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Crandall Canyon Accident Investigation
Summary and Conclusions On August 6, 2007, six miners were killed in a catastrophic coal outburst when roof-supporting pillars failed and violently ejected coal over a half-mile area. Ten days later, two mine employees and an MSHA inspector perished in a coal outburst during rescue efforts.

The August 6 catastrophic accident was the result of an inadequate mine design“. https://arlweb.msha.gov/Genwal/ccSummary.asp

EXECUTIVE SUMMARY
The August 6 and 16 Accidents The Crandall Canyon Mine, in Emery County, Utah, was operated by Genwal Resources Inc (GRI), whose parent company was acquired by a subsidiary of Murray Energy Corporation in August 2006. On August 6, 2007, at 2:48 a.m., a catastrophic coal outburst accident occurred during pillar recovery in the South Barrier section, while the section crew was mining the barrier near crosscut 139. The outburst initiated near the section pillar line (the general area where the miners were working) and propagated toward the mine portal.

Within seconds, overstressed pillars failed throughout the South Barrier section over a distance of approximately ½ mile. Coal was expelled into the mine openings on the section, likely causing fatal injuries to Kerry Allred, Don Erickson, Jose Luis Hernandez, Juan Carlos Payan, Brandon Phillips, and Manuel Sanchez. The barrier pillars to the north and south of the South Barrier section also failed, inundating the section with lethally oxygen-deficient air from the adjacent sealed area(s), which may have contributed to the death of the miners. The resulting magnitude 3.9 seismic event shook the mine office three miles away and destroyed telephone communication to the section.

Federal and local authorities responded to the accident. MSHA issued an order pursuant to section 103(k) of the Mine Act that required GRI to obtain MSHA approval for all plans to recover or restore operations to the affected area. Mine rescue teams were organized, a command center was established, and a rescue effort was initiated. After unsuccessful attempts to reach the miners by crawling over the debris, GRI developed a rescue plan, approved by MSHA, to access the entrapped miners by loading burst debris from the South Barrier section No. 1 entry using a continuous mining machine. These efforts began on August 8 at crosscut 120.

On August 16, 2007, at 6:38 p.m., a coal outburst occurred from the pillar between the No. 1 and No. 2 entries, adjacent to rescue workers as they were completing the installation of ground support behind the continuous mining machine. Coal ejected from the pillar dislodged standing roof supports, steel cables, chain-link fence, and a steel roof support channel, which struck the rescue workers and filled the entry with approximately four feet of debris. Ventilation controls were damaged and heavy dust filled the clean-up area, reducing visibility and impairing breathing. Also, air from inby the clean-up area containing approximately 16% oxygen migrated over the injured rescue workers. Nearby rescue workers immediately started digging out the injured miners and repairing ventilation controls. Two mine employees, Dale Black and Brandon Kimber, and one MSHA inspector, Gary Jensen, received fatal injuries. Six additional rescue workers, including an MSHA inspector, were also injured.

Underground rescue efforts were suspended while a group of independent ground control experts reevaluated conditions and rescue methods, although surface drilling continued. In total, seven boreholes were drilled from the surface to the mine workings. Each successive borehole provided information as to conditions in the affected area and helped to determine the location of the next hole. None of the boreholes identified the location of the entrapped miners. Ultimately, it was learned that the area where the miners were believed to have last been working sustained extensive pillar damage and had levels of oxygen that would not have sustained life.

Explanation of the August 6 Collapse The August 6 collapse was not a “natural” earthquake, but rather was caused by a flawed mine design. Ultimately, it is most likely the stress level exceeded the strength of a pillar or group of pillars near the pillar line and that local failure initiated a rapid and widespread collapse that propagated outby through the large area of similar sized pillars.

Three separate methods of analysis employed as part of MSHA’s investigation confirmed that the mining plan was destined to fail. Results of the first method, Analysis of Retreat Mining Pillar Stability (ARMPS), were well below NIOSH recommendations. The second method, a finite element analysis of the mining plan, indicated a decidedly unsafe, unstable situation in the making even without pillar recovery. Similarly, the third method, boundary element analysis, demonstrated that the area was primed for a massive pillar collapse. Seismic analyses and subsidence information employed in the investigation provided clarification that the collapse was most likely initiated by the mining activity. Information provided by the University of Utah Seismograph Stations (UUSS) and from satellite radar images also helped in defining the nature and extent of the collapse.

The extensive pillar failure and subsequent inundation of the section by oxygen-deficient air occurred because of inadequacies in the mine design, faulty pillar recovery methods, and failure to adequately revise mining plans following coal burst accidents.

