Influence of detonator delay scatter on rock fragmentation by bunch-holes blasting

Bunch-holes blasting (several blastholes arranged in a bunch with short spacing and detonated simultaneously to form a large common blasting crater) is typically used in underground mines to increase blasting efficiency under a single free surface. Considering the low accuracy of pyrotechnic detonators, which are commonly used in Chinese mines, bunch-holes blasting with the same nominal times is actually short-delay blasting, and the actual delay intervals induced by the delay scatter of detonators can lead to unsuccessful bunch-hole blasting, which seriously affects rock fragmentation. In this study, theoretical analysis is firstly conducted to explore the mechanism of short-delay bunch-holes blasting, and the formation time of a new free surface is defined as the critical delay interval of bunch holes. Then, the formula of the new free surface forming time is deduced to determine the critical delay interval of the short-delay bunch-holes blasting. Considering the delay scatter of the detonator, the probability method is used to quantitatively analyse the probability of successful bunch-holes blasting with different delay nominal time detonators. Field experiments are performed to verify the results. This research demonstrates that as the detonator delay scatter increases, the probability of successful bunch-holes blasting generally decreases. In addition, the charge length also has an impact on the probability, which is positively correlated but not as dramatic as the detonator delay scatter. Hence, it is recommended to use high-precision detonators or low-segment detonators for bunch-holes blasting to ensure the quality of the cutting blasting.