Modelling brittle fracture in weak rock with the bonded block method, from laboratory to tunnel scale

This paper presents results from a series of 3D bonded block models of weak rock with properties based on the Ashfield Shale in Sydney, NSW. The bonded block method is a subset of the discrete element method, where intact rock can be represented using an assembly of 2D polygons or 3D polyhedra with strong, stiff contacts. Results from a series of numerical UCS, triaxial, and direct tension tests confirm that the micro-scale breakage of block contacts can accurately reproduce laboratory scale brittle fracture behaviour. The laboratory simulation results are used to develop a tunnel scale bonded block model that explores the Voussoir beam analogue for flat roofed tunnels, with focus on the role of brittle fracture in progressive roof yielding. A simplified synthetic rock mass is constructed by embedding horizontal, cohesionless bedding discontinuities into a bonded block assembly. The bedding discontinuities promote delamination of shale beds and shear failure of bonded block contacts.

By adding rockbolts to the roof, several discrete beds can be stitched together to behave as a thicker equivalent beam. Rockbolt reinforcement inhibits fracture initiation and propagation, helping the rock mass to retain its inherent cohesion and tensile strength and establish a stable compression arch in the roof. The results demonstrate that the bonded block approach can help us to better understand the influence of ground support on progressive brittle fracture for tunnels in weak, horizontally bedded rock like the Ashfield Shale.