Mechanical response mechanism and parameter optimization of double-arch tunnels without middle drift during construction.

Due to the high construction efficiency and low cost, double-arch tunnels without middle drift has been gradually promoted in engineering in recent years. However, the excavation characteristics of the left and right tunnels are easy to cause structural disturbance and deformation out of control. In particular, the surrounding rock disturbance of the front tunnel caused by the construction of the secondary tunnel often leads to lining cracking and structural instability. Based on the Xinmajie Tunnel project in China, combined with similar model experiments and finite element simulation, the structural mechanical response and failure evolution characteristics during the construction process were systematically studied, and the influence of the excavation of the secondary tunnel on the stress mechanism of the front tunnel was mainly revealed. On this basis, a comparative study was further carried out on the influence of different excavation methods, excavation footage and tunnel face offset parameters about the structural stress. The results show that the deformation of the surrounding rock experiences three stages of "rapid increase‒rapid decrease‒slow increase", and the disturbance of the secondary tunnel makes the maximum settlement of the front tunnel arch crown increase by 25.56%. The axial force and bending moment of the right arch waist lining of the front tunnel are the largest, and the stress concentration phenomenon is prominent, which is the most unfavorable part of the force. During the loading process, the cracks of the lining structure are concentrated in the weak areas such as arch shoulder, arch crown and inverted arch. In addition, the excavation method of the middle partition wall (first left and then right) and the control of small excavation footage are helpful to weaken the disturbance, and alleviate the stress concentration. The 12 m staggered distance of the left and right tunnel faces is the best in controlling the settlement and plastic zone expansion. It is recommended as an important parameter reference for similar tunnel layout.