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I Sanborn Creek Subsidence Page 9 September 2, 1997 J interburden (Chanda, 1969). 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). 1 INTRODUCTION 1 The subsidence prediction study for planned longwall mining in 1 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 Hawksnest Mine and the possible overlying landslide hazard area. The longwall panel maps in Appendix A were extracted from the mine map. 1 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 364-ft above the B Seam. 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 1 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 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 1 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. 4