concrete broker
Jun 26 · 3 min read


Maximise your blast efficiency and reduce unwanted side effects

Cover photo courtesy of Muswellbrook Shire Council


The process of rock fragmentation by blasting has been used in the mining industry for the past century. Explosives are employed in the blast hole in a variety of ways to fracture the rock and dislodge the broken rock to enable excavation equipment to remove the rock as easily as possible. Explosives are extremely energetic substances in compact form and the utilisation of the energy released in an effective manner is the aim of all good mining operators. The energy released from the detonation of an explosive charge occurs in millisecond time frame and extracting useful work from this energy is of prime concern. A major portion of the explosive energy is in the form of gas energy and it is this portion that is said to perform the movement of the fragmented rock mass. The transmission of this gas energy into the confining medium in the blast hole will determine the success or otherwise of the blast performance. In other words, when blasting we need to ensure the gas emitted when blasting is confined within the blast hole.

Miners frequently utilise crushed hard rock that is approximately 20 to 25mm in diameter to stem the top of a blast hole in order to prevent this gas from escaping out the top of a blast hole and into the atmosphere, therein reducing the impact of the blast. Stemming has the additional environmental effect of mitigating excess noise levels at the time of a blast.

Photo Courtesy of Concrete & Quarries


Stemming is a material that is put inside of a blast hole to help prevent gases from escaping.

While stemming is typically put in the top of a blast hole, it can also be used to help bridge mud seams or weak layers. Improperly placed stemming can greater decrease fragmentation size. This is because if a stronger layer is stemmed that means their is no explosive to fragment that strong rock. In weak layers the rock is already being easily broken up. When stemming across a seam it is also important to put primers in both powder columns and ensure that both will fire. Stemming is also extremely important in the top of a blast hole. Using proper stemming is one of the best ways to increase your fragmentation and sizing ranges.


Stemming relies on many of the same variables as burden, and because of this we can use a simple equation to calculate the appropriate stemming depth.

Stemming (feet) = 0.7 x Burden (feet)

Stemming (meters) = 0.7 x Burden (meters)

It is important to note that these equations are made for using appropriate sized stemming. A good stemming is crushed rock as it will form a plug in the hole, confining the gases.

Most importantly from a safety perspective the use of stemming material in blast holes prevents fly rock and air blasts.

Concrete and Quarries recommends only stemming material that is cubicle in nature, and not sharp, obtrusive or elongated. This material, currently utilised by large miners including BHP, maximises the efficiency of your blast. By purchasing stemming material from Concrete and Quarries you the shot firer have less chance of cut offs, over pressure, vibration and flying rock during the blast process. Another advantage of purchasing stemming material from Concrete & Quarriesis that the hard rock from which it is sourced, does not exhibit any signs of silica dust, reducing employee exposure to the disease known as silicosis. Silica dust can be located in the crushed sedimentary rock located adjacent to coal seams.

For an obligation free quotation on stemming material for your next mining blast email Concrete & Quarries or click here to visit their website for further information.

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