Stemming in blasting is very important and affect not only the efficiency of the blasting but also the impact of the blasting on the environment.
By simple definition, stemming is a confinement process done by the use of innert materials to confine explosive charge within the drill hole during charging.
The define function of stemming is to:
1. Ensure the explosive charge is confine within the hole socket so as to avoid poor distribution of explosive release shock energy.
2. Prevent the escape of shock wave into the environment and causing airblast and other environmental impact.
Stemming can be top confine or inter-column charge confined.
In case of decking charge pattern, each explosive charge column is separated by innert material to reduce the charge weight and also to ensure effective distribution of shock energy along the hole column.
In the case of decking, stemming is use to confine and to separate upper charge column from the lower charge column.
Stemming materials can be the drill cuttings from the hole or poorly sorted, well-graded gravel or stones.
Use of stones of well-graded nature with particle size ranging from 4mm to 9mm tend to show good confinement than drill cuttings due to it heterogeneity nature that permit the grains interlocking.
Some of the factors that affects stemming length are:
1. Hole diameter
15-30D
2. Bench height/Hole depth
Stemming=0.45-0.55H
3. Burden
Stemming=0.7-1.5B
4. Explosive energy/strength: high strength explosive requires longer stemming length to confine and resist the escape of shock energy to the surface
5. Rock properties: This includes the rock strength, discontinuity nature among others.
Most cases, stemming may be position at point where discontinuity daylight on the hole wall instead of placing booster.
Also, stemming length varies with the strength of rock formation to be blasted.
Hard rocks like granite (>250Mpa) is blast with stemming length varying from 20-30D.
Medium rocks like dolomite:20-25D
Soft rock :15-20D.
Reason why stemming length tend to increase with Rock strength is that, stronger rocks offers higher resistance against the Penetration of released shock energy at detonation. The release wave tends to follow weaker zone instead of fracturing and slabbing the rock mass, in a bit to relieve the detonation pressure, the stemming of short length may be pull out.
This is why, lengthy stemming is needed to confine the Explosive energy within the rock mass.
In the process of selecting lengthy stemming the place of ensuring good and sufficient charge length must also be considered so as to enable good fragmentation.
Blasting with high safety condition, getting good blasting result alongside with good economic factor is what define a successful Blasting
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