Study on construction deformation law and support forms of tunnels in high‑geostress sand‑mudstone interbedded strata based on model tests

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Aiming at the asymmetric large deformation encountered during the excavation of tunnels in sand–mudstone interbedded strata under high in-situ stress, this paper takes the Jianshan Tunnel along the Zhonglan section of the Yinlan High-Speed Railway as the engineering background and adopts a combined approach of discrete element numerical simulation and physical model testing to systematically investigate the surrounding rock deformation laws and support mechanical response of soft–hard interbedded tunnels under different bolt support forms. First, the control effects of symmetric and asymmetric arrangements of long–short bolts on surrounding rock displacement and crack distribution are comparatively analyzed using the discrete element method. Second, based on similarity theory and orthogonal tests, sandstone and mudstone similar materials satisfying the required physico-mechanical indices are prepared, and a self-developed model test system capable of three-dimensional high-geostress loading is established. Finally, under the most unfavorable condition of a 45° dip angle and 1:1 layer thickness ratio, model tests are conducted for two support schemes—systematic equal-length bolts and asymmetric long–short bolts—to monitor the evolution of surrounding rock deformation and circumferential stress during excavation. The results show that the asymmetric arrangement of long–short bolts reduces the maximum horizontal displacement difference between the left and right sides by approximately 40% and effectively restrains crack propagation. The surrounding rock deformation along the tunnel longitudinal direction can be divided into five zones—the unaffected zone, initial influence zone, tunnel face transition zone, deformation development zone, and stable zone—with the maximum excavation influence range extending from 0.5B ahead of the tunnel face to 1B behind it. Under asymmetric support, the circumferential stress range at each position is significantly narrowed and the tensile stress difference between the left and right spandrels tends to be balanced, indicating a notable improvement in the stress condition of the support structure. The findings provide reference for deformation control and support design of tunnels in soft–hard interbedded strata under high in-situ stress.