Mining Contract: Conduct Numerical Modeling to Meet the Requirements of the 2008 Congressional Appropriations Bill Per the Attached Statement of Work
On August 6th, 2007, the Crandall Canyon Mine in Utah collapsed, trapping six miners. Ten days later during the heroic rescue effort, another collapse occurred, killing three of the rescue workers. At the time of the collapse, the Crandall Canyon Mine was performing pillar retreat mining at cover deeper than 1,500 ft.
The safety of deep cover mining depends largely on proper pillar design to ensure that the overburden rock mass is well supported by unmined areas of the mine. In retreat mining, these unmined areas are partially extracted as the last operation before the mining is complete. This extraction process increases the risk of ground failure unless the operation is well-designed. One of the common methods used for the mine planning is a numerical modeling simulation of rock mass response and pillar loading using a computer program called LaModel, so named because of the laminations of the rock layers above the coal seam. The program was originally developed for NIOSH by Dr. Keith Heasley, who is now at West Virginia University (WVU).
The Crandall Canyon catastrophe underscored the importance of design tools like LaModel in providing safe workplaces for miners involved in deep cover pillar recovery. Under the direction of Dr. Heasley, the research team at WVU is developing software wizards to provide improved recommendations for critical LaModel input parameters. Dr. Heasley's team is also back analyzing case studies of deep cover retreat mining and comparing LaModel results with those from the NIOSH-developed Analysis of Retreat Mining Pillar Stability (ARMPS) software. ARMPS is an empirically based assessment of pillar design that utilizes data from past deep cover mining cases to define successful and unsuccessful mining plans. ARMPS provides adequate design solutions for most simple situations, but LaModel provides more flexibility for the complex geometries and multiple seam situations that are often encountered. The verification of the program applications and revised input parameter selection should guide mine planners in the use of LaModel in conjunction with ARMPS, thereby providing optimal pillar designs that will prevent such tragedies as Crandall Canyon from recurring.
Contract Status & Impact
The project has been completed with the exception of workshop presentations. Due to the short project time frame, all tasks were conducted simultaneously by a team consisting of two professors and five graduate students. Eight more case histories than the 40 required case histories were completed. The case histories provided the necessary background to improve the application of the LaModel program as intended. A surprising and significant finding was that topography has a large effect on the results. Since topography is not included in the ARMPS analysis, an idealized LaModel output was developed for comparison with ARMPS. These results correlate well to the ARMPS stability factors for the pillar design. The loading in ARMPS is slightly different than that in LaModel: the Active Mining Zone (AMZ) sees more of the overall load in all loading conditions, and the gob and barrier pillars see less loading in ARMPS. Programming has also been completed for the Rock Stiffness Wizard, Gob Load Wizard, and Pillar Strength Wizard, and these will be included in the final version of the model. These wizards will facilitate the appropriate application of input parameters to ensure proper use of the program by the design engineer.