Mining Project: Integrated Analysis of Coal Pillar and Entry Stability
To reduce ground fall injuries and fatalities in underground coal mines through the application of improved analysis procedures that will facilitate the design of pillars, entries, and ribs as an integrated system.
The stability of coal mine entries cannot be properly analyzed without also considering pillar stability. It is widely understood that coal pillars interact with the surrounding strata. Excessive rib sloughage can lead to increased opening spans which could impact the stability of the mine roof. In contrast, stiffer roof and floor materials can lead to increased rib sloughage.
Although integrated analysis techniques have been discussed by ground control researchers in the past, little effort has been applied to addressing the simultaneous analysis of pillar and entry stability. Current ground control design methods consider pillar, roof, and rib stability as separate problems while they are actually interrelated. These empirical design methods are based on operational statistics that date back more than 30 years, and the methods are becoming less relevant as mines become deeper and operate in more difficult ground conditions.
Every year, unintentional falls of ground result in mine worker injuries and fatalities. Current trends in injury and fatality rates have remained relatively consistent over the past 5+ years. This trend is expected to continue given the status quo of ground control analysis procedures. The National Institute for Occupational Safety and Health (NIOSH) is uniquely positioned to set the tone for ground control research for the next decade and drive current injury and fatality trends further toward the goal of zero injuries and fatalities.
Under this project, NIOSH researchers will address the above issues by way of five research aims and related tasks, as summarized below.
- Develop improved procedures to assess the strength of stratigraphic units above and/or below the immediate mine opening. This aim will be achieved through borescoping and the potential utilization of multiple indirect rock testing methods that are emerging technologies not widely used in the industry, including the borehole scratch test developed by the Pennsylvania State University, seismic analysis, and geophysical methods such as gamma ray logging. A data collection procedure similar to the existing Coal Mine Roof Rating (CMRR) or other rock mass classification system will be developed as part of this research aim.
- Develop improved procedures for the analysis of rib stability. This aim will be accomplished through analysis of additional field monitoring sites, additional field measurements of a Coal Pillar Rib Rating (CPRR), and refinement of a rib factor of safety (RibFoS) being developed through the NIOSH project, “Design Methodology for Rib Control in Coal Mines.” During this process, analytical procedures for determining the potential suitability of rib support will be developed by way of a hybrid numerical-empirical modelling approach. Currently, field instrumentation sites are planned at three mines in Utah and Alabama.
- Develop improved procedures for the analysis of load distribution and pillar response. This research aim will expand the analysis of field monitoring sites to include those from western mines. FLAC3D models will be calibrated to the data collected from the new field sites and used to analyze the load redistribution and pillar response. From these models, a hybrid numerical-empirical approach will be used to determine the potential suitability of a given pillar design. From the subsequent analysis, a methodology suitable for implementation in pillar design will be determined.
- Develop improved procedures for the analysis of entry stability. This research aim will refine two existing analysis procedures developed through the NIOSH project, “Design Procedures for Gateroad Ground Control,” and integrate them with the rib and pillar analysis procedures into the proposed output described below. Stress and displacement measuring instruments will be installed at participating mines to further understand how stress redistribution impacts the mine roof and its interaction with the installed support system. Samples will be collected at as many mines as feasible. Each instrumentation site contains approximately 150 channels of data that will be collected at hourly intervals over six to eighteen months, resulting in approximately 2,000,000 data points collected at each field site.
- Develop an integrated analysis procedure for coal mine entry and pillar stability. This research aim integrates the four previous stand-alone aims. Direct sampling is not proposed for this aim, with the final output being a single user-friendly software tool for coal mine ground control analysis.
The integration of pillar and entry stability analysis resulting from this project could significantly impact the safety of the nation’s underground mine workers. The logical framework proposed would allow mine engineers to consider the stability of their mines with a holistic approach that addresses both global and local stability through one piece of software. This would allow mine engineers to understand the critical design parameters without the requirement of extensive numerical modelling expertise, multiple software packages, or separate analysis techniques. Based on the measure of the resultant software usage, statistics on ground fall accidents and injuries can be monitored after the project to provide insight into product performance.