- The geology of the material to be blasted
- The geometry of the blast and the drilling (blast pattern, depth, diameter, and angle of the holes)
- The blasting experience of the mine personnel
- The initiation systems, the powder factor and the charge per delay
- The loading configuration (explosive type, stemming, etc)
30 CFR Part 77.1303 requires that ample warning shall be given before blasts are fired, and all persons shall be cleared and removed from the blast area unless suitable blasting shelters are provided to protect persons endangered by concussion or flyrock from blasting.
The Institute of Makers of Explosives (IME) has defined flyrock as “the rock propelled beyond the blast area by the force of an explosion”. The Dictionary of Mining Terms gives the same definition. Flyrock comes in different sizes and shapes, ranging in mass from a few ounces to several tons. Persson et al. (in Persson, P.A, Holmberg, R., & Lee, J., 1994, “Rock blasting and explosives engineering”, Boca Raton, FL: CRC Press LLC.) referenced flyrocks weighing approximately three tons thrown to a distance of 980 ft.
Three typical cases
These three well known cases taken amongst dozens from the literature aim to show how dramatic consequences can be but also demonstrate that each of them could have been prevented by the proper application of the regulation or the application of existing mitigating techniques.
- On July 5, 1990, a blaster standing on the top of a 200-ft highwall (~60m) about 505 ft (154m) from the blast site was fatally injured by flyrock. The highwall could not shield him from the flyrock. The employee suffered a massive head injury. The flyrock originated from a toe blast. Explosive energy takes the path of least resistance and blasting small diameter angled toe holes requires special attention. The blaster failed to perceive that flyrock could strike him on the top of a highwall. This accident could have been prevented by using a proper blasting shelter or “matting” the holes. (source: Daugherty J.B. & Frantz, J.A, “Accident investigation report (surface nonmetal mine), fatal explosives and breaking agents accident”, MSHA, mine ID No. 15-16739, The Kentucky Stone Company, Princeton Quarry, Princeton, Caldwell County, Kentucky, July 5, 1990)
- The Chief Inspector of Mines in Queensland, Australia, reported that a blaster was standing behind a steel hopper while video-taping a toe shot in a metalliferous quarry. Flyrock travelled about 246 ft (75m) and seriously injured the blaster. The blaster lost his right eye, his cheek bone was shattered, and his jaw was broken. This accident could have been prevented by using a proper blasting shelter. (Source: Blasting Safety – Revisiting Site Security, by T. S. Bajpayee, Harry C. Verakis, and Thomas E. Lobb, 2005)
- On February 1, 1992, a blaster was fatally injured in a surface coal mine. The blaster positioned himself under a Ford 9000, 2-1/2-ton truck while firing the shot. Flyrock travelled 750 ft (~230m) and fatally injured the blaster. Taking shelter under a pickup truck, explosive truck, or other equipment is not adequate because flyrock can travel horizontally. (Source: Boggs, R.W, & Blevins W., “Report of investigation (surface coal mine), fatal explosives accident”, MSHA, No. 1 surface mine (ID No. 46-07311 E24), Austin Powder Co. (Saft. & Comp.), Wharncliffe, Mingo County, West Virginia, February 1, 1992)
Additional Available Mitigating Techniques
Langefors and Kishlstrom [in “The modern technique of rock blasting”, 1963], Roth, and Persson and al. have postulated concepts and developed theories to compute flyrock range. A blaster may use such concepts, in conjunction with past experience, to determine the size of a blast area.
From 1997, researchers have used numerical simulation techniques to predict blast results by computing the interaction of rock and explosive. Katsabanis & Liu, Favreau & Favreau, Preece & Chung, Dare-Bryan, Wade & Randall can be considered amongst the pioneers in that field. In Europe, Bernard starts developing its own model at the same period though along slightly different concepts. A blaster may be able to improve the design of a blast by using one of the simulation techniques and technologies that finally came out those developments. It is nevertheless interesting to underline how cautious and humble experts remain in front of the fly-rock phenomena. Bernard states on TBTech website that his model, “DNA-Blast, does not recommend or replace safety rules set by law or decree, by the sites itself or coming from experience or recommendation of professional institution or groups. i.e the flyrock simulation does not provide a clearing distance.”
Wireless blasting can also be considered part of the mitigating techniques, having brought a conclusive contribution to blasters’ safety in breaking with the wired initiation system inherent limitations. In the mines that use this system, the blasting point is most likely determined by safety concerns.
To conclude, techniques to mitigate the flyrock risk include “proper blast design, driller-blaster communication, inspection prior to loading and firing the blast, removing employees from the blast area, controlling access to the blast area, and using a blasting shelter” (Source: Bajpayee, Verakis, & Lobb, 2004)