Numerical Investigation of the Influence of Borehole Orientation on Drilling-Induced Core Damage

Damaged and disked core from boreholes are indicators of high stress relative to the intact rock strength at the drilling location. While core disking is mainly used as a means to estimate the in situ stress state, core damage (micro-fractures) and its level need to be identified to allow for the accurate estimation of the in situ intact rock strength and therefore proper design of underground infrastructure such as tunnels and pillars. It is generally recommended that laboratory testing be done on samples from boreholes drilled parallel to the major principal stress. In this study, results are reported from an investigation on the influence of borehole orientation on the drilling-induced core damage and associated strength and modulus reductions. The drilling-induced coring stress paths, for boreholes drilled parallel to σ1 and σ3 within a stress state representative for the 420 Level at the Underground Research Laboratory, were first obtained from an elastic three-dimensional finite element model. The coring-induced stress path for the borehole drilled parallel to σ3 was then applied to a two-dimensional discrete element model previously calibrated to the undamaged Lac du Bonnet granite to create damage in the form of micro-cracks. Once the model was calibrated to both undamaged and damaged LdB granite, it was used to predict the damage level and the unloading-induced micro-crack characteristics of the core from the borehole drilled parallel to σ1. The results confirm the effect of sample disturbance on rock strength and modulus measured in the laboratory and potentially offer a mean to model this process and quantify drilling-induced core damage.

, 2012