Numerical simulation of crack propagation induced by smooth blasting in tunnels

Smooth blasting is widely used to control overbreak in drill-and-blast tunnelling, yet the coupled effects of charged-hole spacing and intermediate empty-hole diameter on contour formation remain insufficiently quantified. In this study, an empty-hole-assisted smooth-blasting scheme was investigated for a shallow-buried tuff tunnel using an equivalent rock-mass Riedel–Hiermaier–Thoma (RHT) model. The model parameters were derived from laboratory and field acoustic data and the generalized Hoek–Brown criterion, and the element-scale tensile strength was verified through mesh and tensile-strength sensitivity analyses. A full-factorial double-hole study (charged-hole spacings of 0.6–0.8 m; empty-hole diameters of 0–62 mm) was conducted. A damage threshold of D > 0.5, corresponding approximately to a 30% reduction in P-wave velocity, was used to separate macroscopic fractures from the retained damaged rock. Empty holes acted as artificial free surfaces that guided point-to-point crack propagation along the contour and suppressed radial cracking. A 0.7 m spacing combined with a 22 mm empty hole (0.7–22) gave the most favourable. a spacing of 0.8 m left rock bridges, whereas empty holes of 42 mm or larger caused additional crushing without further improving crack guidance. The tunnel-scale simulation of the conventional scheme (0.6 m spacing, no empty hole) reproduced the irregular overbreak and deep radial cracking observed in the field. Compared with this scheme, the 0.7–22 scheme reduced the maximum overbreak depth from 0.254 m to 0.099 m (61.0%), the overbreak area by 72.2% and the non-penetrated length of the perimeter-hole line from 26.2% to 5.1%; these reductions remained stable for thresholds of D = 0.3–0.7. The scheme reduced the number of charged contour holes and the contour-hole explosive by 14.3% but increased the total number of drilled holes, so its main benefit lies in contour quality rather than in direct cost savings. The conclusions apply to shallow tunnels with low in situ stress and remain to be confirmed by field trials.