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Elk Creek Mine Subsidence Page 19 February 26, 2003 CHIMNEY SUBSIDENCE POTENTIAL • Chimney (roof collapse) subsidence over the planned "D" Seam longwall panel workings should not breach the ground surface. Similarly, chimney subsidence over abandoned underlying "C" Seam and "B" Seam room and pillar workings should not have broken into the "D" Seam. In addition, no hydrologic paths should develop to the surface through the fractured and fissured zone above the caved rubble above the extracted "D" Seam longwall panels. ^ Theoretically, the maximum height of worst-case conical chimney collapse will not penetrate upward into the Elk Creek ' Mine overburden more than ten times the mining height (Piggott 6 Eynon, 1977). The actual maximum conical chimney collapse height should be less than the theoretical maximum because more ' competent sandstone beds present in the overburden should bridge across the progressively narrower conical collapse geometry. This theoretical maximum is 95-ft above 9.5-ft high gateroad ' entries and 120-ft above the 12-ft high Elk Creek Mine longwalls. Conical chimney collapse can develop above entry and crosscut intersections, as shown on Figure 2, where the collapsed rubble ' can move outward into intersecting entries and crosscuts. It fs more likely that rectangular collapse will develop over above the longwall panels, where the collapse rubble is confined between panel ribsides, as indicated on Figures 3 and 4. Theoretically, ' • rectangular chimney collapse will only penetrate 36-ft of the coal measure roof rock overlying the 12-ft high longwall panels. ' Piggott and Eynon's geometric method for predicting the theoretical worst-case height of chimney development, based on the type of working, the mining height and the percent swell of ' the collapsing rock was applied using the rock type distribution reported for the Elk Creek Mine interburden. Conical chimney collapse. Table 3 presents percent swell for various rocks. If ' twelve feet of coal was extracted and 60 percent the overburden is sandstone (678 free swell) and 40 percent is shale/mudstone (338 free swell), the maximum height of potential conical, ' worst-case, chimney collapse is approximately 67 feet. Therefore, it is not theoretically possible for conical chimney collapse to penetrate through the 210-ft minimum overburden above Panel #1. The same is the case for chimney collapse through the ' approximately 194 feet of interburden between the underlying "C" Seam and the planned "D" Seam workings. Similarly, it should be impossible for conical chimney collapse to reach the surface ' above the planned maximum 9.5-foot high "ff' Seam entries and gateroads or rectangular collapse above longwall panels in the Elk Creek Mine. Kenny (1969) reported the height of waste caving over actual • English longwalls as approaching four seam mining heights. This ' -19-