When looking to establish the elevation mentioned in part 1, we are not concerned about the location of the bottom of the hole, we are concerned where the craters from neighboring holes intersect. Where these craters intersect is where we want the desired grade to be. This is what we design our blasts around and have some general rules of thumb on how to determine where this intersection will be. This point relies on a number of factors including the size of the hole, the spacing between the holes, the explosives products used, the competency or hardness of the rock, as well as many others. The less competent the material, the more forgiving it will be to allow us to dig the bottom out to a consistent grade.
Typically, in drilling and blasting in general, if you are drilling a larger hole your spacing will be more than if you were drilling a smaller hole. For sub-drill, the more spacing you have between holes, the more sub-drill you will need. This is because in order to cover a longer distance, you will need a larger crater. In order to create a larger crater, you will need to drill deeper (more sub-drill). Therefor, in most applications, a larger hole means more sub-drill. This is where the rule of thumb came from where if you are drilling a 3” hole you should add 3’ of sub-drill, if a 4” hole add 4’ of sub-drill, and so on. Too little sub-drill can be disastrous for construction projects if a specific grade is required and the drilling and blasting does not quite fracture all of the material down to that grade. In these cases, there may be only a foot or two of hard rock left which is too little to require blasting, but could take a tremendous amount of time to rock hammer. However, in quarry applications, too much sub-drill is actually worse than too little sub-drill.
In the quarry environment, most mining plans include benches from one level down to the next. As the miners go from one level to another, material is blasted, material is consumed, and benches disappear. It is a very effective system and allows for efficient mining as well as improved safety for the miners. In this benching system, the driller is directed to drill down to the next bench level below so when the bench they are on is blasted and the rock is removed, the level created is even with the existing level, leaving a flat workable area. In order to achieve this flat workable area some contractors turn to drilling excessive amounts of sub-drill. This way they ensure the floor is going to be able to get dug down to the level necessary, even if they have some imperfections in their drilling or blasting process. Not only is this waste, it also creates some major issues down the road that can cost the quarry owner a significant amount of money to fix. It may seem great at the time that the floor is digging and coming out flat and even with the existing floor, however a few hard spots in the floor is much better long term than a floor being overshot, that is too much sub-drill.
Drilling and blasting costs money. Just like everything else, you would not want to pay more money for something than is absolutely necessary, so why would sub-drill be any different? Excessive sub-drill can be a remedy for more than average borehole deviation due to improper equipment, poorly trained operators, or poor blast design. Not only does this add more cost due to the additional drilling of more sub-drilling, but that borehole also needs to be loaded with explosives that are expensive as well. Therefor on a project that should only take 4’ of sub-drill but is using 9’, there are five additional feet that will need to be drilled and loaded. Often times this additional 5’ of blasted material is not excavated and is left in place to produce a flat surface, only needed to be drilled AGAIN when that next bench level is ready to be mined. Therefor, this material is not only being paid for when it is sub-drill, but it is also being paid for when it is at the top of the hole on the next bench shot down.
Paying for this material twice is not only inefficient, but it also can create enormous problems for the next planned drill and blast down. These pockets of loose material that in the 9’ example could result in pockets of 5’ or more of fractured rock. Drilling through fractured rock is not only slow, but it is hard on the drill rig and if not done properly can result in a poor collar which will in turn produce excessive borehole deviation. If you haven’t read the post on borehole deviation, make sure to do so to understand the issues associated with it.
For these reasons, we practice minimal sub-drill in quarrying operations. Although it may result in hard spots in the floor and a little rise to the flat level, dealing with these issues pales in cost and efficiency to the practice of excessive sub-drill. Minimal sub-drill ensures we are only blasting the rock we can mine and we are leaving a bench for the next go around that is going to be relatively hard to the top which is conducive to efficient drilling, good collars, and minimal borehole deviation.
In construction projects, more sub-drill can often be better to ensure the grade mark is made and the rock can be excavated easily to the level necessary for that specific project. Weighing the costs of too little sub-drill to too much sub-drill is required for all drilling and blasting operations in order to produce the most effective and efficient outcome for the customer.