An investigation was made of ground response to pillar retreat mining in a 1.7 meter thick seam at 108 meter depth at a site in northern West Virginia. This paper describes mining-related stress changes and movements at mine level, displacements within the overburden, ground surface subsidence and groundwater level variations. Subsidence approached 40 per cent of the mine height and continued more than a year after mining. Water levels declined in certain deep-lying strata.
Pillar extraction has a long history of use in U.S. coal fields and, even though longwall mining is becoming increasingly popular, still accounts for a sizable proportion of the coal mined. Ground movements associated with pillar extraction are sometimes large enough to disrupt the ground water regime as well as damage buildings and other surface structures. The federal government has recognized the need for a better understanding of subsidence processes so that a greater compatibility can be achieved between coal mining and current and future land use. To further that understanding, GAI Consultants, Inc., under contract to the U.S. Bureau of Mines and U.S. Department of Energy, is conducting a study of subsidence at Kitt Energy Corporation's Kitt No. 1 Mine in northern West Virginia. This paper presents the initial findings. Further reports will follow.
The monitoring site occupies approximately 0.26 square kilometers of rolling grasslands having a maximum relief of 55 meters. The mine is located in the nearly flat-lying 1.7 meter Lower Kittanning Coal approximately 180 meters below ground surface. Overburden strata (Figure 1) are of the Pennsylvanian age Allegheny and Conemaugh Groups and consist of alternating units of claystone, siltstone, shale and sandstone, along with a few thin coal units. One-third to one-and-one half meters of colluvial and residual soils mantle much of the site. Alluvial sediment three to five meters, thick occupies the topographically low, south-central portion of the site.
The room and pillar system employed at the mine permitted almost total (>90 percent) removal of coal from the panel. Developmental (advance) mining consisted of driving a series of 5.2-meter wide entries on 18-meter centers from the mains to the far end of the panel, joining them on 27-meter centers with crosscuts of the same width. Ventilation entries were driven along the south and east margins of the panel. Retreat mining began with Pillar 1 (Figure 2) and progressed row by row toward the main entries. Pillars were extracted in pairs. All coal was removed with a single continuous miner. Developmental mining took nine months; retreat mining six months. The retreat rate was relatively steady. Coal in adjoining panels was unmined. No prior mining had been conducted in seams above or below the panel.
A variety of devices were installed at the monitoring site to measure displacements, stresses and water levels: at mine level -- convergence points, roof extensometers, instrumented roof bolts and stress meters; in the overburden -- inclinometers, time domain reflectomerry cables, multiple position borehole extensometers, and piezometers; at ground surface -- survey monuments and benchmarks. Details are discussed in Speck and Bruhn, 1983.