GRI’s mine design was inadequate and incorporated flawed design recommendations from contractor Agapito Associates, Inc. (AAI). Although AAI had many years of experience at this mine and was familiar with the mine conditions, they conducted engineering analyses that were flawed. These design issues and faulty pillar recovery methods resulted in pillar dimensions that were not compatible with effective ground control to prevent coal bursts under the deep overburden and high abutment loading that existed in the South Barrier section.

AAI’s analysis using the engineering model known as “ARMPS” was inappropriately applied. They used an area for back-analysis that experienced poor ground conditions and did not consider the barrier pillar stability factors in any of their analyses. The mine-specific ARMPS design threshold proved to be invalid, as evidenced by March 7 and 10, 2007, coal outburst accidents and other pillar failures. Despite these failures, AAI recommended a pillar design for the South Barrier section that had a lower calculated pillar stability factor than recommended by the National Institute for Occupational Safety and Health (NIOSH) criteria, lower than established by their mine specific criteria, and lower than the failed pillars in the North Barrier section. AAI performed the ARMPS analysis for the South Barrier section, but did not include these results in their reports that were presented to MSHA in support of GRI’s plan submittal.

AAI’s analysis using the engineering model known as “Lamodel” was flawed. They used an area for back-analysis that was inaccessible and could not be verified for known ground conditions, which resulted in an unreliable calibration and the selection of inappropriate model parameters. These model parameters overestimated pillar strength and underestimated load. AAI modeled pillars with cores that would never fail regardless of the applied load, which was not consistent with realistic mining conditions. They did not consider the indestructible nature of the modeled pillars in their interpretation of the results. Modeled abutment stresses from the adjacent longwall panels were underestimated and inconsistent with observed ground behavior and previous studies at this and nearby mines.

AAI managers did not review input and output files for accuracy and completeness. They also did not review vertical stress and total displacement output at full scale, which would have shown unrealistic results and indicated that corrections were needed to the model. Following the March 10 coal outburst accident, AAI modified the model, but failed to correct the significant submit written reports of these accidents to MSHA or plot coal bursts on a mine map available for inspection by MSHA and miners as required.

These reporting failures were particularly critical because they deprived MSHA of the information it needed to properly assess and approve GRI’s mining plans. Under Federal regulations, a mine operator is required to develop and submit to MSHA a “roof control plan” suitable to the prevailing geological conditions and the mining system to be used at the mine. MSHA has an opportunity to review and approve or disapprove the plan. MSHA had specifically separated the operator’s proposed mining plans into four separate plans, addressing different stages of the mining process, and had asked the mine operator to communicate any problems encountered so that MSHA could evaluate the safety of the plans as mining progressed. MSHA was only to approve the “retreat mining” phases of the project if favorable conditions were observed during development of the sections. However, the operator failed to make MSHA aware of the extent of the violent conditions encountered during mining and did not make MSHA aware of the severity of the March 10 coal outburst. MSHA approved the operator’s plans to conduct retreat mining in the South Barrier, where the fatal accident ultimately occurred, without the benefit of this critical information.

Additionally, GRI continued pillar recovery without adequately revising their mining methods when conditions and accident history indicated that their roof control plan was not suitable for controlling coal bursts. GRI investigations of non-injury coal burst accidents did not result in adequate changes of pillar recovery methods to prevent similar occurrences before continued mining. GRI did not consult with AAI or propose revisions to their roof control plan following the August 3, 2007, coal outburst accident in the South Barrier section, even though pillar conditions were similar to the failed area in the North Barrier section.

Explanation of the August 16 Accident

The August 16 accident occurred because rescue of the entrapped miners required removal of compacted coal debris from an entry affected by the August 6 accident. Entry clean-up reduced confining pressure on the failed pillars and increased the potential for additional bursts. Methods for installing ground control systems required rescue workers to travel near areas with high burst potential. Methods were not available to determine the maximum coal burst intensity that the ground support system would be subjected to. On August 16, the coal burst intensity exceeded the capacity of the support system. No alternatives to these methods were available to rescue the entrapped miners. As a result, only suspension of underground rescue efforts could have prevented this accident.

Prior to the August 16 accident, underground rescue efforts were only likely to have been suspended had definitive information been available to indicate that the entrapped miners could not have survived the accident. Information was not sufficient to fully evaluate conditions on the section prior to this accident. Sufficient resources, including drilling resources, should have been deployed. The rescue attempt imposed greater risks on rescue workers than would be accepted for normal mining. However, the prospect of saving the entrapped miners’ lives warranted the heroic efforts of the rescue workers. The greater risks imposed on the rescue workers underscore the high degree of care that must be taken by mine operators to prevent catastrophic pillar failures.


https://arlweb.msha.gov/Fatals/2007/CrandallCanyon/FTL07CrandallCanyonNoAppendix.pdf

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