WorldWide Drilling Resource®

Controlling Flyrock is Essential Adapted from Information by the Office of Surface Mining Flyrock is the debris ejected from a blast which travels through the air or along the ground. It is not only dangerous, it can also be deadly. Managing flyrock capable of travelling thousands of feet from the blast, can be challenging. The most dangerous cause of flyrock is ejection from a crack or weak zone in the highwall face where gases violently vent. It is similar to a rifle where the expanding gases eject a projectile. This could be why the ejection of stemming from of the top of a blasthole is called rifling. A blast designed to horizontally dis- place overburden material with explosive energy is called cast blasting or “engineered” flyrock, which moves material in a controlled, safe way. The same factors which cause excessive airblast and ground vibrations have the potential to cause flyrock, which is the number two killer in mining operations. This is one of the reasons it is crucial for blasters to understand and control the factors leading to the creation of flyrock. Common Causes of Flyrock: ) Overloaded blastholes with too much explosive materials ) Heavily confined charges or the lack of relief (such as lift blasts) ) Explosives loaded into materials which are less rigid and tend to flow instead of break, like mud seams, fractures, and/or voids ) Insufficient front-row burden, causing front-face blowouts ) Burdens and spacings too close together (results in high powder factors) ) Not enough stemming material ) Inadequate delay between holes in the same row or between rows; detona- tors firing out of sequence ) Deviation of blasthole detonation from the intended sequence ) Changing geology or rock type ) Spacing and burden exceeds borehole depth ) Angled boreholes ) Secondary blasting ) Human error, improperly loaded blasts Controlling Flyrock: ( Accurately measure the burden for each blasthole, and be aware of the true burden for each hole along the free faces. ( 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. ( Effective communication between the drilling and blasting crew is crucial. The drill operator should pass along information regarding unusual geologi- cal conditions, such as mud seams, voids, or other weak layers encountered during drilling. ( Use adequate stemming and stem through incompetent zones, using crushed stone for stemming. ( Place primer lower in the hole, increase delays between rows, and reduce burden in back rows. Remember, flyrock is one of the most dangerous effects of poorly controlled blasts which can cause property damage, personal injury, or death. 42 MARCH 2020 WorldWide Drilling Resource ® The hole in this rock face, shown with yellow dashes, deviated as it intersected the rock joint region. When the hole intersected the joint, it became nearly parallel with it. EXB

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