Abstract: In order to solve the problem of the bottom drum disaster and the continuous collapse of the second lining when the Daliangshan Tunnel crosses the high geostress expansive soft rock formation, the analysis of the failure evolution mechanism of the large deformation tunnel support and the optimization design of the support scheme were carried out by means of field investigation, monitoring experiment and numerical simulation, and the support optimization scheme of deepening the inverted arch + laying the steel flower pipe + thickening the second lining was proposed according to the characteristics of the tunnel disease and the field test test, and the feasibility of the support optimization scheme was verified by numerical simulation, and optimized the support scheme in the experimental application. The main conclusions are as follows: (1) The water-softened surrounding rock at the arch bottom experiences a marked decline in its bearing capacity, while stress concentration emerges at the non-eroded arch foot. The convergence of downward loads transmitted from overburden pressures and upward expansion forces originating from the softened rock base at the arch bottom culminates in localized shear bulge failure at the arch foot; (2) The unilateral bulge of the inverted arch diminishes the horizontal restraint provided by the secondary lining. Consequently, under the horizontal stress imposed by the surrounding rock, the tunnel's lateral flanks are compressed inward, leading to concrete crushing and steel extrusion failures within the arch shoulder to arch crown region, where internal compressive stresses are relatively high. The subsequent loss of horizontal confinement in the surrounding rock enhances the plastic zone extent, particularly towards the arch shoulder on the side of the inverted arch's uplift, exacerbating both the arch bulge and the failure of the secondary lining; (3) The optimized support design fundamentally strengthens the load-bearing capacity of the surrounding rock and refines the internal force distribution within the secondary lining. After optimization, a notable reduction is observed in the extent of tensile and shear failures within the plastic zone of the surrounding rock. Furthermore, disasters such as bottom bulge and secondary lining collapse, triggered by water erosion at the arch bottom, are effectively contained, demonstrating the efficacy of the proposed support enhancement measures.

Key words: expansion soft rock; highway tunnel, numerical simulation, disaster mechanism, support optimization