Progressive Failure of Blocky Rock System: Geometrical-Mechanical Identification and Rock-Bolt Support

Excavation of shallow-buried, hard rock masses usually leads to the instability of rock blocks delimited by discontinuity planes. Classic block theory only deals with the stability of rock blocks on the exposed surfaces. However, in rock engineering practice, it is quite common that when surface unstable blocks collapse along with their constraints to adjacent blocks released, more inner blocks will be exposed to the surface and become unstable or fall down. This process is termed the progressive failure of blocky rock mass. Based on three-dimensional (3D) block-cutting analysis, the removable blocks in progressive failure process can be determined through geometrical and kinematics analysis. This study proposes to further identify unstable key blocks from removable blocks using mechanical analysis. On this basis, the anchoring force required for all of randomly distributed key blocks is calculated, and the spatial distribution of the required anchoring force per unit area of the free surface are analyzed, with suitable probability distribution obtained. Afterwards, adopting reliability-based design concept, the design scheme for systematic rock bolting to prevent progressive failure of blocky rock mass is discussed, and the method is presented for estimating the anchoring force of individual rock bolt, bolt spacing, and anchorage length.