Publication informationEmpirical rock properties and continuum mechanics provide a basis for defining 11 relationships between a variety of mechanical properties, such as strength, friction 12 angle, Young's modulus, Poisson's ratio, on the one hand and both porosity and crack 13 density, on the other. This study uses the Discrete Element Method (DEM), in which 14 rock is represented by bonded, spherical particles, to investigate the dependence of 15 elasticity, strength and friction angle on porosity and crack density. A series of 16 confined triaxial extension and compression tests was performed on samples that were 17 generated with different particle packing methods, characterised by differing particle
[1] We present a new method to implement realistic grain fracture in 3D numerical simulations of granular shear. We use a particle based model that includes breakable bonds between individual particles allowing the simulation of fracture of large aggregate grains during shear. Grain fracture simulations produce a comminuted granular material that is texturally comparable to natural and laboratory produced fault gouge. Our model is initially characterized by monodisperse large aggregate grains and gradually evolves toward a fractal distribution of grain sizes with accumulated strain. Comminution rate and survival of large grains is sensitive to applied normal stress. The fractal dimension of the resultant grain size distributions (2.3 ± 0.3 and 2.9 ± 0.5) agree well with observations of natural gouges and theoretical results that predict a fractal dimension of 2.58. Citation: Abe, S., and K. Mair (2005), Grain fracture in 3D numerical simulations of granular shear, Geophys. Res. Lett., 32, L05305,
[1] We investigate how existing veins interact with extension fractures in rocks using 3-D Discrete Element Method models with a geometry inspired by tension tests with notched samples. In a sensitivity study, we varied (a) the angle between the vein and the bulk extension direction and (b) the strength ratio between host rock and vein material. Results show a range of vein-fracture interactions, which fall into different, robust, "structural styles." Veins, which are weaker than the host rock, tend to localize fracturing into the vein, even at high-misorientation angles. Veins, which are stronger than the host rock, cause deflection of the fracture tip along the vein-host rock interface. Fractures are arrested at the interface from weak to stronger material. When propagating from a stronger to a weaker material, macroscopic bifurcation of the fracture is common. Complex interactions are favored by a low angle between the vein and the fracture and by high-strength contrast. The structural styles in the models show good agreement with microstructures and mesostructures of crack-seal veins found in natural systems. We propose that these structural styles form the basis for criteria to recognize strength contrasts and stress of crack-seal systems in nature.Citation: Virgo, S., S. Abe, and J. L. Urai (2013), Extension fracture propagation in rocks with veins: Insight into the crack-seal process using Discrete Element Method modeling,
[1] The friction of granular fault gouge plays an important role in governing the mechanical behavior and hence earthquake potential of faults. Using numerical modelling, significant progress has recently been made towards understanding the micro-mechanics that drive fault gouge evolution. Despite these insights, many previous numerical models have predicted unrealistically low macroscopic frictional strength. Here we describe modified 3D discrete element simulations of fault gouge evolution. Our particlebased simulations, modelled on laboratory experiments, include breakable bonds between individual particles (or particle clusters) allowing fracture of aggregate grains. With accumulated strain, grains break up, evolving in size and shape to produce a textural signature reminiscent of natural faults. Cluster-simulations, producing pseudo-angular daughter fragments yield realistic frictional strength (0.6). Non-cluster simulations, producing angular and spherical daughter fragments, have much lower friction levels. We therefore demonstrate that gouge fragment shape and resulting interactions dominate the frictional strength of faults. Citation: Abe, S., and K. Mair (2009), Effects of gouge fragment shape on fault friction: New 3D modelling results, Geophys. Res. Lett., 36, L23302,
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