Numerical investigation on crack propagation and coalescence induced by dual-borehole blasting

Dynamic directional fracture of dual-borehole blasting is very vital to the rock fragmentation and final contour of some projects, such as the tunnel and slope. In this paper, a rock-like material, polymethyl methacrylate (PMMA), is utilized to explore the effects of borehole distance d, delayed time td, and guide hole type on crack propagation behavior under dual-borehole blasting through numerical simulations. To improve the deficiency of constitutive model and the post-failure response of brittle material and reduce the mesh dependency, a coupling scheme of Johnson-Holmquist (JH-2) model, failure criterion, and crack softening failure model is adopted here. First, the crack propagation results of single-borehole blasting well reproduced inner and outer actions of blasting, which proved the reliability of this scheme. For the dual-borehole blasting, it was found that the increase of borehole distance indeed penalized the crack coalescence and the length of crack produced at the common guide hole. Compared with the common guide hole, the grooved guide hole at d = 30 cm contributes more crack coalescences, which is independent of the delayed time; at d = 40 cm, the longer cracks are produced at grooved guide hole than common one and the crack coalescence is related to the delayed time. The grooved guide hole was more conducive to the coalescence and lengthening of crack. More importantly, the crack coalescence manner showed mutual, direct, and indirect ones, and two manners could coexist in one model. In addition, guide hole also could enrich the crack coalescence manner. Lastly, the crack stagger depends on the change of tensile stress at one crack tip into the compressive one at another one; the crack coalescence indicates tensile stress fields of two crack tips have no mutual influence.