Numerical Evaluation of Twin Tunneling Effects on Battered Piled Raft Foundations in Soft Clay under Lateral and Combined Loads

The rapid expansion of underground infrastructure in dense urban areas often necessitates the construction of twin tunnels adjacent to existing piled raft foundations. This study investigates the complex soil–structure interaction of battered piled raft systems subjected to lateral and combined (vertical and lateral) loading in soft clay during sequential twin tunneling. Three-dimensional finite-element analyses incorporating coupled consolidation and an advanced hypoplastic clay model with small-strain stiffness were conducted and validated against centrifuge data. The analyses explored three representative tunnel positions relative to the piled raft along the pile shaft [cover-to-diameter (C/D) = 1.33], near the pile toe (C/D = 2.33), and beneath the pile toe (C/D = 3.53) to capture depth-dependent tunneling effects. The results showed that twin tunneling caused lateral displacement, differential settlement, pile deflection, and bending moments, with the response being most pronounced when twin tunneling advances near the pile toe. The first tunnel induced substantial stress relief, and stiffness degradation, while the second tunnel further displaced the foundation toward the first tunnel owing to prior stiffness loss. Combined loading consistently increased tunneling-induced responses compared with lateral-load-only conditions by increasing shear strain in the surrounding ground, degrading stiffness, and reducing both shaft and end-bearing resistances. The raft enhanced overall stability by redistributing load between the piles and the surrounding soil; however, its resistance reduced as tunnels advanced, particularly in deeper cases. Load transfer mechanisms, axial force redistribution between front and rear piles, and localized stress path changes near the pile toe critically influenced the observed deformation patterns. These findings highlight the coupled influence of twin tunneling, loading conditions, and tunnel depth on the serviceability and structural performance of piled raft foundations in soft ground.