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

Offshore piles are subjected to complex loading regimes, which include both rapidly
applied static and cyclic loads. This note describes an experimental investigation
conducted in order to assess the factors influencing the response of offshore piles to these
loading conditions. The tests were performed using instrumented model piles installed in
soft clay. During cyclic loading, the piles demonstrated a transition from stable to
unstable behaviour when the applied loads reached a specific load threshold. Stable
behaviour was defined when increments of plastic displacement decreased as the number
of load cycles increased. During stable behaviour, radial effective stresses at the pile-soil
interface remained constant. During unstable behaviour, pore pressures at the pile-soil
interface rose as the number of cycles increased. This resulted in reduced radial effective
stresses and progressively increasing displacement rates. Due to the presence of these
excess pore pressures, the shaft resistance recorded during static load tests, performed
after a period of unstable cyclic loading, were lower than those measured on piles where
the pore pressure was fully equalised. However, the axial resistance was seen to be rate
dependent. 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 negative
effect caused by cyclic loading.