This paper presents a grain-based discrete element model for simulating the quasi-brittle failure of rocks and associated permeability evolution under hydro-mechanical loading. In this approach, the development of crack along grain boundaries controls the degree of damage and the associated permeability alteration in material. As a result, the overall permeability of the model, which is a function of degree of micro-cracking and crack connectivity, can be calculated at each stress state. Micro-mechanical parameters of the model is calibrated to Lac du Bonnet granite such that the model reproduces the physics similar to that of the rock during compression and tension. The numerical experimentations demonstrate the capability of DEM-Voronoi model to mimic the pre- and post-failure response of materials. The calibrated model accurately predicts, in a quantitative sense, the macroscopic properties of granite such as elastic properties, damage thresholds, peak strengths, triaxial strength envelope, and hydraulic conductivity of granite at critical damage thresholds.

1 Introduction

It is well-documented in geomechanics-related literature that the permeability of the rock is highly stress-dependent (e.g. Souley et al. 2001). This stress-dependency is due to the fact that when rock is subjected to mechanical loading, the rock microstructure can close, open, extend and induce new cracks, which in turn change the mechanical and hydraulic properties of rock. As a result, the stress-dependent permeability is a function of two fundamental mechanisms:

  • microcracking-induced permeability change which is related to growth of the micro-cracks, and

  • stress-induced pore volume change in which the pore volume is a function of interaction between fluid pressure and mechanical stress.

In the former case, damage-modified permeability evolution relates to heterogeneous nature of rock material and presence of internal micro-structure with different sizes and properties. Thus, when the rock is subjected to a compressive load, local tensile stress concentration induced on these internal micro-structures initiates a complex process of fracture propagation in rock. Generation of these crack corridors (channels) modifies the overall permeability of the rock by the extent that is dependent on the opening, density, connectivity of such cracks.

This content is only available via PDF.
You can access this article if you purchase or spend a download.