Structure Optimization for the Support System in Soft Rock Tunnel Based on Numerical Analysis and Field Monitoring

Optimization of the support used when constructing tunnels in soft surrounding rock has long been a hotspot in engineering. To control the deformation of soft rock and ensure construction safety, this paper proposes a support scheme involving weakening the anchor bolts while enhancing the rigidity and strength of the primary supports. This was realized by combining the large deformations that often occurred during the construction of the Youfangping tunnel of the Gucheng-Zhuxi expressway. Moreover, the scheme was analyzed and compared with the original scheme and one involving weakening the anchor bolts. In addition, the displacement deformations, force conditions on the anchor bolts, development of plastic zones, stress on the shotcrete, and force conditions on the secondary lining structure were analyzed via numerical simulation for the different support schemes. Concomitantly, three groups of experimental sections were selected on-site to monitor and measure the deformations and force conditions of the surrounding rock for these three support schemes. The numerical simulations and field-monitoring results show that weakening the anchor bolts has a small effect on the overall support provided by the support system. However, it can simply the process and reduce engineering costs. Moreover, increasing the rigidity and strength of the initial supports can effectively control the large deformations in the surrounding rock. Therefore, a support scheme in which anchor bolts are weakened while the rigidity and strength of the initial supports are enhanced, as presented in this paper, proves to be feasible method of support optimization. It also provides a useful reference for optimizing other similar tunnels along the expressway.