Experimental and numerical studies on progressive failure characteristics of weak surrounding rock mass of tunnel and its anchoring effect

In view of the collapse accidents encountered in the process of tunneling in weak and fractured rock mass, scaled model tests and numerical simulations are developed to study the progressive failure process of surrounding rock mass as well as the characteristics of ground surface deformation and stress of rock mass under different bolt support modes. According to the results, the following conclusions can be drawn: (1) Tunnel excavation may cause the overburden load to transfer from the crown to the both side walls, resulting in the shear failure of rock mass below tunnel waist, therefore, the foot lock bolts can be used to halt this initial failure. (2) The failure of rock mass mostly starts from the tunnel periphery and then develops gradually outwards, which eventually leads to the emergence of two sliding surfaces appeared with an angle of 45°+/2 to the horizontal plane and a natural equilibrium arch formed above the tunnel arch, and the bolt support can effectively reduce this collapsed region. (3) Due to the improvement of stress state in surrounding rock mass, bolt support not only increase shear strength value of rock mass at the tunnel waist but also make the rock mass above tunnel crown bear even larger overburden load before completely failure. (4) The tangential stress of rock mass within the collapsed region drops down sharply, and this feature can be used to determine the collapse range of rock mass and is also regarded as the early warning signal for tunnel collapse.