Distinct Element Method (DEM) Modelling of Laboratory to Outcrop-Scale Fracturing of Natural Rocks

In the Distinct Element Method (DEM), as implemented in PFC, rock is represented as an assemblage of spherical particles that can be cemented together. Cement breaks if its strength is exceeded and sliding occurs at uncemented contacts if Coulomb friction is exceeded. The mechanical behaviour of the model material is not predefined, as in continuum approaches, but rather the user predefines microproperties (particle and cement properties) and determines macroproperties (elastic and strength parameters) from numerical lab experiments. It is this emergent behaviour that makes DEM ideal for investigating mechanical property relations and fracture processes.

3D DEM investigations of the impact of porosity and crack density on rock properties show that DEM is capable of reproducing the lab-scale behaviour and failure criterion of rocks. Numerically derived mechanical property relations (Young's modulus, Poisson's ratio, strength, friction angle) are consistent with those described from continuum mechanics, where available, and those of lab tests on natural rocks. For example, Young's modulus, strength, friction angle and the unconfined compressive strength to tensile strength ratio (UCS/T) decrease with increasing porosity, whereas Poisson's ratio is (almost) porosity independent; the pre-eminent control on UCS/T is, however, crack density.

Recent work shows that 2D and 3D DEM model materials when, calibrated to the rheological properties of rock and subjected to appropriate boundary conditions, are also capable of modelling outcrop-scale fracture processes, such as the propagation of normal faults through mechanically layered sequences. Multilayer fault models replicate many of the structures observed in outcrop, including fault refraction, splaying, normal drag and segment linkage and provide support for empirically derived positive correlations between fault zone width and displacement. These results combined with ongoing modeling studies on other fracture-related phenomena, such as jointing and caldera collapse, provide positive grounds for application of DEM to a broad range of fracture-related issues in geology.