Impact of side-by-side twin tunneling with various tunnel-pile clearances on pile group responses under a fluid-solid coupling effect

In urban subway construction, shield tunneling near pile groups is common, where additional loads may threaten existing structures. This study establishes multiple 3D nonlinear FDM models with fluid-solid coupling to investigate how tunnel–pile clearances (H_c) affect the mechanical response of low-cap pile groups (2 ×2) during side-by-side twin tunneling in composite strata. The advanced CYSoil model, incorporating nonlinearity, strain path dependency, and small strain behavior, is employed to simulate soil response. Results show that tunneling induces up to a ∼66.7 % reduction in pore water pressure, forming a funnel-shaped seepage pattern. As H_c increases from 0.8D to 2.6D, the low-pressure zone shifts from sidewalls to vault and invert, while maximum displacements reduce by up to 14.04 mm (lateral), 5.28 mm (transverse), and 19.68 mm (vertical). Axial force evolution in piles follows a three-stage decline, i.e., rapid, slow, and moderate, with peak shaft resistance concentrated near the tunnel axis. These findings aid in optimizing tunnel-pile configurations and mitigating geotechnical risks.