Seismic response mechanisms of casing-shape composite tunnel lining: Theoretical analysis and shaking table test verification

In high-intensity seismic zones tunnel lining is generally utilized to provide efficient protection against tunnel collapse by withstanding pressure under strong earthquake events. However, this can be a challenging task in the case of fault-crossing tunnels, since tunnel lining follows the deformation of surrounding rock. To address this issue, the casing-shape tunnel lining system is proposed to reduce the constraint effects of surrounding rock on the lining structure by using an internal and external lining with a buffer layer employed in between. This research aims to investigate the rock-structure interaction and seismic response mechanism of casing-shape tunnels, both theoretically and experimentally, to facilitate the adoption of the proposed system. To develop a practical mechanical model for parametric study, the casing-shape tunnel lining is simplified as a Timoshenko composite beam. The theoretical analysis results indicate that the external lining and buffer layer can effectively reduce the earthquake-induced internal forces on the internal lining and there is an optimal length of casing-shape lining to balance cost-effectiveness with seismic resistance protection. To validate the proposed simplified model of casing-shape tunnel lining, a series of shaking table model tests are then performed on large-scale casing-shape lining models with earthquake-induced fault slippage under incremental input intensity excitations. The results demonstrate that: (i) the inclined angle of fault considerably affects the adopted length of casing-shape tunnel lining, (ii) the fault slippage shears the external lining directly leading to severe seismic damage at arch springing and invert, and (iii) the upper part of external lining and the whole internal lining generally exhibit a satisfying level of protection. This highlights the need to select appropriate cross-section shapes and structural materials for the construction of casing-shape tunnel lining systems. The outcomes of this study can provide theoretical foundation and experimental evidence for the seismic response mechanisms of casing-shape tunnel lining in practical applications.