Simulation study on the mechanical evolution law of double-arch tunnel construction without middle drift

The construction method without middle drift can simplify the construction organization process of double-arch tunnels, thereby improving construction efficiency. However, the mechanical evolution of the surrounding rock and support system, as well as the working mechanism of the support structure, remain unclear, which is unfavorable for structural design and construction safety control. Taking the Xinjia Tunnel as a case study, this paper establishes a three-dimensional nonlinear finite element model incorporating the complete construction sequence and the full set of support structures. The mechanical responses of the surrounding rock and support structure are calculated and analyzed under four working conditions with longitudinal spacings of the preceding and following tunnel faces set at L = 10.5 m, 36 m, 60 m, and 90 m. The stress transfer mechanism of the surrounding rock and the mechanical response evolution characteristics of each support component are systematically analyzed. The results show that the stress and deformation of the surrounding rock and support structure in a double-arch tunnel without middle drift exhibit significant asymmetry, and the stability of the goose-wing-shaped surrounding rock zone at the top of the middle partition wall is the key focus of construction safety risk control. A smaller longitudinal spacing between the preceding and following tunnel faces leads to a stronger superposition effect of construction disturbances and a significantly increased risk of support structure damage. The research findings can provide a theoretical basis and technical reference for the design and construction of double-arch tunnels without middle drift under similar geological conditions.