Experimental study on the effect of flexible joints of a deep-buried tunnel across an active fault under high in-situ stress conditions

During dislocation, a tunnel crossing the active fault will be damaged to varying degrees due to its permanent stratum displacement. Most previous studies did not consider the influence of the tunnel's deep burial and the high in-situ stress, so the results were not entirely practical. In this paper, the necessity of solving the anti-dislocation problem of deep-buried tunnels is systemically discussed. Through the model test of tunnels across active faults, the differences in failures between deep-buried tunnels and shallow-buried tunnels were compared, and the dislocation test of deep-buried segmental tunnels was carried out to analyze the external stress change, lining strain, and failure mode of tunnels. The results are as follows: (1) The overall deformation of deep-buried and shallow-buried tunnels is both Sshaped. The failure mode of deep-buried tunnels is primarily characterized by shear and tensile failure, resulting in significant compressive deformation and a larger damaged area. In contrast, shallow-buried tunnels mainly experience shear failure, with the tunnel being sheared apart at the fault crossing, leading to more severe damage. (2) After the segmental structure design of the deep-buried tunnel, the "S" deformation pattern is transformed into a "ladder" pattern, and the strain of the tunnel and the peak stress of the external rock mass are reduced; therefore, damages are significantly mitigated. (3) Through the analysis of the distribution of cracks in the tunnel lining, it is found that the tunnel without a segmental structure design has suffered from penetrating failure and that cracks affect the entire lining. The cracks in a flexible segmental tunnel affect about 66.6% of the entire length of the tunnel, and cracks in a tunnel with a short segmental tunnel only affect about 33.3% of the entire length of the tunnel. Therefore, a deep-buried tunnel with a short segmental tunnel can yield a better anti-dislocation effect. (4) By comparing the shallow-buried segmental tunnel in previous studies, it is concluded that the shallow-buried segmental tunnel will also suffer from deformation outside the fault zone, while the damages to the deep-buried segmental tunnel are concentrated in the fault zone, so the anti-dislocation protection measures of the deep-buried tunnel shall be provided mainly in the fault zone. The results of the above study can provide theoretical reference and technical support for the design and reinforcement measures of the tunnel crossing active fault under high in-situ stress conditions.