Failure investigation of the tunnel lining in expansive mudstone – A case study

•The failure mechanism of the preliminary lining when tunneling in expansive mudstone is analyzed based on a tunnel case in northern China.•The expansive and softening characteristics of mudstone when encountering water are both considered in the calculation model.•The relationship among expansion force, humidity expansion coefficient and initial water content of mudstone for the case tunnel is proposed. When studying the deformation characteristic of tunnel lining in the expansive surrounding rock, most research focuses on the expansion effect of surrounding rock after water absorption, while seldom considers the softening effect caused by water absorption at the same time. Therefore, based on a failure case of a tunnel in expansive mudstone in northern China, this paper investigates the expansion characteristics of the mudstone and the deformation as well as the failure law of tunnel lining through laboratory tests, field monitoring and numerical simulations. The results show that the mudstone exhibits significant water disintegration and softening characteristics, with a noticeable deterioration in strength. The clay mineral content of the mudstone is 68.8 %, and evaluations of various indicators indicate moderate expansion potential. The failure modes at the crown of the preliminary lining are compressive failure, which are coincident with that of field monitoring. The expansion of the surrounding rock leads to a sharp increase in the internal forces of the preliminary lining, significantly weakening its safety and causing large deformation even damage. The maximum deformation of lining reached 449 mm, necessitating the dismantling and rebuilding of the damaged preliminary lining. Therefore, in the design and construction of tunnels in mudstone, considering the expansive potential is essential to ensure the safety of the project.