Abstract
In 2015, after amendments to the explosives regulations, stemming became a mandatory activity for all South African mining operations. There are, however, circumstances in which it is thought stemming has an adverse impact on the blasting outcome. Some of these circumstances include blasting in hot holes, in reactive ground, or when blasting a pre-split. In order to determine when stemming is necessary, its role in the control of adverse blasting phenomena and impact on explosive performance were reviewed. Stemming was found to play a significant role in the fragmentation process and burden movement. Additionally, stemming significantly influences the control of flyrock, air-blast, and toxic fume generation. The review of the literature indicates some motivation for not using stemming for pre-split, trim, hot hole, and reactive ground blasting, provided the benefits associated with not stemming the holes outweigh the risks of stemming them. Best practice for stemming from the literature indicates a stemming length of 0.7 x burden is best for larger hole diameters, and 20 to 30 x 0 for smaller hole diameters. Crushed aggregate appears to be the most effective stemming material. The South African explosives regulations pertaining to stemming were found to be consistent with those of Australia and the USA.
Highlights
The practice surrounding the use of stemming has in recent years become topical and, in some cases, a controversial subject
Should a mine require the use of stemming plugs, the efficiency of these can be maximized by using an air deck as well as small-diameter boreholes with the appropriate spacing and a highly decoupled charge with the correct split factor
It is recommended that stemming should be used during pre-splitting to reduce the adverse effects such as flyrock and air-blast associated with firing unstemmed charge holes, unless the mine can present a good case for exemption, e.g. due to the mine’s remoteness, where flyrock and air-blast will not influence the areas of concern addressed in the regulations
Summary
The practice surrounding the use of stemming has in recent years become topical and, in some cases, a controversial subject. Zhang et al (2020) state ‘up till stemming has not been widely used in underground blasts, such as production blasts in underground mining, blasts in tunnelling and drifting, and blasts in other types of underground space construction using explosives’. They attribute this to the lack of quantitative studies demonstrating the necessity and importance of stemming in rock fragmentation. The South African regulations stipulate that the purpose of stemming is to reduce the hazards associated with unconfined explosives, blowouts, flyrock, and harmful gases escaping the shot-hole
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