A revised Broms’ theory to estimate the ultimate capacity of laterally loaded piles driven in chalk

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4. A revised Broms’ theory to estimate the ultimate capacity of laterally loaded piles driven in chalk

Chalk is a weak rock often encountered at foundation level in the North Sea, where several offshore wind turbines supported by monopiles are under development or at planning stage. Monopile driving causes significant damage in weak rocks like chalk, and the damaged area tends to dominate both axial and lateral pile behaviour. This was investigated through extensive testing on piles driven at the low-to-medium density St. Nicholas at Wade chalk site (UK), which was conducted as part of the ALPACA Joint Industry Project (JIP). Based on this testing, pile lateral loading was modelled through advanced 3D finite element analyses undertaken as part of the ALPHA research project. These numerical analyses allowed to identify the key ingredients that dominate the lateral behaviour of piles driven in chalk.

Leveraging the lessons learned through field and numerical investigations, the present paper introduces a revised Broms' theory that provides reasonable estimates of the ultimate capacity of laterally loaded piles driven in chalk. The proposed approach accounts for the gap opening observed on site during lateral loading in active areas, providing representative ultimate soil reactions mobilized at the soil-pile interface. The modifications suggested to Broms' theory also allow to predict realistic depths for the rotation points, hence providing representative failure mechanisms. The revised Broms' theory presented in the paper was validated against field data collected during the ALPACA JIP, showing reasonable predictions not only in terms of ultimate pile capacities, but also in terms of maximum bending moments acting on the pile. The proposed approach offers a valuable tool to estimate the ultimate reactions attained in p-y curves, the latter extensively adopted in 1D numerical models for pile design.

21st International Conference on Soil Mechanics and Geotechnical Engineering (Vienna)

TC209