Study on Blasting Vibration and Fragmentation Control of Underground Powerhouse based on Time-sequence Effect of Millisecond Delay Initiation

During underground powerhouses blasting excavation, extended inter-hole delay times are commonly employed to safeguard critical structures like rock-anchored beams by preventing blast vibration peak superposition. This method, however, results in elevated boulder rates. To address this issue, vibration control and fragmentation optimization studies were conducted using millisecond blasting techniques. Using the linear superposition principle for blast vibrations, this study established a vibration amplification factor model that incorporates the number of holes and rock mass attenuation properties, and derived a computational formula for critical inter-hole delay times that meet vibration safety thresholds at varying distances. Field validation tests were conducted during the excavation of Xulong Hydropower Station′s underground powerhouse, confirming the theoretical model′s practical applicability. Results demonstrate that the optimized 50 ms inter-hole delay scheme, compared to the conventional 100 ms approach, maintains peak particle velocity at rock-anchored beam benches below 2.8 cm/s while achieving superior fragmentation performance. This configuration reduces the median fragment size from approximately 75.5 cm to 11.3 cm, significantly improving size-distribution uniformity and effectively minimizing boulder occurrence. The research establishes both theoretical foundations and practical methodologies for determining critical delay intervals to reconcile vibration safety and fragmentation quality in underground blasting operations.