Abstract: When the Daliangshan tunnel crosses through expansive soft rocks with high geostress, the tunnel bottom uplifted and the secondary lining continuously collapsed. To address these issues, field investigation, monitoring experiment, and numerical simulation methods are employed to analyze the support failure evolution mechanism of large deformation tunnel and optimize the design of the support scheme. A support optimization scheme of deepening the inverted arch + laying the steel flower pipe + thickening the secondary lining is proposed according to the tunnel disease characteristics and the field test. The feasibility of the support optimization scheme is validated through numerical simulation, and the optimized support scheme is applied in the experimental section. Main findings 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: highway tunnel, expansive soft rock, numerical simulation, failure mechanism, support optimization