Analytical 3D failure envelopes for RC pile groups under combined loading: A generalised design approach

The paper presents a generalised design approach for pile groups subjected to combined (VHM) loading. The Limit Equilibrium method is extended to pile groups under general planar loading, considering the axial load dependency of bending stiffness and strength of reinforced concrete (RC) piles. 3D failure envelopes are analytically derived for different layouts. The key simplifying assumptions are critically discussed, and the failure envelopes are verified against finite element (FE) analysis results. Key features of pile group response, such as plunging vs. non-plunging axial pile response and the role of transverse reinforcement, are identified and quantified. A key novelty of the proposed method lies on the consideration of coupling between the lateral–bending resistance of the piles. The method further incorporates well-recognised features of pile group response regarding the axial load dependency of bending moment capacity and stiffness of the RC piles. The latter results in redistribution of bending moments and shear forces along the piles of the group. These features allow the reproduction of key aspects observed in demanding FE simulations and centrifuge tests, regarding the ultimate resistance under VHM and in addition the load path dependent failure modes. The proposed method offers a practical engineering tool for preliminary design and optimisation. •Analytical failure envelopes of RC pile groups under combined VHM loading.•Full coupling between the lateral–bending resistance of the piles.•Axial load dependent bending moment capacity and stiffness of the RC piles.•Redistribution of bending moments and shear forces along the piles of the group.•Simple design method based on closed form expressions predicting failure modes.