Ground movements associated with the extraction of underground coal pillars are sometimes large enough to damage surface structures. This paper presents the results of a study of related mine-level ground movements at a mine in West Virginia, located in the northern Appalachian Coal field of the eastern United States. Convergence points and roof extensometers placed in selected mine openings and stress meters installed in coal pillars measured the evolution of displacements and stresses during the successive stages of mining. The recovery of coal pillars led to distribution of the overburden load, resulting in a dramatic increase in stress in remaining pillars due to load transfer from the mined-out area and culminated in local bearing failure of the underclay floor and roof instability in the ventilation entries. This was accompanied by roof deflections and a variation of convergence depending on local differences in the load-carrying capacity of the roof/pillar /floor system.
Pillar extraction has a long history of use in United States coal fields and still accounts for a sizable proportion of the coal mined, even though longwall mining is becoming increasingly popular. Ground movements associated with pillar extraction are sometimes large enough to damage buildings and other surface structures. To foster a greater compatibility between coal mining practices and current and future land use, GAI Consultants, Inc., under contract to the U.S. Bureau of Mines and U.S. Department of Energy, conducted a study of ground movements at an underground mine in northern West Virginia (Bruhn and Speck, 1986). This paper presents the findings of the mine-level portion of at study.
The mine is located in the nearly flat-lying Lower Kittanning Coal Seam located approximately 183 meters below the ground surface. Within the subject mine-panel the coal thickness varied from 1.4 to 1.9 meters in thickness. A 2.5 to 5.0-centimeter thick bed of argillaceous coal divides the coal into two units. Overburden strata are of the Pennsylvanian age Allegheny, Glenshaw, and Casselman Formations and consist of alternating units of claystone, shale, siltstone, and sandstone, along a few limestone and coal units. Residual soils and alluvial sediment up to 4.6 meters thick mantle the site. The immediate mine roof is composed of 4.9 meters of silty and calcareous shales overlain by a 2-meter hard, gray limestone. Rock Quality Designation (RQD) values for these strata varied from 69 to 88 percent, which indicates the rock quality to be fair to good. The mine floor is composed of 9 centimeters medium hard underclay which overlies 46 centimeters of soft to medium soft underclay. Silty and sandy shales separate the underclay from the 0.3 meter thick Clarion coal located about 5 meters below the mine floor.
Mining was conducted using a single Joy 14 CM continuous miner, along with a scoop and 21 SC shuttle cars. The room and pillar, advance and retreat, mining method permitted almost total (> 90 percent) removal of coal from the panel.