PURPOSE:  Investigate the complex relationships between fragmentation, rock scaling, ground support, and safety in mines that use drilling and blasting as the primary excavation method.

RESEARCH SUMMARY:  Rock falls are one of the most serious causes of fatal injury to underground miners. From 1998 through 2002, the fatal injury rate in underground mines was 56.2 (i.e., the number of fatalities per 100,000 full-time-equivalent workers). Of these, the Mine Safety and Health Administration (MSHA) classified 43.3% as being caused by falls of ground.

Metal and stone mines typically excavate rock using explosives. If too much is used, the rock surrounding the opening can be excessively fractured, and workers can be endangered by loose and falling rock when they enter the workplace. Fractured, weakened, and/or loosened rock must be either scaled or supported. Both of these activities are extremely hazardous.

The research problem addressed by this project was whether advanced fragmentation techniques could be used to minimize the hazards associated with rock falls. These techniques include various controlled blasting methods and new products, such as electronic detonators and string emulsion, that have the potential to reduce excessive fragmentation of surrounding ground and preserve more of the inherent strength of the rock. Such techniques have been applied in many civil engineering projects and have been introduced in some sectors of the mining industry. Although these methods promise better ground control safety, the benefits have rarely been quantified.

In this project, researchers seek to quantify the interaction of fragmentation, rock scaling, and support methods, allowing them to be optimized as a single system. The first step in this research is to study and classify the dynamic response of rocks. A 60-mm split Hopkinson pressure bar was set up to measure the extent of fracturing in different types of rock. Assessment methods are being developed to evaluate the extent of rock fracturing. A follow-up project that builds upon this work is planned that will use a systems approach to look at not only blasting, but also the drilling, scaling, and support portions of the mining cycle. Future work includes single-hole blast testing, development of an RMR-based blasting model, improved mathematical models, field verification tests, and demonstrations at mine sites. This information will be used to reduce hazards related to rock falls at mining operations.