Assessment of the optimized stemming length considering rock fragmentation and escape of explosive gases using actual large-scale results

• This study provides a comprehensive evaluation of the impact of stemming length on rock fragmentation and blasting safety. • The role of stemming length has not yet been clear enough to use in rock fragmentation by blasting. • The impact of the S t /B ratio on the fragment size and blasting safety was investigated for different geomechanical conditions. • The optimized value of S t /B = 0.67 was found considering rock fragmentation and blasting safety. • For S t /B < 0.67, both the median fragment size and the amount of escape of blasting gases increased. • For S t /B ≥ 0.67, the median fragment size increased with an increase in the S t /B ratio with a normal escape of explosive gases. To avoid the discharge of explosive gases during blasting and utilize its energy for rock breakage, the explosive is confined within the rock mass, and the top part of the hole is blocked by the stemming materials. In blasthole design, the stemming length (S t ) plays a crucial role as it directly impacts important parameters such as rock fragmentation, blasting safety, amount of charge, and powder factor. Long ago, it was proposed that the stemming length should be equal to one-third of the blasthole length. However, subsequent research has demonstrated that the stemming length is influenced by the burden (B), and different values for the S t /B ratio have been reported in different resources. The impact of the S t /B ratio on rock fragmentation and escape of explosive gases has not yet been investigated to clarify a suitable value of the S t /B ratio. Since the blastability index (BI), which represents the rock mass parameters affecting rock fragmentation, varies from one zone to another, this study investigates the impact of the S t /B ratio on the fragment size of blasted rocks under different geomechanical conditions with varying blastability index values. The results showed that for S t /B < 0.67, both the median fragment size (X 50 ) and the amount of escape of blasting gases increased. For S t /B ≥ 0.67, X 50 increased with an increase in the S t /B ratio. The optimum value of S t /B = 0.67 was obtained considering both the rock fragmentation and safety due to the excessive escape of blasting gases. X 50 was minimized at S t /B = 0.67, and the amount of escape of explosive gases was also within acceptable limits based on actual large-scale results. Therefore, S t /B = 0.67 can be used as an optimized ratio for blasthole pattern design in practice. The sensitivity analysis based on the blastability index (BI) indicates that the sensitivity to stemming length variations increases with higher BI values, emphasizing that rock masses with higher BI are more reactive to changes in stemming length. These findings underscore the critical importance of precise stemming length adjustments to maximize fragmentation efficiency and operational performance.