Failure interaction mechanisms for large piled rafts under undrained conditions: how can soil–pile–raft coupling be taken into account?

This paper presents a novel framework for geotechnical ultimate limit state verification of large piled raft foundations, where raft width is comparable with pile length, typical of existing viaducts and modern high-rise buildings. Available verification methods identify piled rafts with pile groups, neglecting the raft contribution and thus underestimating bearing capacity under vertical and combined loading. The proposed approach originates from a new mechanical interpretation of system behaviour under combined loads. Particularly innovative is the interpretation under horizontal loads, where the pile–raft–soil assembly rotates rather than purely translates, in contrast with the fixed-head pile group assumption. These insights form the basis for defining the soil–piles–raft coupled interaction domain, filling a gap not yet addressed in the literature. The domain is formulated for undrained conditions by combining classical bearing capacity theory with the limit equilibrium method. By including direct raft contribution, the method provides an accurate tool for the reassessment of existing foundations, reducing the risk of unnecessary retrofitting. Comparison against non-linear finite element analysis results also defines its applicability range. The outcome is a simple closed-form formula for pseudo-static seismic design and verification, requiring only geometry and standard geotechnical parameters, and offering a practical alternative to complex numerical analyses.