Centrifuge Modeling for Seismic Response of Fixed-End Model Piles Considering Local Scour

Abstract

The soil around offshore foundations usually suffers from local scour, which is widely believed to undermine the bearing capacity and serviceability of the foundations. However, the extent to which this undermining effect could be less clear when considering the foundation is subjected to an earthquake. This paper presents a set of centrifuge modeling tests on single piles, with a prototype diameter (D) of 1.5 m and a first-order natural frequency highly comparable to that of a prototype wind turbine. Two model piles with the same dimension and stiffness, but installed with and without a scour hole (2D in depth), were used to investigate the local scour effect on the seismic response of the pile and the soil around through comparison. The seismic pile–soil interaction was interpreted in terms of dynamic p–y curves derived from the measured bending moment along the pile shaft and acceleration at different soil depths. It was found that the existence of the scour hole reduced the first-order natural frequency of the pile by 15%, increased the bending moment along the pile by up to 90%, and increased the lateral displacement amplitude of the pile head by 37%. The scoured pile had lower seismic stiffness than the benchmark pile during shaking, and the seismic stiffness of both piles finally decreased to only 20% of the static stiffness. These results highlight the necessity to treat the scour effect on both dynamic and static responses of pile foundations with great caution. Owing to the facts that the state of the soil around the scoured pile transitions from normal consolidation to overconsolidation and that the presence of soil above the scour base inhibited the accumulation of excess pore pressure, the current design principle that all soils above the scour base should not be accounted for is deemed overly conservative.