Abstract

On August 6th, 2007, the Crandall Canyon Mine in Utah collapsed entrapping six miners. It appeared that a half square mile area of pillars in the western section of the mine had bumped in a brief time period, filling the mine entries with coal and entrapping the six miners working there. Ten days later, during the heroic rescue effort, another bump occurred thereby killing three of the rescue workers and injuring six others. This paper details a back-analysis of the August 6th, 2007 collapse using the LaModel boundary-element program along with the best available geotechnical information. For the back-analysis, an initial model of a previous bump at the mine was used to calibrate the input rock mass, coal and gob properties. Then, the calibrated input was used to model the mining scenario at the time of the collapse. Ultimately, it was determined that the fundamental cause of the collapse was the large area of equal size pillars with near unity safety factors in the collapse area. It is hoped that the enhanced understanding of the collapse presented in this back analysis will foster improvements in future mine designs to eliminate similar type events.

THE CRANDALL CANYON MINE

The Crandall Canyon Mine was located in Emory County, Utah. It was a drift mine into the Hiawatha coal seam of the Blackhawk formation in the rugged topography of the Wasatch Plateau. At the time of the accident (August, 2007), the mine was operating one continuous miner section pulling remnant pillars in the last phases of operations. The immediate geology above the seam typically consisted of 0 – 0.6 m (0 - 2 ft) of interbedded siltstone, shale, and/or sandstone [1]. This immediate roof was overlain by 200 m (650 ft) of the Blackhawk formation which consists of interbedded siltstones and sandstones. Above the Blackhawk formation is 75 m (250 ft) of the relatively massive Castlegate Sandstone which is known to form cliffs in the area. Also, directly below the Hiawatha seam is the massive Star Point Sandstone [1].

The recent mining operations began at the Crandall Canyon Mine site in 1983. Initial operations were room-and- pillar mining with continuous miners, including several retreat sections employing continuous haulage. In 1995, a longwall system was installed and it operated successfully until the longwall reserves were exhausted in 2005. With the end of the longwall operation, pillar recovery commenced in the various remaining main and barrier pillars, primarily in the South Mains area [1]. In the last quarter of 2006, resource recovery moved to the Main West area of the Crandall Canyon Mine (see figure 1). The Main West section was initially developed in 1995 with 5 entries and pillars on 27.4 X 28.0 m (90 X 92 ft) centers. This section was developed with a continuous haulage system with 6.1 m (20 ft) wide entries, rounded pillar corners and an average 2.4 m (8.0 ft) extraction height.

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