Print6.3.2: Drift Round Blast Design
For drift rounds, we are talking about driving openings, i.e., drilling, blasting, loading, and hauling, in an underground mine. These openings are horizontal or nearly so in many cases, but can be on fairly steep grades in others. Think of advancing a tunnel through a mountain – we are driving an opening through the mountain, and depending on our needs, the tunnel may be inclined or nearly horizontal.
Although the calculations that we just covered are generally applicable, designing a blasting round for drifting is more difficult. There are two reasons for this. Primarily, it is because we only have one free face. In bench blasting, we almost always have at least two. Recall, what is happening in a bench blast. As we initiate the round, the blasted and expanding material closest to the highwall is able to move freely upward and outward in the direction of the highwall. When rock is blasted, it expands by 30% or more, so it needs somewhere to expand. Furthermore, the expanding gases from the detonation of the explosive are rapidly propelling this blasted rock. After the first row of holes has been fired and the fragmented material is moving out of the way, the next row is fired, and the round evolves in an orderly sequence based on the timing delays used in the pattern. If we don’t give the blasted material the space to expand and move, we’ll have a big problem. The fragmentation will be adversely impacted, of course, but the bigger problem is that the fragmented material will tend to stay in place, and then it will be very difficult to dig or load. This is the challenge we face in drifting, because we have only one free face.
We overcome this limitation by creating a second free face; and we do this by putting cut holes in the center of the opening that we are driving. We’ll look at these cut holes in the next lesson, but, essentially, they are unloaded holes drilled in the center of the center of the opening; and then, when this new free face is created, we set off the remainder of the holes. The blasted rock from those holes is then free to expand inward towards this new free face that we created in the center of the opening, and then outward.
We can use the same formulas for drift rounds as we did for designing a blast round for a bench. However, the patterns will be significantly different. And that’s what we really need to look at next – the patterns, i.e., hole placement and timing delays that are used in bench and drift blasting. We’ll do that in the next lesson.
Before moving on to that topic, let me finish one last detail here. I said that drift blasting was more challenging for two reasons, and we just talked about the primary reason for this. The other reason is overbreak, which is fragmentation that occurs beyond the intended space. Suppose you were blasting a tunnel with a rectangular cross-section of 30’ wide by 20’ high. Your goal will be to limit fractures in the rock to the 30’x20’ opening. It’s unlikely that you could maintain a perfect opening, but limiting the fracturing beyond those boundaries to less than several inches is doable. We aren’t as particular about this in many surface applications as we are in underground mines. Why? We touched on this earlier.
These fractures are likely to become future ground control problems, costing us time and money. The fractures or cracks are likely to destabilize over time, and if there is ground water seepage, the process will accelerate. Accordingly, we have to design a round to minimize overbreak. Unfortunately, few engineers do this for lack-of-knowledge; but it is not that difficult, and we will learn how to do it in the next lesson.