Numerical Simulation on Optimization of Support System for a Tunnel through Debris Flow Deposits

Based on Nianggai village tunnel in Nyingchi-Lhasa section of national highway 318, three optimized measures proposed were aimed at deficiencies of steel frame and shotcrete net support for a tunnel through debris flow deposits. The numerical models with complete consistence were established to analyze the effects of each optimized measure comparatively. The results show that three optimized measures are of effective benefit for controlling vault settlement and peripheral convergence, and the effect of grouting behind initial support, optimized bolt and double-layer densifying reinforcement mesh are in descending order. The backfilling grouting is prone to cause the uplift of inverts, therefore, the grouting amount and pressure must be strictly controlled. The axial force of feet-lock bolt is increased to 110.3% compared to the original one after removing the systematic bolts. The distribution of positive and negative bending moment for the initial support is more uniform, and then it is more advantageous to the structure stress. The bending moment of the support is increased to 1.63 times of the original one when double-layer densifying reinforcement mesh is adopted. A larger range of surrounding rock around the cavern is mobilized to participate in bearing capacity together. Furthermore, the bending moment of supporting structures and axial force of the bolts are reduced to 67.6% and 79% compared to the original one. It is indicated that the proposed optimized measures are significantly effective to control tunnel deformation and reduce the stress of supporting structure. The research conclusions are supposed to provide reference for the selection of tunnel support method in debris flow accumulation.