Micromechanical modeling of inherently anisotropic rock and its application in borehole breakout analyses

Abstract

Excavation of boreholes, tunnels, caverns and underground galleries in anisotropic rock usually has a higher risk of failure than in isotropic rock since beddings act as weak planes which cause redistribution of stress and serve as fracture initiators. However, in previous studies underground constructions are usually modeled by simplifying the anisotropic condition to transverse isotropic condition without rigorous consideration of the effect of rock anisotropy on the stress distribution, failure mechanism/pattern, and preferred instability. In this study, the mechanical behaviors of inherently anisotropic rock were investigated at grain-scale with the use of discrete element method (DEM) under different stress conditions. To achieve this goal, a novel numerical modeling technique is developed based on DEM, in which the rock matrix is represented as bonded particle model and the existence of inherently anisotropy is explicitly represented by imposing individual smooth-joint contacts into the bonded particle model with the same orientation.

The research was first started with proposing a new set of micro-mechanical analyses linking the transmission of load to the evolution of micro cracks which allow us to extract innovative features emerged from the stresses and fracture propagation in later simulations. True triaxial compression tests were simulated on isotropic rocks to evaluate the effect of intermediate principal stress 𝜎2. In the brittle regime, micro-cracks concentrate in a band striking sub-parallel to 𝜎2. With the increase of 𝜎2 and 𝜎3, the distribution of contact force becomes more homogeneous which results in the distribution of cracks transform from localized to be distributed.

Systematic parametric studies were conducted to assess the effect of smooth-joint parameters on the mechanical properties of anisotropic rock samples under both uniaxial compression test and Brazilian test conditions. After that, step-by-step procedures for the calibration of micro parameters were recommended. The numerical model was calibrated to simulate the behaviors of rocks with different degrees of anisotropy. Various failure modes i.e. elastic mismatch, sliding wing crack and compression induced tensile crack, were found to be the dominant mechanism with different anisotropy angles. The new numerical approach was then applied in the simulation of stress-induced borehole breakouts in anisotropic rock formations at reduced scale. Effects of different factors including the particle size distribution, borehole diameter, far-field stress anisotropy and rock anisotropy on the stress distribution and borehole breakout propagation were systematically evaluated. Spiral-shaped shear fractures can be obtained in relatively homogenous model while three V-shaped concentrated fractures arise in the more heterogeneous model as wider particle size distribution results in the local stress perturbations which cause localization of cracks. Rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress. Far-field stress anisotropy plays a secondary role when drilled parallel with beddings. This study provides insights on how to orientate structures in a preferential direction and to make safer and more economic designs of underground structures.