Divergence between CPT based method predictions for the axial capacities of large offshore piles driven in sand

Most offshore structures are founded on large driven piles and reliable prediction of their axial capacities is often a key design requirement. When compared to the ICP-05 and UWA-05 design methods, the recent Unified CPT based design approach for sands gives broadly similar capacities, but significantly less scatter for the tests considered in its calibration which only includes one pile with a diameter exceeding 810mm. The PAGE JIP sought to address this and focused on evaluating the time-dependent capacities of 25 full-scale offshore piles. It showed greater divergence between Unified and ICP-05 predictions and a tendency for the new method to under-predict the shaft resistances assessed at ages exceeding 30 days after driving. This divergence for large diameter piles was confirmed by a parametric study presented in this paper which revealed that one of the main contributing factors lies in the inclusion of a vertical effective stress term in the ICP-05 method, which is not included in the Unified method's formulation.