A discrete element exploration of V-shaped breakout failure mechanisms in underground opening

V-shaped breakouts, which may appear in underground opening during excavation, are the results of two different failure mechanisms: tensile spalling and shear fracturing. This study uses discrete elements in exploring the conditions that would lead to different breakout mechanisms under plane strain conditions. The test tunnel of the Mine-by Experiment in Lac du Bonnet granite batholith is adopted as the base problem. In order to carry out the study, some fundamental issues need to be addressed. First, an exponential softening bond that enables the incorporation of fracture energy is adopted. In order to obtain a reasonable ratio between the uniaxial compressive strength, σc, and the uniaxial tensile strength, σt, discrete disc particles are tied together to form an irregular shape clump as the basic discrete element. This effort is supported by a successful reproducing of test results from Lac du Bonnet granite in DEM modeling. The issue of sensitivity of discrete particle size on results is examined. The reduction of strength with increase in specimen size is also modeled. After the calibration work is completed, the Mine-by tunnel behavior is studied. Finally, this study shows that a reduction in σc/σt ratio, under the same setup, would cause the failure mechanism to transit from tensile spalling to shear fracturing in V-shaped breakouts.