Background
In the mid-1990’s, the number of injuries from rock falls increased. This increase could be attributed to (1) an increase in new, inexperienced miners, who have shown to have a much higher rate of accidents than older, experienced miners, (2) an increase in the total number of underground workers, and (3) mining in weak rock surrounding the ore deposits, which resulted in an increase in rock falls.
A research project was started in 2002 to develop tools to evaluate the relationship between geology, opening span, and support requirements. Thirty-four data collection points were established at eight mines in Nevada and four mines in Idaho and Montana. A manual of underground mine design for weak rock masses in hard-rock mines was developed. This manual contains information that relates span curves, rock mass rating (RMR) values, stope design curves, and equivalent linear overbreak/slough (ELOS) values and is used to select adequate support for the particular mining geology and selected opening span. It is being used by 75% of the active underground mines in Nevada and 50% of the underground mines in Montana and Idaho.
During 1999-2005, the mine design curves and applicable nomograms resulting from this research were transferred to the mining industry through 11 technical papers and 9 presentations. This research was also the focus of three workshops. A handbook was developed that is being used to design openings and select supports.
Potential Outcome
Since 2002, the critical information for mine design and the nomograms contained in the design manual have been adopted at most mines in Nevada, Idaho, and Montana. After these interventions began in 2002, ground fall injuries in Nevada mines have dropped, as shown in the bar graph. This injury reduction was due to the improved methods for the design of mine openings and selection of supports for metal mines with weak rock. Although many other factors must also be considered as contributors to this trend, the intervention by NIOSH was a factor for the reduction in ground fall injuries.
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