Design implications of composite tunnel linings with varying stiffness and thickness in saturated sand: Insights from large-scale 1g shaking table tests

Composite tunnel linings constructed in saturated sand may experience complex seismic soil–structure interaction owing to the combined effects of wave propagation, excess pore water pressure buildup, and liquefaction-induced ground deformation. This study presents a series of large-scale 1g shaking table tests to investigate the seismic response of a shallow-buried composite double-layer tunnel lining system with varying inner lining stiffness and thickness. Sinusoidal sweep waves, low-amplitude recorded earthquake motions, and progressively intensified Tianjin earthquake motions were applied to examine acceleration amplification, lining strain redistribution, excess pore water pressure development, and macroscopic liquefaction-related phenomena under controlled saturated ground conditions. The results show that the predominant frequency of the saturated sand governs both acceleration amplification and pore pressure buildup, with pronounced responses occurring when the input frequency approaches the soil predominant frequency. Increasing the inner lining stiffness or thickness reduced lining acceleration by approximately 15–25% and outer lining strain by up to 30%, although excessive thickness occasionally caused localized amplification under high-frequency excitation. Liquefaction consistently initiated beneath the tunnel invert when the input PGA approached 0.3 g, regardless of lining configuration, and was accompanied by ground settlement, sand–water ejection, surface cracking, and slight tunnel uplift. The findings demonstrate that stiffness enhancement and thickness increase are not mechanically equivalent: increasing stiffness mainly redistributes seismic demand toward the inner lining, whereas increasing thickness modifies the response through a combined stiffness–mass effect. These results provide experimental evidence for understanding the seismic response of composite tunnel linings in saturated and liquefaction-prone ground.