Overcoming Oversize: Oversize and confinement (Part 4)
The timing factor
The final evolution of our 4D energy model is achieved by advancing the model from three dimensions (the physical blast layout such as bench height, burden and spacing) to the fourth dimension, which now introduces the blasthole timing.
While hole-to-hole timing does impact fragmentation, it is beyond the purview of our analysis, which is currently focused on the results of overconfinement in the development of oversize. As such, expanding this model to the fourth dimension will focus solely on the row-to-row timing of the blast.
Let’s assume a blast has five rows of holes, each lined up directly behind the other. In a proper blast, the first hole, which has a designed burden to the free face, will fire first, if the proper energy is applied, then the face will begin to bend and move starting off slowly, accelerating to a maximum speed, and then slowing again before coming to its final resting place.
While this happens, new fractures in the rock increase the rock volume while decreasing the rock mass density, meaning the rock will take up more space after the blast than before. Furthermore, the rock mass will break from the free face and break back toward the borehole under flexural failure. This results in the front of the rock moving faster at all times than the back of the rock.
If each row is fired completely independent, one would expect the fragmentation of each row to be identical. We shall then conduct a thought experiment to consider: What if the fifth row of holes is detonated without any other row firing? This fifth row would “feel” the burden of the entire rock mass in front – or, effectively, the burden would be five times the typically designed burden.
Remember back to Johnsson and Hofmeister’s experiment: When not enough energy is applied, a certain effective maximum burden is reached, and the entire rock mass cannot be broken. However, a large amount of energy is still in the borehole, and the stemming design assumes a typical time for breakage of the face.
In that effect, the energy will go upward and crater the shot, leading to large oversize and no breakage in the burden of the blast. If the fifth row is fired before the first four rows, then the energy is too small and the rock mass would not break (i.e., the blast would have a burden larger than the maximum effective burden).
In the majority of cases, the maximum effective burden is less than two times the designed burden. This means that if the second row fired before the first row, the same results would occur for those blastholes because the energy would be too small for the rock mass in front.
This is all fairly intuitive for anyone who has worked in blasting. The major question then becomes: What if the second row fires and the first row begins to move but is not fully broken or out of the way? In this case, does the second row not feel an increase to the burden or confinement?
Looking at the problem another way, if the first row fires and the row breaks, but the back of the rock – which moves slower than the front of that rock – has not moved before the second row of holes fires, does that second row not “feel” a burden larger than its designed burden?
The answer is, of course, yes. The second row would have an increased burden based on the resistance of the rock in front. This means that if the row-to-row timing is too fast and the first row does not fully relieve the entire path of motion of the second row’s rock mass, then the end result is overconfinement, as the second row has too little energy to move its rock mass and the first row’s rock mass. The result is the same as other situations of overconfinement: oversize present in the muckpile.
Final thoughts
This 4D energy framework for looking at a blast allows one to simply make decisions on ways to reduce oversize.
If the stemming blows out, then the explosive energy is less than 100 percent and the explosive has too little energy for the rock, resulting in more oversize. If the burden is expanded, then the explosive energy is reduced and oversize will occur.
If the row-to-row timing is too fast, or the sequencing of boreholes is incorrect, then the holes firing will have too little energy for the rock mass in front, and oversize will result.
All of this assumes that the previous three parts to the “Overcoming Oversize” series are followed and that the entirety of the blast is properly designed. If the rest of the blast is properly designed and oversize is still an issue, check the energy. An energy increase is likely required to reduce oversize further.
Anthony Konya is vice president at Precision Blasting Services, consulting around the world in rock blasting and vibration from blasting. He is also the founder and CEO of Academy Blasting, an explosive engineering education company, and the host of AcademyBlasting.TV podcast.
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