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"D" Seam Longwall Subsidence Page 59 August 17, 1998 flexure and associated extension of the ground surface may open as • the result of longwall panel mining. Prediction of the maximum horizontal compressive strain line is less important because compression ridges represent only a minor hazard. The maximum predicted vertical subsidence at the final recovery room and starter room for each individual panel in each panel group conservatively assumed that the gateroad pillar crushing subsidence at the local depth was added to the single panel subsidence at that depth. Table 13 presents the predicted subsidence at the ends of each panel in each panel group. Table 1 presented the maximum predicted worst-case subsidence and horizontal strain at the critical locations alongside and at the ends of the panel groups. The worst-case maximum predicted horizontal tensile (+E) and compressive (-E) strains for each of the five panel groups will result in observable subsidence effects. The magnitude of the maximum predicted tensile strains would result in tensile fractures at the ground surface. These fractures will be most pronounced at the shallower southern end of the panel groups. The absolute maximum predicted tensile strain is 59950µe, at the southern end of Panel #7. The predicted open surface fractures will approach 2-ft in width. Outside the southern panel mining limits these open fractures may intercept seasonal runoff • water and divert it toward the east or west, depending on the local hillside slope direction. In effect, the east-west open surface fractures will facilitate the flow of runoff toward the nearby natural drainages. The predicted north-south open surface fractures will tend to channel runoff water downslope, generally toward the south. The channelization of the seasonal surface runoff will tend to fill the open fractures and possibly temporarily increase erosion along the subsidence fracture channels. The impact should, however, be temporary as indicated by the absence of any such subsidence induced drainage channel diversions from the previous advance-and-retreat high-extraction room-and-pillar mining. Advance and retreat room and pillar mining at the shallow mining depths should have produced similarly high horizontal ground surface strains. No open fractures produced by shallow room and pillar mining are currently visible. The natural vegetation has apparently thrived in the subsidence areas in the past and should do the same in the future. The predicted maximum worst-case compressive strains will produce compression ridges at the ground surface within the projected outlines of the panel groups, at the locations indicated on the plates. The height of the worst case compression ridges, less than 1-ft, in relation to the hillside slopes at the ground surface should not impede the normal seasonal runoff. The absolute • maximum predicted compression strain is 15750µe, also at the southern end of Panel #7. 54