Investigation of the influence of segment thickness on the anti-flotation stability of twin shield tunnels in water-rich strata

Abstract

Anti-flotation stability is a critical concern for shield tunnels constructed in water-rich strata, particularly for twin shield tunnels, where excavation-induced disturbance interacts with groundwater buoyancy. This study investigates the influence of segment thickness on the anti-flotation stability of twin shield tunnels through three-dimensional numerical simulations and physical model tests. Five segment thicknesses were considered for the Xi’an Metro Line 8 tunnel section. The results show that increasing segment thickness improves anti-flotation stability primarily by increasing the self-weight and stiffness of the segmental lining, thereby enhancing resistance to groundwater-induced uplift and reducing deformation of both the lining and the surrounding soil. When segment thickness increased from 200 mm to 400 mm, the maximum ground surface heave decreased from 13.0–11.1 mm, and the vertical displacement of the soil at the tunnel invert decreased by 14.7%. The vertical stress response at the tunnel invert was more sensitive to segment thickness than that at the tunnel vault, indicating that the invert is the critical location governing uplift stability. However, the benefits became less pronounced when segment thickness exceeded 300 mm, indicating a diminishing marginal effect. Considering deformation control, lining stress reduction, and economic efficiency, a segment thickness of 300 mm is recommended for the water-rich twin shield tunnel conditions investigated in this study. These findings provide a basis for optimising segmental lining design and anti-flotation control in similar shield tunnel projects.