46 FEBRUARY 2025 WorldWide Drilling Resource® What Causes Flyrock and How to Control it Adapted from Information by the Office of Surface Mining Reclamation and Enforcement When material is ejected from a blast site and travels through the air, or along the ground, it is referred to as flyrock. Flyrock can be rock or soil and any size flyrock is capable of damaging property or injuring people. In fact, it is the single factor of surface mining likely to cause a fatality, making flyrock control essential. Flyrock can be thrown thousands of feet from the blast and is the second highest cause of death in mining operations. The most dangerous source of flyrock is ejection from a crack or weak zone in the highwall face where gases violently vent. This action is akin to a rifle where the expanding gases eject a projectile; frequently the ejection of stemming out of the top of a blasthole is called rifling. It is crucial for blasters to understand and control the factors which can create flyrock. Let’s start by taking a look at some of the common causes of flyrock: j Overloaded blastholes with excessive amounts of explosives j Heavily confined charges or the lack of relief (eg. lift blasts) j Explosives loaded into incompetent materials (eg. mud seams, fractures, and/or voids) j Insufficient front-row burden, causing front-face blowouts j Burdens and spacings too close together (resulting in high powder factors) j Inadequate/insufficient stemming material j Inadequate delay between holes in the same row or between rows; detonators firing out of sequence j Deviation of blasthole detonation from the intended sequence j Changing geology or rock type j Spacing and burden exceeds borehole depth j Angled boreholes j Secondary blasting j Human error, improperly loaded blasts Flyrock traveling in the air or along the ground is not allowed to travel more than half the distance to the nearest dwelling or occupied structure, beyond the “area of control” (the blasting area), or past the permit boundary. The blaster is responsible for securing the area around the blast site where flying debris may occur. So what can the blaster do to control flyrock? 1. Accurately measure the burden for each blasthole, and be aware of the true burden for each hole along the free faces. 2. Be aware of the powder factor and total charge-weight loaded, so holes are not overloaded; always measure explosive quantities or tape the holes while loading. 3. Insist on effective communication between the drill operator and the blasting crew, so the drill operator conveys information regarding unusual conditions (for example, mud seams, voids, or other weak layers) during drilling. 4. Use adequate stemming and stem through incompetent zones. Use crushed stone for stemming. 5. Place primer lower in the hole, increase delays between rows, reduce burden in back rows. Unfortunately, flyrock rolled downslope from a mine andright through this home. Although children were playing in the front yard at the time, no one was injured. EXB
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