Experimental Study on Tunnel Bottom Deformation Trend in Gently Inclined Layered Shale Based on the Energy Index

Influenced by the anisotropy and water-softening characteristics of gently inclined layered shale, many tunnels have encountered bottom deformation issues during construction and operation, which severely impact the safety of tunnel structures. The energy evolution law during rock deformation and damage can provide support for the assessment and prediction of structure deformation. However, most studies have been conducted on enstatite, granite, and sandstone with limited research on shale. In this study, both conventional and single-cyclic loading-and-unloading uniaxial compression tests were conducted on shale specimens with varying dip angles of the structural plane (Dφ) and water content (Wc) in addressing the most typical layered shale in the Chaoyang Tunnel. The energy evolution features of rock samples at each stage of the tests were analyzed to determine the discriminating indicator (SC) for tunnel bottom deformation tendency. The indicator was based on the elastic strain energy (U) and the post-peak dissipation energy (U). The results demonstrated that the Dφ and Wc directly affected the energy storage and dissipation process of rock specimens, which in turn enabled them to exhibit different damage evolution features. The U and the total input energy (U) satisfied a linear relationship, which was determined by the Dφ and Wc of rock specimens. The energy evolution-based indicator SC can accurately characterize the bottom deformation of the tunnel constructed in a gently inclined layered shale stratum. The findings can offer a scientific foundation for rational evaluation of the structure deformation of tunnels under construction.