LAUSR

Abstract The flat-jointed (FJ) contact model implemented in the particle flow code resolves the long-standing issue of the code producing unrealistically low ratios of uniaxial compressive strength (UCS) to direct tensile strength. Despite the improvements introduced with the FJ model, an FJ sample calibrated to the UCS still produces a crack initiation stress much lower than the expected value of approximately 0.4 UCS, as commonly observed in laboratory compression tests for most rocks. In this study, we investigated the role of initial crack volume on the crack initiation stress of a rock. We found that the microcracks among the mineral grains and their associated void space played a major role in controlling the stress magnitude associated with crack initiation. We also observed that the closure of these cracks between mineral grains was the primary driver of the crack closure phase in the compression test, and the elastic bimodularity was due to the crack density. We developed a methodology to incorporate the initial microcracks using the FJ contact model to match the crack closure stress and bimodularity of a laboratory rock sample. By introducing microcracks and voids among the mineral grains, we automatically capture the crack initiation stress of Lac du Bonnet granite measured at approximately 40% of the UCS.