Stemming Charges to Break Boulders

Using Stemming Charges to Break Cap Rock

Stemming is widely used to ensure loaded boreholes achieve optimum blast results, but can this stemming be creating boulders in your blast?

The utilization of stemming is now commonplace in mine and construction blasting projects as it leads to large decreases in air overpressure and ground vibration and increases to fragmentation and muckpile control. Oftentimes when too much stemming is used large boulders can be left after a blast at the top of a muckpile. The common response to this is a blaster will reduce the stemming length and in many cases the same amount or more boulders appear throughout the blast. This is because the shot has now lost confinement and the explosive energy is not being fully utilized. A good blaster will then resort to the previous stemming length, and in many cases blame geology for the caprock – accepting that up to 10% or more of the blast may be large boulders that require secondary blasting or that cannot be used.


But…what if this cap rock can be eliminated in the blast while allowing for proper function of the stemming?

The question then becomes, “how can a blaster modify a blast to reduce or eliminate any boulders from the stemming zone without impacting other aspects of the blast?” Good Question, and yes there is a way to do this through the use of a stem charge. A stem charge is a small explosive that is placed in the stemming which is detonated with the rest of the blast and can normally cut boulders way down, if not removing them entirely. This stem charge will often be a cap sensitive emulsion with detonating cord used to initiate it from the main column. This is done by wrapping detonating cord around the emulsion charge, and stringing the detonating cord down into the main column. Now when the column of explosives is detonated, the detonating cord will set off the emulsion charge at the same time.


Do you want to learn more about the application of stem charges and how to break boulders once they are in the pit? Check out Anthony Konya and Dr. Calvin J Konya latest article in the Engineering and Mining Journal on Breaking Boudlers in the Top Burden


Want to learn more about blast design, stemming, and optimizing blasts? Try taking the online “Mechanics of Rock Blasting” course