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Blasting Theory_082120.pdf

The document discusses blasting theory, explaining that detonation of an explosive in rock causes 3 stages: initial crushing from high pressure, propagation of compressive stress waves that cause cracking, and expansion of cracks by high pressure gases. Proper blast design is needed to produce rock fragmentation of a size suitable for the intended use, such as crushing or construction, with the three main factors controlling fragment size being explosive quantity, distribution, and rock structure.

Engineering◦
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Presented by: Dr Clement Kweku Arthur
Learning Objective  Having worked through this chapter, the student will be able to:  Discuss blasting theory
Background  In the preceding lectures we have been discussing topics related to explosives and blasting in general.  In this lecture however, we shall take a brief look into what happens when the explosive is detonated in the blasthole in the rock mass: Blasting Theory
Introduction  The sudden release of energy and the reaction products at high pressure by the rapid chemical reaction in an explosive contained in a blasthole in rock gives rise to compression waves in the explosive and in the surrounding rock material.  These compressive waves are called detonation waves or shock waves, depending on whether they involve chemical reaction or not.  They play a central role in the functioning of the explosive and in the fragmentation of the rock mass.  On detonation of an explosive in a rock mass the rock is affected in 3 principal ways or stages:
Stage 1  In this stage, and starting from the point of initiation, the blasthole expands by the crushing of the walls immediately surrounding the explosive due to the development of the high pressure, followed by a development of radial cracks
Stage 2  At this stage compressive stress waves emanate or radiate in all directions from the blasthole with a velocity equal to the sonic wave velocity in the rock mass.  New cracks are developed while old ones are enlarged. When these compressive stress waves reflect against a free face in the rock mass, they cause tensile stresses in the rock mass between the blasthole and the free face.  If the tensile strength of the rock is exceeded in the process, then the rock breaks in the burden area, which should be the case in a correctly designed blast, otherwise the compressive stress waves will just disappear in the rock mass and the rock does not break.
Stage 2
Stage 3  In this stage, large volumes of gases are produced under very high pressures.  These are dissipated into the cracks formed during the first and second stages of the process and expand the cracks.  If the distance between the blasthole and free face is correctly calculated, the rock mass between the blasthole and the free face will give way or yield and be thrown forward, thus fragmenting the rock.
Stage 3
Conclusion on Blasting Theory  It must be noted that the explosive reaction in the blasthole is very fast (in 1000th of a second or milliseconds) and the effective work done by the explosive is considered complete when the volume of the blasthole has been expanded to 10 times its original volume in approximately 5 milliseconds.
Rock Fragmentation  Very often the end purpose of rock blasting is to produce material that fulfils the objective of the blast. For example: crushing for mineral beneficiation, road construction, dam building, erosion control etc., etc.  In the case of mining and construction fragments of rock should not only be small enough for the loading and transport equipment,  but should also be small enough to pass through the crusher opening: a state on which the economic success of the mining operation hinges, since the primary crusher becomes a bottleneck, choking the flow of ore into the beneficiation plant.
Rock Fragmentation  Sometimes it is necessary to produce quantities of large and regularly shaped fragments, as is the case in dam building or constructing wave breakers.  Even where the main purpose of excavation is to create an empty space surrounded by rock walls such as in tunnels, drives, etc., in underground mining the fragment size is important because it influences the speed of loading/mucking and the capacity of the transport equipment.  In all cases therefore the degree of fragmentation influences the economy of the excavation job.
Rock Fragmentation  Basically, 3 main factors control the fragment size distribution of a blast. These are:  Quantity of explosive used  Its distribution in the rock mass, and  The rock structure
Rock Fragmentation  The ideally fragmented rock is that which does not need further treatment after the primary blast.  The parameters of the subsequent operations are therefore, the guidelines for deciding on the desired fragmentation of the formation.  Since the fragment size is of the utmost importance to the operation all possible efforts should be made to keep the size down, without making the blast do the work of the crusher.
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