Stemming, the inert material packed on top of an explosive charge in a blast hole, plays a pivotal role in the effectiveness and safety of blasting operations [1]. The choice of stemming material directly influences blast performance by determining how well the explosive energy is contained and utilized for rock fragmentation. A well-designed stemming column is also a critical safety measure, significantly mitigating hazards such as flyrock and airblast [1].
The primary function of stemming is to confine the rapidly expanding gases produced by the detonation of explosives. This confinement is crucial for maximizing the energy directed into the rock mass, which in turn governs the degree of fragmentation [1]. The effectiveness of this confinement is largely dependent on the physical properties of the stemming material [2].
Key material properties and their impact
- Particle shape and angularity: angular, crushed materials are significantly more effective than rounded particles or fine drill cuttings [3]. The sharp edges of angular aggregates interlock with each other and the borehole wall, creating a much stronger seal that resists the upward force of the blast [4]. This interlocking mechanism is crucial for holding the stemming in place for the few critical milliseconds required for the explosive energy to fracture the surrounding rock.
- Particle size distribution: a well-graded mixture of particle sizes is ideal. Larger particles provide structural integrity and resistance to being ejected, while smaller particles fill the voids between the larger ones, increasing the overall density and preventing the easy escape of high-pressure gases. Materials dominated by fine particles, such as drill cuttings, are easily fluidized by the blast and ejected prematurely, leading to a significant loss of explosive energy.
- Density: denser materials provide greater inertial resistance to the upward thrust of the explosion. While not the primary factor, a denser stemming column will be more difficult to displace, contributing to better energy confinement.
- Cohesiveness: for certain applications, materials with some cohesive strength, like moist clay or specialized plaster-like products, can form a solid plug that provides excellent confinement. However, these may not be suitable for all situations and can be more complex to install.
By optimizing these properties, effective stemming leads to:
- Improved fragmentation: better energy confinement results in more extensive and uniform fragmentation of the rock mass. This is economically beneficial as it reduces the need for secondary breaking of oversized boulders and improves the efficiency of loading and crushing operations [4].
- Increased heave and muckpile displacement: proper stemming ensures that the explosive energy is effectively used to push the fragmented rock forward, creating a well-displaced muckpile that is easier and safer to excavate.
Mitigating hazards: the safety imperative of proper stemming
Ineffective stemming is a primary contributor to two of the most significant hazards in blasting: flyrock and airblast.
- Flyrock: this refers to rock fragments that are propelled beyond the designated blast area, posing a severe risk to personnel, equipment, and surrounding infrastructure [5]. When stemming material is weak or improperly installed, it can be ejected from the borehole at high velocity, and in some cases, the explosive gases can channel through the stemming and launch surface rock into the air [5]. The use of angular, well-graded stemming material significantly increases the resistance to ejection, thereby reducing the likelihood and severity of flyrock [6].
- Airblast: an airblast is a shock wave that travels through the air from the blast site. It can be caused by the rapid release of explosive gases into the atmosphere, a phenomenon exacerbated by poor stemming [7]. This sudden release of pressure can cause damage to nearby structures and is a significant noise concern [8]. By effectively containing the gases within the borehole, proper stemming minimizes the energy that escapes directly into the atmosphere, thus reducing the intensity of the airblast [8].
To sum up, the selection and proper application of stemming materials are not minor details in blasting operations but are fundamental to achieving both optimal performance and a safe working environment. The use of high-quality, angular, and well-graded stemming materials is a critical investment that pays dividends in improved fragmentation, more efficient operations, and, most importantly, the prevention of hazardous incidents.
Reference
[1] “o-pitblast.com/blog/stemming-control-in-blasting-a-crucial-factor-for-efficiency-and-safety#:~:text=This seemingly small detail can,explosive energy generated during blasting.” Accessed: Oct. 01, 2025. [Online]. Available: https://www.o-pitblast.com/blog/stemming-control-in-blasting-a-crucial-factor-for-efficiency-and-safety
[2] Y. Ko and K. Kwak, “Blast Effects of a Shear Thickening Fluid-Based Stemming Material,” Mining, vol. 2, no. 2, pp. 330–349, June 2022, doi: 10.3390/mining2020018.
[3] H. Cevizci, “A new stemming application for blasting: a case study,” Rem: Rev. Esc. Minas, vol. 66, pp. 513–519, 2013, doi: https://doi.org/10.1590/S0370-44672013000400017.
[4] Editor, “The impact of stemming in blast outcomes,” Quarrying Africa. Accessed: Oct. 01, 2025. [Online]. Available: https://quarryingafrica.com/the-impact-of-stemming-in-blast-outcomes/
[5] T. Szendrei and S. Tose, “Flyrock in surface mining Part II – Causes, sources, and mechanisms of rock projection,” Journal of the Southern African Institute of Mining and Metallurgy, vol. 123, no. 12, pp. 557–564, Dec. 2023, doi: 10.17159/2411-9717/2583/2023.
[6] K. Baluch, H.-J. Park, J.-G. Kim, Y.-H. Ko, and G. Kim, “Enhancing Rock Blasting Efficiency in Mining and Tunneling: A Comparative Study of Shear-Thickening Fluid Stemming and Plug Device Performance,” Applied Sciences, vol. 14, no. 13, p. 5395, Jan. 2024, doi: 10.3390/app14135395.
[7] M. Mpofu, S. Ngobese, B. Maphalala, D. Roberts, and S. Khan, “The influence of stemming practice on ground vibration and air blast,” Journal of the Southern African Institute of Mining and Metallurgy, vol. 121, no. 1, pp. 1–10, Jan. 2021, doi: 10.17159/2411-9717/1204/2021.
[8] A. B. Richards, “Predictive modelling of airblast overpressure,” Mining Technology, Dec. 2013, Accessed: Oct. 01, 2025. [Online]. Available: https://journals.sagepub.com/doi/abs/10.1179/147490013X13639459465619
