One common factor in all blasts is back or side break. Simply put, back or side break is breakage that occurs OUTSIDE of the drill pattern. For example, if we are drilling on a 10 x 10 pattern, we are basically designing the blast to have each hole break 5’ in each direction. Therefor, the sides and back of the pattern should break depending on the hole size between 5-10’. However, there are times when the blast will produce 2 to 4 times the amount of back or side break as designed.
There are many reasons why back or side break can be excessive, and some are avoidable, some are not. Commonly referred to as “free rock” by quarry operators, back break is generally not a good thing. Although it can produce rock that is outside the area typically quantified for a blast, we end up paying for this rock in other ways down the road. Back break or side break, depending on the rock geology, is rock that is not within the designed blast area. Since it was not in the pattern, the rock will typically not be fractured consistent with the rest of the blast. Back and side break are common causes of oversize due to the fact the rock is outside of the pattern. Since the material is outside of the pattern, it does not have any boreholes drilled through it. This almost always results in oversize, or rock that is larger than we intended it to be. The only time this will not result in oversize is if the rock geology already contains fracture lines where the rock does not need to be broken but simply separated at these joints.
What issues do back and side break cause?
Not only do back and side break generate oversize for the initial blast, they also cause several issues for subsequent blasts. Since back and side break typically only occur in the top half of the bench, many challenges are created for adjacent blasts. For example, if a bench is 40’ tall and we detonate a blast that for whatever reason generates back break, the back break will not impact the entire 40’ back wall. The back break may only dislodge the top 15’ of the back wall, leaving an uneven back wall once the shot rock is removed.
The inconsistent wall left over from the previous blast will in turn generate additional oversize on the following blast. Typically, the more inconsistent the face of a blast the more oversize it will generate. An inconsistent face also requires a more involved blast design that could mean additional blast holes drilled to remove a toe. These additional holes are typically an added expense with not a lot of benefit due to the small amount of crusher feed rock they produce. Their main function is to allow for proper relief of the main body of the blast, so we do not compound the problem on subsequent blasts.
Keep this in mind the next time someone is looking for “free rock” from back break or side break. It may not cost anything at the time, but there will be a price to pay eventually, whether it be additional drilling that would be otherwise unnecessary or more oversize generated.
What other factors can cause back and side break?
An important point to understand is that back break and side break from a blast are not the only culprits for causing these problems. Over digging by an excavator or dozer operator can create these issues just as easily as a blast. Make sure to inform aggressive equipment operators that even though they may be generating cheap rock at the time by digging outside the parameters of the blast, they are creating more issues for the following blasts, which will ultimately result in added time and money. It is very important for the driller, blaster, and quarry operators to be on the same page regarding where the blast started and stopped in order to mine as efficiently as possible.