Sub-Additive Lateral Stiffness and Embedment-Controlled Synergy of Composite Caisson–Pile Foundations Under Combined V–H–M Loading

The lateral serviceability of bridge foundations is critical in scour-prone alluvial rivers under combined vertical, horizontal, and overturning-moment loading, in which neither a standalone caisson nor a pile group alone can adequately resist rotation and axial load. This analytical study evaluated the lateral behavior of composite caisson–pile foundation (CCPF) as a hybrid foundation system integrating the passive confinement and rotational rigidity of a large-diameter caisson with the flexural anchorage and redundancy of an internal pile group. Using three-dimensional finite element simulations, the lateral response of CCPFs was compared against caisson-only and pile-only reference systems by varying caisson embedment depth and pile number. Four novel performance indices were introduced: Lateral Synergy Ratio, Lateral Displacement Reduction Ratio, Critical Embedment Ratio, and Lateral Economy Index. The analysis demonstrated that lateral loading forces both components to compete for the same near-surface passive resistance zone, thereby inducing consistently sub-additive stiffness interactions across all configurations. CCPFs reduced service lateral deflection to 44%–93% of the matched-depth caisson-only value and 17%–91% of the pile-only baseline. These findings advance understanding of composite foundation behavior under lateral serviceability demands and provide design-oriented metrics for resilient bridge foundation solutions in riverine alluvial settings.