Cyclic and Rapid Axial Load Tests on Displacement Piles in Soft Clay

摘要

Offshore piles are subjected to complex loading regimes, which include both rapidlyapplied static and cyclic loads. This note describes an experimental investigationconducted in order to assess the factors influencing the response of offshore piles to theseloading conditions. The tests were performed using instrumented model piles installed insoft clay. During cyclic loading, the piles demonstrated a transition from stable tounstable behaviour when the applied loads reached a specific load threshold. Stablebehaviour was defined when increments of plastic displacement decreased as the numberof load cycles increased. During stable behaviour, radial effective stresses at the pile-soilinterface remained constant. During unstable behaviour, pore pressures at the pile-soilinterface rose as the number of cycles increased. This resulted in reduced radial effectivestresses and progressively increasing displacement rates. Due to the presence of theseexcess pore pressures, the shaft resistance recorded during static load tests, performedafter a period of unstable cyclic loading, were lower than those measured on piles wherethe pore pressure was fully equalised. However, the axial resistance was seen to be ratedependent. Fast loading of the pile resulted in reductions of pore water pressure at the soil-pile interface and enhanced shaft resistance which could overcome the negativeeffect caused by cyclic loading.