Experiment Study on Tunnel Crown Collapse and the Bolt Anchoring Effect in Weak and Broken Rock Mass

摘要

Tunneling in weak and broken rock mass will inevitably disturb surrounding rock mass, which results in instability failure due to the excessive deformation or the high stress generated in rock mass. Therefore, anchor bolts are always used as early support in practical engineering. In this paper, the weak and broken rock mass of grade IV according to Chinese codes is taken as the reference prototype. Subsequently, the trapdoor model test was carried out to simulate tunnel excavation in such type of rock mass, for the purpose of studying the progressive failure characteristics, the failure mode, and the bolt anchoring effect. The test results show that tunneling will inevitably form a disturbed zone around the tunnel, but only part of the rock mass in this zone collapses into tunnel arch shape. Secondly, due the support effect of bolt, the rock mass above tunnel crown presents the trend of damaging in partition. Thirdly, the damage zone mainly concentrated in a fan-shaped region, and the angle between this zone boundary and the horizontal plane is about 45°+(Φ/2. Finally, after the tunnel excavation, an arch-shaped bearing belt with high tangential stress will appear in the rock mass, about 1.0 to 1.25 B away from the tunnel crown.