Failure Mechanism and Active Support of Large-Span Tunnel with Unstable Rock Blocks: A Case Study.

Unstable block collapse is an important factor affecting the safety of tunnel construction process. Based on the collapse accident of a block in Qingdao, the failure mechanism of a large-span tunnel with unstable blocks and the control effect of the length, prestress and spacing of pre-stressed rock bolts on the surrounding rock are studied through the model test and combination of numerical calculation, the on-site support optimization scheme is proposed. It is found that the surrounding rock on both sides of the vertical joint has a slight dislocation first, and then the joint opens, and the rock on both sides roughly forms a relative cantilever structure. Pre-stressed rock bolt has the function of limiting block slip and providing radial binding force, and pre-stressed rock bolt restricts the opening of joints and enhances the bearing capacity of surrounding rock. The bolt length is the key factor affecting the bolt support effect, followed by prestress force, and finally the bolt spacing. [ABSTRACT FROM AUTHOR]