Investigation on radial fracturing around borehole under combined static stress and blasting

Abstract

Blast-induced radial fracturing is generally subjected to geo-stress. This study experimentally, theoretically and numerically investigates the radial fracturing behaviours around a rock borehole under combined static stress and blast loading by combining blast testing, elastic mechanics analysis and finite element modelling. The study begins with 11 blast tests on 100 mm cubic granite samples under various uniaxial and biaxial stress states based on a biaxial hydrostatic loading system. The fracture networks are subsequently image-processed in ImageJ where the morphology and fractal characteristics of radial fracturing in the rock surface are examined. Then, the variations in fracture patterns are analyzed using elastic mechanics, and the corresponding crack extension is numerically simulated in LS-DYNA. The mechanisms of the influence of static pressure on blast-induced radial cracking are revealed. The results demonstrate that the static stress greatly suppresses the blast-induced radial fracturing, resulting in fewer and shorter radial cracks with lower propagation velocity. This leads to a reduction in the fractal dimension of fractures and rock damage, which is attributed to the initial stress offsetting the blast-formed tangential tensile stress. Meanwhile, uniaxial pressure governs the propagation direction of radial fractures, as the cracks initiate at the borehole wall where the maximum hoop tensile stress presents and propagate along a path with the smallest circumferential compression. At last, the implications of the current results for practical blasting are discussed. It is advisable to leverage the directional cracking advantage offered by blasting under anisotropic in-situ stress to achieve a smoother excavation profile.