Date of Award
Master of Science
Background: The support of intersections in coal mines is an important safety issue in the U.S., as intersections are by far the most common area for unplanned falls of ground. A relatively comprehensive, nation-wide study of falls of ground is coupled with a national survey to mines about their support methods to determine common characteristics of failure and recommend changes to improve stability, and recommendations for future research. Methods: Over 600 fall of ground reports were collected from nine of the 11 Mine Safety and Health Administration (MSHA) District offices, and data was compiled to determine common characteristics of these unplanned falls. Statistical analysis was conducted on the data to examine which variables affected fall dimensions. To obtain data on current support usage, mail and phone surveys were collected with responses from 70 underground coal mines, representing approximately 235 million tons of annual production, or 66% of the U.S. total. These surveys provided a national snapshot of what support mines are using as well as typical extraction height, intersection width, and other details. Rocscience's Phase2 software was used to model a typical coal mine intersection and examine possible stability changes with different support options. Results: Surveys from underground mines revealed that the current industry average for intersection width is 20ft, average bolt length is 6ft, average distance from pillars to the first row of bolts is 3 to 4 ft, and a great majority of mines do not angle bolts over pillars. The fall of ground study confirmed that most falls are thicker than the average bolt length of 6 ft. and tend to be massive, extending past the intersection width of 20ft. The study also showed that falls with longer roof bolts installed typically had thicker falls which broke above the anchorage zone. Statistical analysis found a few questionably significant interactions, with the most prominent being the effect of roof type on fall height. Immediate roof geologies of dark shale and thinly laminated shale resulted in higher roof falls than other types. 2D modeling was unsuccessful at replicating the type of massive shear failures that have been commented on by MSHA personnel and that the study data suggests. It is the author's opinion that Phase2 and 2-D modeling in general may not be powerful or comprehensive enough to capture the true shear behavior of the rock strata in the roof beam because it cannot effectively model failure and dilation. Conclusions: Increasing bolt length may not be the most effective solution to reducing massive intersection failures. Rather, installing angled bolts over pillars may increase the strength of the system at the crucial roof-pillar edge. Weathering of bolts and/or rock are likely contributing to the significant number of cutter failures happening months or years after excavation. Recommendations for future action include 3D modeling of cutter failure and benefits of angled bolts over pillars. More consistent and thorough MSHA 7000 50a forms will enable more accurate statistical analysis and a better understanding of massive failure characteristics.
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