Study on pile cap effects of pile group foundations under linear wave action by a hybrid BEM and EEM method

Abstract

For pile group foundations (PGFs) subjected to ocean wave action, diffraction waves from the large-scale pile cap can remarkably change wave loads on slender piles below the pile cap (i.e., pile cap effects). However, relevant influencing factors and the variation pattern of wave loads on piles are still unclear. To this end, this study develops a three-dimensional hybrid method for calculating wave loads on slender piles in PGFs under linear wave action. The presented method is implemented by combining the boundary element method (BEM) and the eigenfunction expansion method (EEM) and can consider the effect of diffracted waves from the pile cap. The proposed method is then applied to a truncated cylinder and a PGF with a square pile cap, and the calculated results are respectively compared with the analytical and numerical results. Excellent agreements verify the validity of the presented method. The effects of various factors, including cap shapes, cap dimensions, cap drafts, wave propagation directions, wave numbers, pile slopes, and pile tilt directions, on wave loads of piles are investigated using the presented method. The results show that wave numbers and cap horizontal dimensions have a very strong effect on wave forces on slender piles in PGFs, and wave forces on piles decrease substantially as they increase. However, the influence of cap drafts is weaker. The wave propagation direction determines the distribution pattern of wave load magnitudes at different pile locations, and this distribution pattern varies with different cap shapes. For tilted piles, the wave propagation direction and the pile tilt direction jointly affect (enhance or weaken) wave loads on piles, and the pile slope can amplify this effect. Wave loads on the external piles of the pile group are generally stronger and these external piles need to be reasonably strengthened. This research can assist with the design and optimization of PGFs for offshore bridges.