STUDY ON CONTINUOUS-ALTERNATIVE DESIGN SCHEME OF ADVANCED ROCK BOLTS ON TUNNEL FACE

The instability of tunnel face is quite common in weak surrounding rock tunnels. The advanced rock bolts are effective supporting structure for the tunnel face. According to the conventional installation scheme, the advanced rock bolts have to be cut off and shortened with the excavation of tunnel, and their supporting effect can become weaker and weaker. The rock bolts can not be reinstalled until their length is cut to a predetermined value. The discontinuous installing of rock bolts leads to significant fluctuations in the supporting effect. In order to solve this problem, a continuous-alternative design scheme is proposed for advanced rock bolts on tunnel face. In the new scheme, the rock bolts on tunnel face are divided into several parts equally. The rock bolts are successively installed during the tunnel advancing process, so that the rock bolts supporting state can be more uniform and stable. In order to evaluate the feasibility of the new scheme systematically, a case analysis is carried out based on the Muzhailing Tunnel Project of the Weiwu Expressway. First, both conventional discontinuous scheme and continuous-alternative scheme are designed under the condition of the same amount of rock bolt. The finite difference software FLAC^3D is used to establish numerical model. In this way, the deformations of tunnel face and ground are compared for two different design schemes. The results show that the supporting effect of rock bolts is greatly influenced by the changes of the supporting states in the conventional discontinuous installation scheme. The continuous-alternative installation scheme can contribute a stable supporting effect on tunnel face without increasing the cost. Moreover, the supporting effect is much higher than the minimum level of conventional installation scheme. This study provides a new method for the arrangement of the advanced rock bolts on tunnel face.