Characterising the damage developed around piles by percussive driving in low-to-medium density chalk

This paper explores how the effects of pile driving installation on low-to-medium density chalk may be chacaterised. The damage developed around eight open and closed, steel and concrete piles driven at the St Nicholas at Wade test site in Kent, UK, was investigated by carefully logging and micro-sampling chalk from around their shafts after long ageing in situ. The observations identified relatively thin annuli of de-structured 'putty' chalk left around the pile shafts. Related numerical studies have confirmed that, after re-consolidation in situ, this Zone A material largely controls the piles’ axial load-displacement behaviour. Measurements of the reduced Zone A water content profiles around piles with different geometries, scales and materials confirm that their thicknesses scale primarily with pile wall thickness tw. A second, more extensive, annular Zone B was also identified, which manifests far more frequent fracturing than the natural chalk. Earlier numerical analyses have shown that its degraded properties largely control the piles’ lateral loading responses. The Zone B annular thicknesses, and degrees of damage within them, depend on both pile diameter D and tw. These observations are crucial to both the numerical modelling of piles driven in chalk and any lateral loading design calculations.