Analysis on the Construction Timing and Stability of Double-layer's initial supports of Soft Rock Tunnel Based on Convergence-Confinement Method

Many tunnels face large soft rock deformation problems in Southeast China. Double-layer's initial support could effectively solve the problem of large deformation in soft rock tunnels, which can flexibly control the timing and ensure support stability. Combined with the Yuntunbao Tunnel of Chengdu-Lanzhou Railway, the timing and stability of the double-layer's initial support in soft rock tunnel were studied based on the convergence-confinement method. The ground reaction curve was calculated under different models, and the longitudinal deformation profile was evaluated using numerical simulation. Empirical equations helped to compute the support characteristic curve. The convergence-confinement curve was drawn to discuss the second layer's initial support timing according to the results mentioned above. After the verification of field data, the effectiveness of numerical simulation was initially verified. The results show that: 1. The ground reaction curve obtained by the H-B model is closer to the engineering reality than the curve obtained by the M-C model. 2. Compared with the bench method, the complete section method is unfavorable to the support, and the support safety factor is lower. 3. The first layer's initial support should be implemented immediately after excavation, and the second layer's initial support should be executed when the first layer reaches 70% of its limit deformation.