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Elk Creek Mine Subsidence Page 13 February 26, 2003 <br />INTRODUCTION <br />• The subsidence prediction study for planned longwall mining <br />in the Elk Creek Mine `D" Seam was undertaken at the direction of <br />Mr. Robert L. Koch, Chief Mining Engineer. Oxbow Mining, LLC <br />provided the planned longwall panel locations, seam thicknesses, <br />panel mining height, gateroad entry and crosscut height, <br />overburden depths, drillhole logs, overlying surface drainages, <br />underlying mine workings and the overlying landslide hazard area. <br />^ The surface subsidence predictions for the individual <br />planned longwall panels were made using the British National Coal <br />' Board's (NC$) "Subsidence Engineers' Handbook (NCB, 1975). This <br />conservative experience based longwall subsidence prediction <br />method includes consideration of longwall mining height, panel <br />' width, panel depth and the area of previous room and pillar <br />mining in the underlying "C" Seam and `B" Seam. <br />' The NCB longwall surface subsidence predictions <br />conservatively utilized the 35° angle of draw measured for the <br />Carboniferous strata in Great Britain. The 35° angle of draw is <br />' conservative because smaller angles of draw, less than or equal <br />to approximately 25°, have been measured for the late Cretaceous <br />and Tertiary Mesaverde western U.S. coal-bearing formation <br />' • (Pendleton, 1985; Gentry and Abel, 1978; Abel and Lee, 1984). <br />The larger, 35°, NCB assumed angle of draw conservatively extends <br />subsidence to a greater distance beyond the probable longwall <br />' mining limits, approximately 50$ further. Appendix C contains <br />the NCB graphs used in calculating surface subsidence over <br />individual and adjacent to the longwall panels using the 35° <br />angle of draw. <br />The subsidence resulting from unlikely, but possible, <br />' complete crushing of the gateroad pillars after longwall mining <br />the coal in immediately adjacent longwall panels was predicted <br />using the experience based subsidence prediction method for <br />low-extraction room-and-pillar mining by Abel and Lee (1984). <br />' This method includes mining depth, percent extraction and the <br />width and height (9.5-ft) of interior gateroad pillars between <br />adjacent longwall panels. <br />1 <br />'~ -13- <br />