Hybrid lattice/discrete element analysis of spalling failure in rock tunnels

A framework for the discontinuum-based simulation of spalling failure in rock tunnels is proposed. This innovation involves the combined application of the damage-initiation and spalling-limit (DISL) approach with the hybrid lattice/discrete element method (LDEM), a special bonded-block modeling technique that incorporates Rigid Body Spring Network lattice bonds between stochastically generated Voronoi cells. First, verification analyses are conducted to understand the deformability and strength behaviors of the LDEM models via unconfined-compression, triaxial, and direct-tension tests on numerical granite and limestone models. The results reveal that LDEM does not require a calibration procedure to define Young's moduli, Poisson's ratios, tensile strengths, and damage-initiation strength envelopes of the models. Moreover, an equation that relates tensile strength and fracture toughness with the mesh size of the model was derived from tension tests on center-cracked specimens. Subsequently, the DISL-LDEM was applied to two rock tunnel spalling cases, i.e., a square tunnel in Cobourg limestone and the circular Mine-by Experiment tunnel in Lac du Bonnet granite. In-situ observations, finite-element analyses, and empirical data were used to compare the simulated failure modes and spalling-breakout dimensions. Using the results obtained from the study cases, a framework for DISL-LDEM simulations of underground excavations was established. Notably, this novel approach does not require trial-and-error simulations and establishes a clear one-to-one relationship between each numerical input parameter and model behavior. Therefore, the proposed framework acts as a practical numerical tool for discontinuum-based simulations of spalling in rock tunnels.