A Retrospective Assessment of Ground Control Research and Applications in the Coal Industry

This paper presents a retrospective assessment of ground control research and its successful application by CONSOL Energy over the past 25 years. Recognizing the need to improve the understanding of coal pillar design mechanics under shallow, medium, and deep covers, an extensive underground coal pillar instrumentation program was initiated in the mid 1980s. Combined with sophisticated numerical modeling technique and laboratory tests, results from this instrumentation program were used to advance the concept of coal pillar design and to demonstrate that numerical modeling can be used to predict in situ coal pillar strength, especially under non-ideal conditions where interface friction and roof and floor deformation are the primary controlling factors. To evaluate the impact of longwall mining, a 10-year subsidence research program was initiated in 1986 that encompassed surface, subsurface, and underground measurements, as well as a sophisticated numerical modeling technique. Results from the research revealed that interface friction and shearing between rock layers within the overburden had a profound effect on surface subsidence development and the resulting strain. Recognizing the impact of horizontal stresses on CONSOL?s mining operations, extensive horizontal stress measurements were conducted using the USBM Borehole Deformation Gage and the CSIRO Mininfrac system in the mid to late 80s. Roof extensometers and CSIRO HI Cells were used to measure roof displacement and roof stress concentration in longwall headgates and tailgates, which resulted in the definition of right-hand and left-hand longwall panels. Pillar design modifications, combined with cable bolt application and innovative standing support installation, were then successfully employed to mitigate horizontal stress impact on the right-hand longwall headgates and left-hand longwall tailgates. An in-house floor bearing capacity test apparatus was fabricated and employed underground to determine mine-specific floor bearing capacities which, along with other geotechnical parameters, were then used in the mine-specific shield design. To promote caving of thick, massive sandstone immediately behind longwall face supports and to improve face conditions, the hydraulic fracturing technique was successfully employed by CONSOL to pre-fracture the sandstone prior to longwall mining. Face monitoring and numerical modeling confirmed the effectiveness of the hydraulic fracturing technique. A roof geology reconnaissance program was initiated by CONSOL in the early 1990s to improve roof control, safety, and production. Information obtained from the reconnaissance program was used successfully to guide mine planning decisions and daily roof support practices.