Numerical study on the reinforcement effect of underwater vibro-compacted sand piles under different wave conditions

To evaluate the dynamic response and reinforcement performance of underwater vibro-compaction sand-pile composite foundations under different wave conditions, a fully coupled wave-structure-seabed numerical model was established on the FSSI-CAS platform, using the nearshore breakwater project at Jinpai Port, Lingao, Hainan, as the prototype. Three representative wave cases were examined, and the spatial distributions and time-history responses of displacement, pore water pressure, strain, and effective stress were compared. Results show that the shallow zone near the structure–seabed interface is the key response region. Horizontal displacement is governed by cyclic oscillation, while vertical displacement shows cumulative settlement. Under Case 1, the peak horizontal displacement reaches about 1.33 × 10−2 m and the maximum settlement about 6.95 × 10-³ m. High dynamic pore water pressure is concentrated beneath the structure and in the shallow underlying foundation, then decays rapidly with depth. The maximum dynamic pore water pressure amplitude is about 1.12 × 105 Pa and occurs near the centre of the structure base. Effective stress varies slowly overall but is superimposed by high-frequency fluctuations. Stronger combined wave cases produce more pronounced effective-stress redistribution and fluctuations, indicating a stronger dynamic response of the foundation system.