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"D" Seam Longwall Subsidence Page 8 August 17, 1998 <br />• INTRODUCTION <br />The subsidence prediction study for planned longwall mining in <br />the Sanborn Creek Mine "D" Seam was undertaken at the direction of <br />Mr. Walt Wright, Mine Manager. Oxbow Carbon and Minerals, Inc. <br />provided the planned mine map, including seam thicknesses, <br />overburden depths, planned longwall panels, drillhole logs, <br />overlying surface drainages, underlying mine workings and the <br />possible overlying landslide hazard area. <br />The surface subsidence predictions for the individual planned <br />longwall panels were made using the British National Coal Board's <br />(NCB) "Subsidence Engineers' Handbook (NCB, 1975). This <br />conservative experience based longwall subsidence prediction method <br />includes consideration of longwall mining height, panel dimensions, <br />panel depth and previous ground subsidence from mining in the <br />underlying "C" Seam and "B" Seam. <br />The subsidence resulting from crushing of the gateroad yield <br />pillars after longwall mining the coal in the immediately adjacent <br />panels in the groups of three individual panels was predicted using <br />the experience based room-and-pillar mining subsidence prediction <br />method by Abel and Lee (1989). This method includes mining depth, <br />percent extraction and the width (50-ft) and height (6.5-f t) of the <br />widest, 50-foot, gateroad pillars. <br />• The NCB longwall surface subsidence predictions conservatively <br />utilized the 35° angle of draw measured for the Carboniferous <br />strata in Great Britain. The 35° angle of draw is conservative <br />because smaller angles of draw, less than or equal to approximately <br />25°, have been measured for the late Cretaceous/Tertiary Mesaverde <br />western U.S. coal-bearing formation (Pendleton, 1985; Gentry and <br />Abel, 1978; Abel and Lee, 1984). The larger, 35°, NCB assumed <br />angle of draw conservatively extends subsidence to a greater <br />distance beyond the longwall mining limits, approximately 50~ <br />further. Appendix B contains the NCB graphs used in calculating <br />surface subsidence over and adjacent to the longwall panels and <br />gateroad pillars using the 35° angle of draw. <br />LJ <br />e <br />