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DRAFT <br />height in the rock above different mining geometries, i.e. rectangular collapse over large area <br />panels, wedge collapse over long narrow entries and conical collapse over four-way roadway <br />intersections. The calculation simply is for what height of roof rock must collapse and expand to <br />fill an underlying mined void applying three types of collapse geometry. Once the void and <br />chimney are filled with caved rock (gob), it is assumed that further roof collapse will be <br />prevented by the broken rock fill. <br />Gray, Bruhn and Turka (1977) tabulated data on 126 chimney collapses above room-and-pillar <br />workings in the nominally 6-foot thick Pittsburgh Seam to the overlying ground surface. The <br />relative cumulative frequency curve, Figure 11. Cumulative Percent of Chimney Collapse <br />Height, suggests that there is a very small probability, 0.8 percent, that a collapse chimney of <br />any type will progress upward through 200 feet of Pennsylvanian formation coal overburden to <br />the ground surface, irrespective either mining geometry or collapse geometry. Gray et. al. <br />recorded the elapsed time after mining that chimneys, sinkholes, breached the ground surface <br />and pillar collapse troughs dropped the ground surface, shown on Figure 12. Time Interval <br />Between Mining and Surface Breached or Dropped. They indicate that the time interval can <br />be as much as 100 years. The McClane Canyon Mine has extracted approximately 36% of <br />Cameo Seam coal by advance room-and-pillar mining at approximately 225 feet of depth, <br />apparently without any chimney collapse to the overlying ground surface. This can be seen on <br />Figure 6. McClane Canyon Mine Workings. This is common practice for operating coal mines <br />because the roof is reinforced as it is exposed and can be re-supported as required during the <br />operating life of the mine to prevent progressive chimney collapse. After a mine is closed <br />progressive deterioration of the roof can result in chimney failures, which at shallow depths can <br />and frequently do breach the ground surface. Areas where the overburden thickness is less <br />than 200 feet above the Cameo Seam in the Proposed Coal Lease area should be considered <br />at risk for long-tern chimney collapse to the surface. The 200-foot overburden contour is shown <br />on Figure 13. Overburden and Outcrop Map for the Project Area. The 200-foot overburden <br />contour extends approximately 360 feet upstream from the outcrop line in Big Salt Wash and <br />approximately 550 feet upstream from the outcrop line in Garvey Canyon. Long-term protection <br />from chimney subsidence to the overlying ground surface can be provided in such shallow <br />overburden by partially backfilling the entries in these two areas upon final closure of the Red <br />Cliff Mine. <br />4.3 Lithology and Angle of Draw <br />The purpose of the reasonably nearby drilling through the Mount Garfield formation for <br />Dorchester Coal Company's Fruita Project was to explore for potential mining of the Main <br />Cameo which ranged from 10 to 29 feet thick at depths of up to 1600 feet in their proposed <br />lease area. The reported Lithologic distribution of rock types above the Main Cameo from 19 <br />drillholes, which individually penetrated between 67 and 1316 feet of overlying rock, for a total <br />of 13,880 feet of drilling is presented in Table 5. Lithologic Distributions for Dorchester <br />Project Overburden. The overall average percentage of sandstone in the overburden is <br />approximately 46%. Abel and Lee (1984) collected data on the relationship between measured <br />angles of draw and the Lithologic distribution in the overburden above several coal seams. <br />Figure 14. Estimated Angle of Draw in Relation to Percent Sandstone and Limestone <br />presents the relationship. The Dorchester Project drilling indicated 46% sandstone and no <br />limestone and predicts a 19° angle of draw (a). The Dorchester drilling indicated considerable <br />lateral rock type variation. Therefore, it should be anticipated that there will be a similar variation <br />in the angle of draw. The range of sandstone percentage as determined from the drillholes was <br />from 28 to 65%, suggesting a range for the angle of draw from just over 15° to <br />Page 18 of 57 <br />