The axial behaviour of piles driven in chalk

This paper describes research into the poorly understood axial behaviour of piles driven in chalk. Comprehensive dynamic and monotonic axial testing on 27, mostly instrumented, piles undertaken for the ALPACA joint industry projects is reported and interpreted covering: diameters between 139 mm and 1 center dot 8 m; lengths from 3 to 18 m; different pile material types; tip and groundwater conditions; and ages after driving. The experiments show the factors that influence resistance most strongly are: (a) pile end conditions; (b) slenderness ratio and flexibility; (c) shaft material; (d) age after driving; (e) relative water table depth; and (f) whether loading is compressive or tensile. Varying the factors systematically identified a remarkable average long-term shaft resistance range from below 11 kPa to more than 200 kPa for piles driven at the same low- to medium-density chalk test site in Kent (UK). Dynamic and static analyses demonstrate that soil resistances to driving were generally well predicted by the Chalk ICP-18 short-term formulation. Considering the piles' long-term behaviour, the Chalk ICP-18 approach over-predicted capacity, while the widely used CIRIA approach proved over-conservative for most cases. The research enabled the development of a revised 'ALPACA-SNW' long-term capacity assessment method that matches the test outcomes far more faithfully.

Automated summary

This paper presents research on the axial behaviour of piles driven in chalk. Comprehensive dynamic and monotonic axial testing on 27 piles was conducted, covering various factors such as pile diameter, length, material type, tip and groundwater conditions, and age after driving. The experiments reveal that the factors that exert the strongest influence on resistance are pile end conditions, slenderness ratio and flexibility, shaft material, age after driving, relative water table depth, and loading type. The research leads to the development of a revised method for long-term capacity assessment.