2017-01-04_PERMIT FILE - C1981044A

Home Browse Search

Where partially extracted room -and -pillar workings exist, maximum observed subsidence averaged up to 10 feet. Surface cracking has been associated with this subsidence. In general, the cracks tended to parallel the surface topography. Surface cracking was best developed where sandstone beds lie at or near the ground surface or the overburden is less than 400 feet thick. Predicted Subsidence Phenomena The development and magnitude of surface subsidence are dependent on 1) overburden depth, 2) geology, 3) surface topography, and 4) mine layout. Overburden Depth. The magnitude of vertical subsidence is very dependent upon both the depth of overburden above the extracted area, denoted as height (h), and the dimensions of the extracted area, denoted as width (w) and length (1). The length of the extracted area is critical when the length is less than 1.4 times the width. The extracted area's width and the overburden depths are used to calculate the width: depth ratio (denoted as w:h). Depending on the value of the w: h ratio, excavations can be classified as critical, supercritical, or sub -critical. Critical subsidence conditions normally result in a U or V shaped subsidence trough. For these conditions, one (1) point, usually at the center of the extracted area, reaches critical or maximum subsidence, (SmAx). Supercritical subsidence conditions exist when the w:h ratio is greater than the critical conditions. Super -critical conditions normally result in a flat -bottom trough. An area on the ground surface reaching the maximum or critical subsidence characterizes this trough. Sub -critical subsidence conditions exist when the w:h ratio is less than that required for critical conditions. Sub - critical conditions normally result in a V shaped subsidence trough. The maximum subsidence measured for these conditions is less than the critical subsidence. Using National Coal Board of Great Britain (NCB) values (NCB, 1975), a summary of w:h ratios for typical mining geometry's in the EC permit area have been calculated. These w:h ratios are tabulated on Table 82, Width:Depth Ratios for Longwall Panel No. 1 (600 Foot Extraction Width), and Table 83, Width:Depth Ratios for Multiple, Adjacent Longwall Panels. Based on the values found on these tables, the ratio of subsidence to extracted seam height will generally be sub -critical to critical for individual longwall panels. However, multiple, adjacent longwall panels will generally result in super- critical conditions for most mining geometry's except those where the overburden is greater than 1400 feet. Basically, the tables indicate that the maximum subsidence that can develop will develop when three (3) or more adjacent longwall panels are mined. Overburden depth can affect the magnitudes of horizontal strains. As the overburden depth increases, subsidence decreases and the effects of subsidence are distributed over a much larger area. Surface strains then tend to be reduced as depth increases. Maximum compressive strains should occur near panel centerlines. Maximum tensile strains should occur over ribsides separating mined out areas from solid coal. Geology. Investigations in Australia (Kapp, 1974), New Mexico (Gentry, Stewart, and King, 1981), Colorado (Dunrud, 1976), and Utah (Bom, 1982) indicate that the stratigraphy and geologic structure of the rock overburden are factors which control the magnitude of subsidence. Stronger strata apparently occupy a greater volume as failure occurs. This results in a reduction in magnitude of vertical subsidence movement. Test borings in the permit area indicate that the overburden contains approximately 39% thick sandstone beds, 8% thin sandstone beds, 11% sandy siltstones, 35% very fine-grained classics (siltstone, mudstone, and shale), and 7% coal. Subsidence measurements at the EC Mines (Bogenreif, 1984 and Elkens, 1986) indicate that geology does have an impact on the magnitude of maximum subsidence that occurs at the mine site. The large percentage of thick sandstone units significantly reduces the magnitude of maximum subsidence resulting from mining. Based on similar geologic environments, subsidence in the planned permit area should be similar to that experience at other locations Permit Revision 04-34 2.05-58 Revised 7/2/04