52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: Bonded-particle models (BPMs) provide a synthetic material consisting of a packed assembly of rigid grains joined by deformable and breakable cement at grain-grain contacts, and whose mechanical behavior is simulated by the distinct-element method. Contact- and parallel-bonded PFC2D and PFC3D models of circular and spherical grains suffer from the limitation that if one matches the unconfined-compressive strength of a typical compact rock, then the direct-tension strength of the model will be too large. This limitation has been overcome by creation of the flat joint contact model. This contact model provides the macroscopic behavior of a finite-size, linear elastic, and either bonded or frictional interface that may sustain partial damage such that the flat-jointed material can mimic the microstructure of angular, interlocked grains. Partial interface damage with continued moment-resisting ability (to resist grain rotation) is a microstructural feature necessary to obtain the relatively large compressive- to-tensile strength ratio of most compact rocks. The ability to match this ratio is demonstrated by creating a 3D flat-jointed material for Lac du Bonnet granite that matches the elastic modulus, direct-tension strength, and compressive strengths up to 6-MPa confinement.
What is an appropriate model for rock masses on the scale that appears in engineering and mining work? Such rock masses are broken up by structural features, notably joints, that have an important influence on the mechanical behavior, and lead us to consider the rock mass as being composed of intact rock and joints. Particle models that mimic the microstructural features of intact rock as well as the larger-scale joints can be created and executed in reasonable times on standard desktop computers. Such models are based on the premise that the mechanical behavior of a rock mass is controlled by the orientations and properties of the joints as well as the microstructure of the intact rock material, and complex macroscopic behaviors, such as rupture and sliding along joints as well as fracture and flow of intact rock, arise from structural and microstructural interactions; thus, if one could replicate the joints, the microstructure and the microstructural interactions within a model, then that model should reproduce the macroscopic behaviors. For the purposes of studying the dominant fracture and failure behaviors of intact rock in the brittle regime, a representation at the grain scale should be sufficient, because the damage processes either occur at this scale or their effects can be mapped to this scale. The flat-jointed bonded-particle model for rock is such a particle model, and its development has been guided by the premise that a closer match to the structural and microstructural features will provide a closer match to the real macroscopic behavior.
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