Resolving explosives inefficiency

Stemming is normally a rock or sand material that is generally placed on top of the explosive in a blasthole.

Many blasters use stemming, as it can reduce the air overpressure levels from a blast by more than 98 percent when properly contained. Additionally, proper stemming can lead to large decreases in fragmentation and reduction of mucking cycle times by more than 18 percent.

This is because stemming is what ensures the explosive is breaking the rock and not making noise. The proper stemming will maximize the efficiency of the explosive.

If blastholes are throwing stemming into the air, there is a good chance the blastholes are under-stemmed. This leads to large increases of up to 6 decibels (dB) in air overpressure and can increase the P80 size by more than 10 percent in an average blast.

Conversely, if the shot is over-stemmed the amount of explosive in the blasthole is reduced and the fragmentation size is also increased. Over-stemming a blast can also lead to large boulders forming from the top of the bench.

In order to achieve efficient explosive use, stemming must be carefully designed and evaluated as a major blast design variable.

Stemming materials

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 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.

A liquid that is used is water when blastholes are loaded with emulsion. These provide almost no retention of the explosive gases and function extremely poorly as a stemming material.

Solids are materials such as concrete and plaster. While these materials are strong, they can be expensive and take a long time to solidify. They also are not extremely effective at stemming alone because they break loose from the wall and are ejected as one large unit at very high velocities.

Sands and gravels (river gravels) are classically defined as materials that have a high sphericity but can be considered anything that is round regardless of the size. Drill cuttings are generally included in this category because of their similar function. These materials provide a longer general retention time of the explosive gases yet do not form a plug and typically blow out, releasing the explosive gasses prematurely.

Crushed rock is considered the best for stemming and creates a plug while holding the stemming for a long period of time. It is 40 percent more efficient than sands and gravels (and drill cuttings).

In almost all cases, the use of properly sized crushed rock stemming can facilitate the design of blasts in which no rock is thrown above the bench. This is frequently achieved in the field where many lock and dam projects the U.S. Army Corps of Engineers regulates cannot have any rock go 20 to 40 ft. above the bench.

Generally, this is also one of the cheapest and easiest stemming methods to use, as most sites have the proper crushed rock in their stockpiles.

Stemming sizing

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 size of the stemming is actually of significant importance where the total amount of stemming used can be decreased by more than 30 percent. 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 1/4 in. to 1/2 in. will serve as the best stemming size for blastholes up to 10 in. in diameter.

When a dirty stemming material is used, such as unwashed or well-graded material, the P50 can generally be used to make material considerations.

Stemming plugs

A stemming plug is a device that can be put into the stemming of a blasthole that is made to help stemming hold longer. These devices come in a number of different shapes, sizes and materials, with the goal of making non-ideal stemming such as sands, gravels and drill cuttings perform better.

The majority of these stemming plugs will make this non-ideal stemming hold longer and possibly retain the stemming completely like a crushed rock. In addition, by adding solid material into the middle of stemming columns of non-ideal stemming, the retention time can also be increased.

All these methods that increase the efficiency of non-ideal materials, however, cost money per blasthole and are normally more expensive than using a crushed rock. These non-ideal materials often still need more total length of stemming than crushed rock alone. Considering this, crushed rock stemming may be the best type of stemming material currently available.

What about using stemming plugs and solids with crushed rock stemming? While there has been no real data presented that the total depth of stemming could be decreased with this method, it is possible and would require testing by the individual mine site to determine if it is economical.

Stemming depth

After the proper stemming material has been selected, the total depth of stemming must be designed in order to achieve efficient explosive utilization. As a precursor to design, this stemming design assumes the rest of the blast, including burden and timing, are properly designed. An improperly designed blast cannot be saved with proper stemming design – only the efficiency of a proper blast can be greatly increased.

The stemming depth can then be designed as a relationship of the burden. This is because calculation of both the burden and the stemming rely on the same variables: borehole pressure, rock type and structural geology, to name a few. If the stemming can hold until the burden is broken, it will depressurize and completely hold for the duration of the blast.

Stemming, in general, will only blow out when the borehole is pressurized and the burden has not fully broken. The stemming design for crushed rock can generally be considered as 70 percent of the burden or calculated by using the formula below:

T = 0.7 x B
Where: T = Stemming depth (ft.)
B = Burden distance (ft.)

If crushed rock is not an option then drill cuttings may be used. However, using drill cuttings for stemming will greatly increase the chance of stemming blowout. This will decrease the total efficiency of the explosive. However, it is still better than no stemming at all.

In the case of a non-ideal stemming, the goal will be to increase the retention time of the blasthole to allow for maximum utilization. At the same time, over-stemming the blasthole will result in decreased fragmentation and boulders.

In order to create a best-case situation for non-ideal stemming material, the depth of the stemming will equal the burden or can be calculated with the formula below:

T = B
Where: T = Stemming depth (ft.)
B = Burden distance (ft.)

Loading practices

In many cases, drill cuttings are used because the blaster can easily and quickly shovel the drill cuttings back into the borehole. While this method may be quick, the results can be detrimental and costly for a mine.

While wheel barrels and shovels can be used for the loading of crushed rock, using small- to medium-sized loaders can provide a very fast and practical way to load blastholes. With a properly designed blast, the explosive should come up to the point where the stemming will start (continuous charge). This practice ensures complete utilization of the borehole. In this case, the stemming just needs to be slowly dumped into the drillhole and filled to the top.

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, a decrease of fragmentation variability, and an 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 dB of air overpressure, reducing the noise from blasting. These stemming improvements are normally easy to introduce, and the results can be seen immediately.

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Anthony Konya is an explosive engineer for Precision Blasting Services who consults around the world in rock blasting and vibration from blasting. Dr. Calvin J. Konya is the president of Precision Blasting Services and director for the Academy of Blasting and Explosive Technology, consulting and training worldwide in rock blasting, vibration and emulsion manufacturing.