Abstract

Distinct Element Method (DEM) was used to investigate the influence of the stress path resulting from the excavation of a tunnel on the amount of rock mass damage generated (i.e., stress fracturing) leading to v-shaped notch formation. The DEM model was calibrated to the laboratory properties measured on specimens of Lac du Bonnet granite in undamaged (intact) and damaged (by drilling) state. Stress paths from a two-dimensional (2D) finite element model at points on and near the tunnel wall were applied to the calibrated undamaged DEM model. It was found that the amount of damage (i.e., number of micro-cracks) in the models decreased rapidly with increasing distance from the excavation wall (i.e., increasing confinement). This suggests that the depth of fracturing is controlled by the confinement-dependent rock strength characteristics. It is concluded that the depth of stress fracturing and failure occurring near the excavation boundary (i.e., spalling and breakout formation) can be predicted by applying a 2D stress path to a DEM model calibrated to the true intact rock strength of a material that takes into consideration the grain-scale geometric heterogeneity and progression of the failure process.

1. Introduction

The excavation of underground openings causes a stress disturbance in the vicinity of the excavation. If the ratio of induced stress to rock strength is high, this excavation-induced stress may lead first to rock damage, then to stress fracturing, and eventually to spalling that evolves to form a v-shaped notch. The stress path nearby an advancing excavation may involve significant stress changes in both magnitude and orientation. Two main factors influencing the stress path are the ratio of far field principal stresses, and the direction of excavation relative to the orientation of these in situ stresses.

Figure 1 compares the stress path of a point inside a core during drilling from a borehole parallel to minimum principal stress (σ3), and the stress path of a point at the wall of a tunnel excavated parallel to intermediate principal stress (σ2). These stress paths were obtained from an elastic three-dimensional finite element model (Bahrani et al., 2012) with properties of the undamaged Lac de Bonnet (LdB) granite and in situ stresses representative for the 420 level of the AECL's Underground Research Laboratory (URL) located in the Lac du Bonnet (LdB) granite batholites in Manitoba, Canada (Martin, 1997).

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