Ground control monitoring of retreat room–and–pillar mine in Central Appalachia

In order to study pillar and overburden response to retreat mining, a ground control program was conducted at a Central Appalachian Mine. The program consisted of several monitoring methods including a seismic monitoring system, borehole pressure cells in the pillars, and time-lapse photogrammetry of the pillar ribs. Two parallel geophone arrays were installed, one on each side of the panel with the sensors mounted 3 m into the roof. A total of fourteen geophones recorded more than 5000 events during the panel retreat. A MIDAS datalogger was used to record pressure from borehole pressure cells(BPCs)located in two adjacent pillars that were not mined during retreat. A series of photographs were taken of the pillars that had the BPCs as the face approached so that deformation of the entire rib could be monitored using photogrammetry. Results showed that pillar stability and cave development were as expected. The BPCs showed an increase in loading when the face was 115 m inby and a clear onset of the forward abutment at 30 m. The photogrammetry results displayed pillar deformation corresponding to the increased loading. The microseismic monitoring results showed the overburden caving inby the face, again as expected. The significance of these results lies in two points,(1) we can quantify the safe manner in which this mine is conducting retreating operations, and(2) we can use volumetric technologies(photogrammetry and microseismic) to monitor entire volumes of the mine in addition to the traditional point-location geotechnical measurements(BPCs).

Microseismic monitoring and analysis of induced seismicity source mechanisms in a retreating room and pillar coal mine in the Eastern United States

A microseismic monitoring system was installed in an underground room and pillar coal mine in the Eastern United States to analyze the occurrence and characteristics of induced seismicity during the retreat of two panels in the mine.This study is the first microseismic monitoring effort at an underground coal mine in nearly 30 years.During the retreat of the first panel,an array of eight uniaxial geophones,installed 10 ft.into the roof,recorded events and their magnitudes.The second panel was monitored using an array of twelve uniaxial geophones and two triaxial geophones,also installed 10 ft.into the roof.Comparing the results of these studies,it has been found that the magnitude of seismic events is minimally affected by immediate roof geology or depth of cover.However,it was observed in both studies that the rate at which seismic events occurred did vary with changing roof geology and depth of cover.Using the seismic data from the second panel retreat,focal mechanism solutions were generated for 50 hand-picked events in order to determine if the failure was in compression,tension,or shear.Results of the focal mechanism solutions show that stress relief resulting in dilational events occurs at significant depths,150-200 m in this case,beneath the active mining face.