NUMERICAL INVESTIGATION OF DRILLING-INDUCED CORE DAMAGE IN HARD BRITTLE ROCKS

Abstract

At different design stages of underground excavations, cored samples are taken and tested to obtain geotechnical design parameters. When samples are retrieved from high-stress environments, damage in the form of micro-cracks may result in incorrect estimates of these parameters. This research investigates the impact of coring and overcoring on damage formation and the subsequent implications for geotechnical design parameters, using the well-documented case of the Underground Research Laboratory (URL) in Canada. It involves the generation of two-dimensional (2D) models: a) continuum-based heterogeneous (four mineral types) and homogeneous (one mineral) models; and b) hybrid continuum-discontinuum models consisting of triangular and Voronoi grains. The 2D models are calibrated against the laboratory properties of Lac du Bonnet (LdB) granite and subjected to an unloading (coring) stress path obtained from a 3D continuum model. The simulation results using continuum models highlight the importance of grain-scale property heterogeneity in the formation of unloading-induced damage. The simulated grain and grain boundary damage in the heterogeneous model result in a reduction in Young’s modulus and Unconfined Compressive Strength (UCS) by up to 29% and 22%, respectively. Using the hybrid method, it is demonstrated how grain-scale heterogeneities promote tensile stresses, leading to micro-crack initiation and opening, and subsequent nonlinearity in the stress-strain curve due to crack closure. In the next step, core drilling and overcoring are explicitly simulated using three-dimensional (3D) continuum and discontinuum models for various in situ stress magnitudes and borehole orientations at the URL. The tensile strength and crack initiation stress level of intact LdB granite are used to assess the potential for damage in the 3D continuum models. This, combined with the number of micro-cracks from the 3D discontinuum models, demonstrate an increase in potential for core and overcore damage with increasing depth, with fewer micro-cracks observed in samples drilled parallel to the major principal stress.