Recent years have seen a rapid increase in the use of Bonded Block Models (BBM) for studying the rock fracturing process, both at the laboratory-scale and at the field-scale. In laboratory-scale BBMs, the blocks are analogous to constituent mineral grains while the contacts between neighboring blocks correspond to grain boundaries. It is known that during a compression test on granite, heterogeneity-driven pre-peak damage initiates first along grain boundaries, but as the load is increased up to and beyond peak, intragranular fracturing starts to play an increasingly important role. This is especially true in triaxial tests where the confining stress suppresses the formation of local tensile stresses within a specimen. It follows that a BBM developed to simulate rock behaviors over a wide range of confinements should allow for both grain boundary cracking as well as cracking or damage within the grains (or blocks). Since grain boundary cracking is an inherent feature of BBMs, this study instead focuses on the approaches for simulating the grain damage process. In particular, grain damage in UDEC BBMs can be explicitly modeled by introducing failure pathways within each block or it can be implicitly modeled using inelastic blocks. To identify which of the two approaches is superior from a phenomenological and a computational standpoint, a comparison is made between an elastic BBM (no grain damage), an inelastic BBM and an elastic BBM with explicit fractures within each block in terms of their ability to reproduce the geomechanical attributes (unconfined and confined) of a granitic rock. It was found that the elastic BBM with no provision for grain damage overpredicted the peak strengths for high confinement conditions. The introduction of inelasticity or explicit fractures within the blocks, both led to a good match in peak strengths for the entire range of confining stresses tested, although the latter failed to properly replicate the confinement dependency of peak dilation angle and the post-peak response.

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