2011-09-23_REPORT - C1980007 (8)

Home Browse Search

Subsidence Prediction Based on Measurements Appendix A at West Elk Mine Page 2 The height of the zone of fracturing is a function of lithology and layer thickness, according to Peng (1992, p. 6 -8). For example, the zone of fracturing commonly is higher for strong, thickly - bedded, brittle sandstones than it is for thinly layered, soft, shales and claystones. Liu (1981) reports ranges of heights of the zone of fracturing for various rock types as follows: 1. Heights of 20t to 30t are reported in strong brittle rocks, such as siliceous sandstones and limestones; a value of 28t was reported for overburden containing 70 percent sandstone. Also, because of hardness, fractures do not close as readily in brittle rocks as they do in soft rocks during recompression. 2. Heights of 9t to 11 t are reported where all the rocks consist of soft shales and claystones. The fractures also commonly close again under stresses associated with static conditions, and become impermeable again. According to the senior geologist at the West Elk Mine, 10t to 20t is a good projection for the height of fracturing in the Apache Rocks and Box Canyon mining areas. Within the South of Divide mining area, the fracture zone may become less continuous in the caved zone with increasing height because of the alternating sequence of harder and brittle rocks and softer and yielding rocks. The height of the fracture zone, therefore, will likely be less —by possibly 10 to 20 percent —than the height predicted for the Apache Rocks and Box Canyon mining areas because of the presence of more shale above the E -Seam mining in the South of Divide mining area. Fractures near the top of the caved zone, therefore, will likely become less continuous with increasing height in the zone of fracturing. However, a projected fracture height of 30 times the coal extraction thickness (30t) may locally occur (Koontz, oral communication March 2004). Also, with increasing height in this zone, and as lateral and vertical constraints increase, fracturing that affect water bearing zones will tend to occur more in zones of convex upward curvature, along separated bedding planes toward the center of the panel, and along local cracks in zones of convex downward curvature (Figure A). Fracturing within the expected zone of fracture may cease completely where soft shales and claystones occur as alternating sequences with sandstones. Drainage, however, may cease after mining is complete and any water bearing zones present may be restored. This is particularly likely in the upper part of the fractured zone in shale sequences between sandstone layers, once subsidence is completed and the separated beds re- compress and close in response to overburden load (Figure A). Evidence of restored water levels has been measured and reported in some wells in the West Elk Mine subsidence monitoring area after mining and subsidence were complete. Continuous Deformation Zone and Near Surface Zone These two zones are discussed together because the ground surface is where nearly all subsidence measurements are made that monitor subsidence processes active in the zone of continuous deformation. 831 - 032.690 Wright Water Engineers, Inc.