Development of grain-based model for crystalline rocks and geological applications to faulting and seismic activities

Abstract

The two-dimensional grain-based model (GBM) implemented in PFC2D (particle flow code, GBM-PFC2D) is an advanced modeling approach for simulating the mechanical behaviors of crystalline rocks. GBM-PFC2D mimics the rock material as an assembly of deformable, breakable polygonal grains cemented along their adjoining sides. Despite the distinctive advantages of GBM over many other rock models in mimicking the crystalline structure and allowing grain breakage during rock deformation, two key issues in the conventional GBM have been identified: (1) using a Voronoi-tessellated network is insufficient to capture the geometric complexities of real mineral grains, and (2) GBM approach is currently confined to laboratory-scale simulations. This thesis aims at developing a robust GBM for more realistic simulation of the microstructures as well as the mechanical behaviors of crystalline rocks, and establishing a potential bridge between geological investigations of faulting mechanisms and the theoretical context of microfracture growth. In the GBM-PFC2D formulation, both stochastic and deterministic methods for more realistic characterization of rock microstructures are developed. Based on the results of thin-section examination, the stochastic method is demonstrated to be capable of reproducing some salient microstructural features in dominating rock mechanical behavior, namely grain size, grain shape and the shape-preferred crystallization orientation. Using the digital image processing technique, the deterministic method provides an algorithm to directly map the image-based rock microstructures into GBM-PFC2D. Meanwhile, the effect of initial microcracks on the mechanical behavior is incorporated into the GBM by the application of a discrete fracture network model. Another advancement of the GBM is to incorporate the coupled thermo-mechanical mechanism. Based on experimental and numerical results, this study demonstrates that the rock would be strengthened rather than weakened upon heating in the mild temperature range 25-200℃. As the temperature of the earth's crust rises with the increase of the burial depth, making it a vital factor governing the mechanical behavior of rock. This finding is of significant importance to a wide range of underground rock engineering, such as geothermal energy extraction and nuclear waste repository. The developed GBM has been applied to throw light on the faulting and seismic activities: (1) fault damage zones in field are characterized, by uniaxial compression tests on rock specimens containing different configurations of en-echelon fractures, which are indicative of fault evolution. The mechanisms of en-echelon fractures evolution into visible damage zones are revealed. (2) The stress-induced local fracturing processes under biaxial compression are quantified with respect to the location, moment and degree. (3) The seismicity activity and its correlation with local stresses along sheared smeared fault interfaces are investigated, which generates valuable insight into the mechanisms of fault reactivation and seismic hazard migration.