Vibrations induced by time-delayed double blastholes in underground rocks: Experimental study and theoretical analysis

Time-delayed blasting is widely utilized in engineering to mitigate induced vibration hazards and enhance fragmentation. The underlying vibration reduction principle is the decrease of the charge weight per delay, while the potential for further vibration reduction remains debated, largely due to unclear underlying mechanisms. In light of the popularization of electronic detonators and the representativeness of double-hole configurations for multiple blastholes, it is essential to investigate the vibration characteristics induced by time-delayed double blastholes. Therefore, a series of double-borehole experimental blasts was conducted in an underground roadway to clarify the variation in vibration from single-hole to dual-hole conditions. Based on the experimental data and inherent limitations, an exact full-field theoretical model was further employed to systematically analyze the effects of delay time, charge length, and borehole inclination angle on vibrations induced by various double-hole configurations. The experimental data and theoretical analysis reveal that the general scaled distance effectively predicts vibrations in delayed blasting but does not reflect vibration reduction. Increasing delay time causes fluctuating PPVs, which stabilize slightly above single-hole PPVs as delay times exceed a certain value. The delayed blasting primarily reduces near-field frequencies. Longer charge lengths in double boreholes increase PPV levels and attenuation rates within a certain length, and the vibration behavior of combined long and short charge lengths is governed by the long blasthole. Larger blasthole inclination angles enhance vibration amplitude and reduce PPV attenuation rates. Optimizing inclination angles is more critical than adjusting delay times, and parallel boreholes offer the best vibration control.