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| Ground control problems created in the seam 170 feet above an extracted longwall panel |
Background
Studies show that 70% of underground coal mines face multiple-seam mining situations where the likelihood of encountering hazardous ground conditions is high. Knowing the location of prior mining, planning engineers may seek to access and mine new reserves above or below old workings. In such situations, two common questions arise:
- Will workings above or below cause excessive stresses in the proposed workings that could lead to rock falls, pillar bumps, or water inundations?
- Will subsidence from workings below cause destructive ground control conditions in the upper seam?
Whether an adverse multiple-seam mining situation exists or not depends on numerous factors, including mining geometry, mine design, and geology. Project researchers have collected well over 250 case histories of multiple-seam mining interactions. Case parameters include geometric factors (depth, seam thickness, and interburden thickness), geologic factors (roof rock CMRR, percentage of sandstone in interburden), mining factors (Analysis of Retreat Mining Pillar Stability factor, or ARMPS, panel width, percentage of extraction), and finally a rating that describes the degree of interaction, from "none" to "mining abandoned." Using the boundary-element program LaModel, project researchers are presently examining numerous case histories to quantify the stress and displacement conditions that may lead to adverse multiple-seam mining interactions.
Potential Outcome
Based on statistical analyses of the case history database and numerical models of failure mechanics, design tools will be developed for evaluating and controlling potential multiple-seam mining interactions in 1 year. This design tool will take information about mining geometry, mine design, and geology and evaluate the potential for a multiple-seam mining interaction. In addition, the design tool will forecast the likely magnitude of the interaction. This tool will likely take a form similar to the successful ARMPS program and is tentatively named ARMPS-MSM (for multiple-seam mining).
These design tools will be widely distributed to our stakeholders through classes at the MSHA Academy and open-industry briefings throughout the coal fields. It is expected that many mines exposed to multiple-seam interactions will employ ARMPS-MSM to recognize hazardous conditions before an event occurs, and either avoid them or implement control strategies.