The surface damage and evolution of gouge materials in rock fractures or faults undergoing shear can change fracture properties in terms of shear strength and dilation, fluid transmissivity and retardation for contaminants. In order to conceptually understand fracture surface damage and gouge behaviors in fracture voids, the particle mechanics models were used to simulate the process of gouge particle plowed off from fracture asperities and subsequently their evolution in a fracture segment undergoing shear. The results show that significant abrasion and damage occurred by wearing the contact asperities and cracking the fracture surfaces. Gouge particles behave in two different ways under low and high normal stresses, respectively. Under low normal stress, gouge particles mainly roll with the moving fracture walls, with little surface damage. Under high normal stress, gouge particles can be crushed into a few major pieces and a large number of minor comminuted particles, accompanied by more severe damage (abrasion and micro-cracking).
It is experimentally observed that damage and wear usually occur at the rock fracture surfaces undergoing compression and shear, which is the main cause of gouge material generation (the sheared-off or broken mineral particles) from asperities of opposite fracture surfaces in contact. The gouge particles can be further crushed into even smaller pieces with increasing shear displacements or normal stresses, during which the gouge particle size, position and distribution can change significantly (Sammis et al. 1987; Scholz 1987; Haggert et al. 1992; Pereira & de Freitas 1993). Four basic mechanisms including adhesion, abrasion wear, corrosion and surface fatigue were classified (Belem et al., 2009). The fracture surface damage and generation and evolution of gouge material in rock fractures can play a key role in fracture mechanical and transport properties in terms of friction coefficient, shear strength, fluid transmissivity, solute retardation coefficient and etc. (Jing & Stephansson 2007; Zhao et al. 2012). However, the progress of research on fracture surface damage and gouge evolution have been relatively slow, mainly due to the technical difficulty in quantitatively measuring the micro-crack development, rate of gouge production, movement and distribution of gouge materials experimentally.
The main objective of this study is to understand the fracture surface damage and gouge particle evolution (movement and breakage) in a rough rock fracture undergoing direct shear, using a particle mechanics model. The modeling results that exhibited similar behaviors observed in experiments provide implications for macroscopically hydro-mechanical behaviors of rock fractures.