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"D" Seam Longwall Subsidence Page 3 August 17, 1998 • The conservatively predicted maximum, worst-case, trough subsidence effects are presented in Table 1. The predictions may be approached but will not be exceeded. Predicted maximum panel group vertical subsidence ranges from 10.02-ft (Panels #10 - #12) to 19.39-ft (Panels #7 - #9). Predicted maximum panel group horizontal tensile strains at the ground surface range from 6385µe (Panels #10 - #12) to 38970µe (Panels #7 - #9). The predicted locations of these maximum tensile and compressive horizontal strain lines are shown on Plates 6 - 10. Tensile strains on the order of 39000µe will result in open fractures approximately 2-ft in width at the ground surface, generally outside the horizontal limits of the individual panel group. Predicted maximum horizontal compressive strains range from 5010µe (Panels #10 - #12) to 30570µe (Panels #7 - #9). Compressive strains of• the magnitude of 30500µe will result in compression ridges being thrust up at the ground surface approximately 250 feet inside the panel group boundaries projected upward to the ground surface. These ridges will approach a maximum height of approximately 1-ft above the subsided ground surface trough. A portion of a landslide area overlies the southwest corner of planned Panel #3 (Plates 1 and 6) and the southeast corner of planned Panel #4 (Plates 2 and 7). This landslide area was identified on the Geologic Hazards Map for the North Fork of the • Gunnison River by the W.R. Junge (1978). The predicted maximum possible subsidence of the affected landslide area for those longwall panel groups is shown on Plates 1 and 2. The affected part of this landslide area will be tilted back into the slope by the planned longwall mining, because mining starts at the north end of each panel and proceeds toward the south. The overall stability of the landslide mass should, therefore, be enhanced by the subsidence accompanying longwall mining of Panels #3 and #4. Some portion of all sixteen planned "D" Seam longwall panels overlying abandoned "C2" Seam, "B" Seam and "B2" Seam room and pillar mine workings, as indicated by the outline of the mine workings shown on Plate 11. The underlying mine workings are present over the shallower southern part of all the planned "D" Seam longwall panels. Planned "D" Seam longwall mining in the Sanborn Creek Mine should not be adversely impacted by either chimney subsidence above room and pillar mine openings or stress concentrations over barrier pillars in the underlying workings. The interburden thickness should be sufficient to eliminate any possibility of a collapse chimney connection between the underlying room and pillar mining (Piggott ~ Eynon, 1977). The stress concentrations above underlying barrier pillars should be laterally distributed and, thereby, dissipated by the thickness of the interburden and the high proportion of sandstone in the • interburden, roughly 60~ (Chanda, 1989). 3