Determination of Optimized Excavation Pattern and Execution of Tunnel Support in the Weak Rocks (A Case Study)

Excavation of a tunnel can cause changing stress concentration and development of probable displacements in the surroundings of excavated area. In the excavation process، excavation of tunnel head is the most critical step and if excavation of this part is not performed properly، then possibility of instability will be increased. In the Top-heading and Bench operation of tunneling، in addition to geo-mechanical properties، extension of plastic zone depends upon the operation circumstance. In this regard، advance rate of the tunnel head and bottom are very effective in the tunnel stability. To determine these parameters، numerical and experimental approaches can be employed. At present، numerical methods are successfully utilized in the stability analysis of the tunnels and for this purpose، three dimensional numerical modeling has given more realistic results in simulation of tunnel face and its effect on the support system. In this paper، stability analysis is performed for a tunnel which is excavated with Top-heading and Bench method having advance rate of 1 m and 2. 5 m for the tunnel head and bottom، respectively. Software was employed for stability analysis and the results are compared to the experimental methods RMR and Q. On the basis of the obtained results، it was concluded that for preventing instability and reducing pressure on the support systems، it is necessary to consider advance rate of 0. 5 m for both the tunnel head and bottom. Considering the obtained results، it was practically observed that the amount of displacements and destructive effects on the support systems were noticeably reduced in the case study tunnel.