable effect on axial response. During lateral cyclic tests, piles initially accumulated displacement slowly and largely recovered on unloading. However, gaps formed between the pile and surrounding chalk, producing a softer response on reloading; stronger cycling then caused more permanent displacement and reduced cyclic stiffness. In axial tests, low-level tension cycles caused little permanent movement, whereas high-level two-way tension-compression cycling could lead to rapidly increasing displacement and failure. A representative long-pile compression test reached a total load of 2,400 kN at failure.<br /><br />The project also demonstrated the value of high-resolution fibre-optic measurements for analysing pile driving and estimating shaft resistance. ALPACA and its follow-on programme aim to use these results to develop better design guidance for offshore foundations in chalk, including the effects of pile scale, water-table position, material, and cyclic loading.

Keywords

ALPACA project

driven piles

offshore chalk

axial shaft resistance

pile ageing

lateral cyclic loading

fibre optic sensors

Fibre Bragg Grating

pile load testing

offshore foundations