In this two part post we are going to talk about sub-drilling. What is sub-drilling, why we do it, when we do it, and how much we do it.
When explosives are detonated, they are expanding rapidly and are ultimately going to take the path of least resistance. This is one of the known constants of drilling and blasting with commercial explosives. We know we are able to confine the explosives and give them one way to go, and we are able to control their path of movement by giving them this out. By understanding the nature of this path of least resistance, we have the tools to design and engineer the most efficient ways to take full advantage of the amazing power of explosives in order to complete specific goals (like blowing stuff up).
As we have talked about in previous posts, our goal as a drilling and blasting contractor, much like any other business, is to complete our objectives with as little wasted resources as possible. Typically our objectives are to fracture the rock to a specified size, move the rock in a desirable direction and velocity, and lastly, to impact the exact amount/area of rock we designed to break. This last point of impacting the area we designed is similar to back and side break. If you have not read our blog post on back break and side break, click here, to refer back to it. In that post we discuss the importance of containing the impacted area of the blast to within the designed parameters.
In addition to back and side break, people often believe blasts can impact the area below the pattern. That is, if we are drilling an area where the holes are 30’ deep, people often think blasts will not only have backbreak, but shoot downward as well, impacting the area below that 30’ mark. If we have back and sidebreak, is it really that crazy to think we can have underbreak, or breakage underneath the pattern? Well yes, it is crazy. Granted we would never say never, but we are discussing overall general rules to drilling and blasting. If we are drilling in hard, solid rock, the path of least resistance is always up and out, not down. For a blast in solid rock to shoot down to create a crater below the deepest holes, this simply goes against anything physically possible. Again, the area of release for the energy is going to be where it meets the least amount of struggle, which is going to be open space, up and out, as opposed to a hard, solid rock mass below.
An exception to this rule is if below the hard, solid rock there is a void, cavern, or some other inconsistency in the solid rock mass that is less competent material. This is a major concern for drilling and blasting as we must deal with the variable of inconsistencies in the rock and plan accordingly as this creates a weak point in the rock and will ultimately be used as an escape route for the explosive energy if given the opportunity. That is, if we drill holes 30’ deep and there is a void or a cave, less competent material, or some other weakness below the hole, all of the energy will not blast upwards. In this scenario a portion of the energy at the bottom of the hole could escape downwards causing all sorts of issues in blast performance. This is another full topic for a post we will cover at a later time.
When the blast hole detonates, the path of least resistance is most likely up and out the face. For this reason, each hole creates a sort of crater where it was drilled. Much like a crater from any other type of explosion, each hole does not create a perfectly flat circle at the bottom of the rock mass. There is typically a low spot not much larger than the hole diameter in addition to an upside down cone, or crater, around the bottom of where the borehole was drilled. Therefor, the bottom floor of the blast, once the blasted material is excavated, can resemble that of the craters of the moon. Due to this cratering effect, we must take additional steps in order to achieve a flat grade for a given project. Many times we are brought in to drill and blast rock down to a specified elevation in order for our customers to lay pipe in a trench, pour a foundation for a building, or even establish a flat working surface in a commercial quarry. However due to the cratering nature of explosives in drilling and blasting, we must add on sub-drill in these situations where a specific flat grade surface is required.
In order to get the cratering holes on a spaced out pattern to generate a flat surface, we must drill the holes a certain distance deeper than the desired grade, this is what we call “sub-drill”. We are drilling our holes deeper than the required grade in order to ensure the client is able to dig down to the elevation they need. If we did not do this, there may be areas they are able to dig to the grade (most likely at the bottom of where each hole was drilled) however the entire area would not be able to be flattened out at that desired elevation.