Improving Explosive Efficiency with Proper Stemming
The use of proper stemming for decreasing noise and increasing fragmentation in blasting.
Stemming is a material that is poured into a borehole after the explosive has been loaded in order to confine gas pressure. Many blasters acknowledge that stemming can reduce air overpressure, often cutting it over 98% compared to an unstemmed borehole. However, stemming is also critical in improving fragmentation, pulling the toe of the blast, and reducing ground vibration. How can stemming effect all this?
It starts with understanding the principles of using explosives to break rock. Breaking rock does not use ‘shock energy’ but instead uses the confined gas pressure that is created by the explosive. The stemming is what holds this pressure in and causes it to act outward towards the burden, instead of escaping out the top of the borehole and breaking the top of the shot.
- Proper stemming has led to over 18% improvements in fragmentation and muck cycle times by allowing the gas energy to break the burden more efficiently. It has also resulted in a decrease of the P80 of a blast by over 10%
- Proper Stemming leads to better shearing at the toe of the bench, leading to reduction and elimination of toes from a blast. This also helps to eliminate cantilever bending of the bench.
- Proper Stemming increases the efficiency of the explosive allowing it to more easily break the burden and reduces the ground vibration from over-confinement
The choice a mine makes for the material that is used for stemming is one of the most important considerations for blasting. Stemming materials typically come in four different types; Liquids, Solids, Sands and Gravels, and crushed rock. These materials vary in their ability to form a ‘plug’ and the time that they can hold the gas pressure. A plug is formed when the stemming material is crushed together, forming a solid rock plug across the entire area of the borehole, this being one of the most important parts of stemming. Without the proper stemming material the stemming will quickly vent and release the gas pressure in a blast prematurely. This leads to dramatically negative results on fragmentation, muckpile throw and placement, ground vibration, and air overpressure.
After the decision to use a crushed rock stemming material has been made the next question is which stockpile to take this material from, or more importantly what size crushed rock is the best. The sizing of the stemming is actually of significant importance where the total amount of stemming used can be decreased by over 30%. This allows more explosive to be placed into the borehole, resulting in better breakage or an expansion of the blast dimensions (burden and spacing). In general, crushed rock that is ¼” to ½” will serve as the best stemming size for blastholes up to 10 inches in diameter.
When a dirty stemming material is used, such as unwashed or well graded material in general the P50 can be used to make material considerations.
The use of proper stemming material and depth of stemming can lead to large economical and community advantages for a mine. The economic advantages of proper stemming come from an increase in fragmentation, decrease of fragmentation variability, and increase of the utilization of borehole for explosives. The community impact of proper stemming design is often seen in a reduction of up to 6 decibels of air overpressure, reducing the noise from blasting. These stemming improvements are normally easy to introduce and the results can be seen immediately.
Want to learn more about stemming? View Anthony Konya’s and Dr. Calvin J Konya’s recent article in the May 2017 edition of Pit and Quarry by clicking here
Want to learn more about stemming and other aspects of rock blasting? Take the Mechanics of Rock Breakage course by clicking here