Combined loading of RC pile groups in clay accounting for N–M interaction

Abstract

Motivated by the need for retrofit of existing bridges, this paper explores the nonlinear response of pile groups in clay under combined loading. The problem is analysed numerically, employing a kinematic hardening model for soil, and the Concrete Damaged Plasticity (CDP) model for the reinforced concrete (RC) piles. Using a reference 2 × 1 pile group, it is shown that three resistance mechanisms are mobilized: axial pile loading (Max), pile bending (Mb), and pilecap resistance (Mcap). While current design practice typically considers only the first mechanism, it is shown that allowing for strongly nonlinear soil response and full mobilization of all three mechanisms may lead to a significant increase of pile group moment capacity. The analysis also reveals the need to account for axial force–bending moment (N−M) interaction of the RC piles, which is feasible through the CDP model. Especially for existing pile groups, more realistic estimation of ultimate moment capacity may facilitate the development of rational retrofit strategies. A parametric study is subsequently conducted, exploring the effect of the safety factor against vertical loading (FSv), the moment to shear (M/H) ratio, interface modelling, and pilecap contribution. The results are generalized by analysing more complex 3 × 1 and 4 × 1 pile group typologies and by deriving 3D failure envelopes in the V – H – M space. Thanks to the contribution of the pilecap, the decrease of FSv leads to an expansion of the failure envelope, which can be of relevance for existing pile group assessment after bridge widening. Finally, the interaction diagrams are normalized, leading to a unique non-dimensional 3D failure envelope for all examined pile group typologies.