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_ Sanborn Creek Subsidence Page 9 September 2, 1997 i_ interburden (Chanda, 1989). Subsidence accompanying the planned B Seam longwall mining will probably, however, progressively drain • any water that may be impounded in the overlying mine workings because of the proximity of the overlying abandoned mine workings - and the magnitude of the predicted ground strains (Neate and ~ Whittaker, 1979). L r ~- The subsidence prediction study for planned longwall mining in r the Sanborn Creek Mine B Seam was undertaken at the direction of ,_ Mr. Walt Wright, Mine Manager. Oxbow Carbon and Minerals, Inc. provided the mine map, including existing Sandborn Creek Mine - workings, planned longwall panels, drillhole logs, overlying drainages, overlying mine workings in the Oliver No. 1 Mine and the J Hawksnest Mine and the possible overlying landslide hazard area. C The longwall panel maps in Appendix A were extracted from the mine map. The surface subsidence predictions for the individual planned longwall panels were made using the British National Coal Board's ~-• (NCB) "Subsidence Engineers' Handbook (NCB, 1975). This conservative experience based longwall subsidence prediction method includes consideration of longwall mining height (12-ft), panel ..~ dimensions, panel depth and previous ground subsidence from mining in the overlying Oliver No. 1 Mine in the D Seam, approximately 238-ft above the B Seam, and the Hawksnest Mine in the E Seam, __ approximately 369-ft above the B Seam. C The subsidence resulting from crushing of the gateroad yield pillars after longwall mining the coal in the adjacent panels in the groups of three individual panels was predicted using the ,- experience based room-and-pillar mining subsidence prediction method by Abel and Lee (1989). This method includes mining depth, " percent extraction and the width (50-ft) and height (8.5-ft) of the C widest gateroad pillars. The NCB longwall surface subsidence predictions were modified to a 25° angle of draw from the 35° angle of draw measured for the L Carboniferous strata in Great Britain. The 25° angle of draw conservatively conforms to the smaller measured angles of draw for the late Cretaceous/Tertiary Mesaverde western U.S. coal-bearing L formation (Pendleton, 1985; Gentry and Abel, 1978; Abel and Lee, 1984). Appendix B contains the NCB graphs used in calculating surface subsidence over and adjacent to the longwall panels and gateroad pillars using the 35° angle of draw. L~ 4