Why is borehole deviation so important? If the holes are off just a couple of feet will it really make that big of a deal? The answer is a resounding YES. We design blasts on the surface of the material. We lay the holes out, measure the area, mark the spots where the holes are to be drilled, and get to work. Once the holes are drilled it is easy to look at a straight row of open holes from the top and imagine all of the holes are straight. This is not the case. The ideal scenario is if you were to flip the area being drilled upside down, it would look exactly the same. That is, all of the holes would be in the spot where you marked them, and they would still have the same spacing from hole to hole and row to row. Due to borehole deviation we are not able to achieve this.
Almost every blast has bit wander. Since the holes are not the same distance apart at the bottom as they are on the top, this causes inconsistencies at the bottom of the shot. This can result in high spots in the floor, areas with excessive fines, and inconsistent fragmentation throughout the muck pile. All of these issues pale in comparison to the implications of having borehole deviation on the face row of a blast.
As discussed in the face profiling post, we carefully measure each bench face to determine where to drill each face hole to produce an ideal outcome. However, after the blast is designed and planned, we must be able to execute. If a face hole is designed to push 10’ of solid rock, and we know this 10’ is the amount of rock that needs to be in front of that loaded borehole in order to avoid fly rock but also move out the face row and create an ideal muck pile, but when drilled that face hole deviates 5’ towards the face, we now how a borehole that was designed to push 10’ now executed to only push half of that amount. This is where a blast ends up producing fly rock. There is insufficient face burden, or padding, for that quantity of explosives and it will cause a blowout. The blast could be perfectly designed with great care but if the hole is not drilled and executed properly, it could cause a disaster. The issue with drilling these boreholes, as we mentioned earlier, is there is no way of knowing whether the borehole is where it was intended to be drilled. However, we now have the tools needed to safeguard this process and ensure our holes are where they were intended, this instrument is called a Borehole Tracker, or Bortrak machine, which we will cover in a later post.