Mining Project: Assessing Elevated Stress Due to Full Extraction Mining

Research Summary

Dynamic rock and coal failures are a significant hazard during full extraction mining. These events are known as coal “bumps” or “bursts.” According to Mine Safety and Health Administration (MSHA) accident data, there were approximately 380 coal burst events reported from 1983 to 2013. Of those, 20 resulted in fatalities with 33% in longwall mines. There were two additional fatalities in a single event in 2014 during room and pillar retreat mining. The two most recent coal burst events that resulted in fatalities occurred in room and pillar retreat mining operations with distinct multi-seam interactions specifically at the accident sites.

Numerous research endeavors during the past 25 years have identified and implemented improved mine design and operational practices that have successfully reduced the rate of occurrence of burst events to the point where catastrophic events today are very rare. Nonetheless, the potential for such events and the ongoing challenge of prevention mandate that continued research be conducted to fully eliminate them.

To address this need, this project had four research aims, as follows:

1. Determine the effect of gob development and resulting stress redistribution due to full extraction operations through field monitoring.
2. Develop numerical models that can simulate stress redistribution and associated pillar and ground response resulting from full extraction mining, and conduct parametric studies to examine controlling factors.
3. Determine the critical overburden and floor strength and stiffness relative to elevated stresses that drive dynamic failure or failures that lead to burst conditions.
4. Develop a standardized procedure to predict the elevated stresses due to full extraction mining.

This project’s research aimed to use the best available geological data and improved identification of potential areas of elevated stress associated with full extraction coal mining to provide the most accurate risk assessment relative to full extraction mining, given current geotechnical practices in the U.S. coal mining industry. Providing an improved picture of the stress regime will enable future enhancements in geological characterization, which will be developed through other efforts to further refine the identification of conditions and mechanisms that control bursts and lead to further risk assessment enhancements. Ultimately these two research agendas—stress assessment and geological characterization—will combine to provide the best opportunity for developing prevention measures and avoidance forecasting designed to eliminate bump catastrophes. This project primarily benefits research efforts related to coal bursts, but is also applicable to all ground control issues and may help to reduce other safety hazards in underground coal mining.

Prior to completing the intended research aims of this project, the project was cancelled due to staffing and resources issues. Comprehensive instrumentation and monitoring sites were completed at several mines: a longwall gateroad, a room and pillar retreat section, and three bleeder entry sites. The data from these sites are being utilized in concurrent NIOSH research projects—Design Procedures for Gateroad Ground Control and Design Methodology for Rib Control in Coal Mines—to improve the stress redistribution due to full extraction, with a focus on longwall gateroads and bleeder entries.