Numerical investigation of the mechanical behaviours of a pair of rock bridges at diverse scales in direct shear.

Rock bridges have long been recognised to provide crucial resistance against rock mass failure. Multiple rock bridges with diverse scales are widespread in natural rock masses. Despite prior positive contributions to a single rock bridge, mechanical behaviours of diverse-scale rock bridges remain elusive. In particular, whether rock bridges interact and how their failure processes evolve are still open questions. To fill in this gap, a block-based discrete element method, specifically the Universal Distinct Element Code, was employed to simulate diverse-scale rock bridges in granite subjected to direct shear to investigate their mechanical properties, stress and displacement fields, cracking processes, and acoustic emission characteristics. Results reveal a fundamental linear correlation between peak shear resistance and the proportion of rock bridges. As failure progresses, shear stress tends to concentrate at rock bridge tips, more prominently for the one farther from the shear loading end, attributed to the rotation of confining pressure plates. In addition, the irregularity of displacement distribution follows an arc-shaped configuration near rock bridges, and larger rock bridges display lower gradients in nearby displacement fields. Furthermore, wing cracks initiate from rock bridge tips in a tensile stress environment, with a greater length for rock bridges closer to the shear loading end. It is also found that rock bridges rupture in tension near the shear loading end and in shear further away. Expanding upon these findings and considering stress thresholds, we acquire new insights into the interaction patterns between diverse-scale rock bridges: when their sizes are similar, the rupture of the rock bridge closer to the shear loading end will expedite damage in the one farther away from the shear loading end towards its volume-expansion point; conversely, the rupture of a significantly larger rock bridge will overwhelmingly affect the smaller one. The identified interaction patterns provide significant proof of physical precursor patterns for reliably predicting the progressive failure of multiple rock bridges.