2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (43)

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4.2 Lithologic Factors Affecting Subsidence Different lithologies (rock types) have differing strengths and therefore differing swell potential when broken. As indicated on Table 4. Bank Density, Swell Factor and Percent Free Swell for Selected Rocks and Soils, there is considerable variation in the percent swell between rock types and within rock types. The height of caving above the mine workings is reduced where the roof rocks consist of strong (high percent swell) sandstones compared to weak (low percent swell) shales, mudstones or soft siltstones. However, the height of rock fracturing above mined openings is greater for strong, brittle sandstones compared to weak, more yieldable shales, mudstones and soft siltstones. The mean percent swell of the overburden rocks controls the potential maximum height of the collapse zone upward in the immediate roof above a longwall panel, an entry or an intersection between an entry and a crosscut. Figure 10. Potential Collapse Heights Above Different Mine Opening Geometries by Piggott and Eynon (1977) provides a percent swell based method for predicting the maximum collapse height in the rock above different mining geometries, i.e. rectangular collapse over large area panels, wedge collapse over long narrow entries and conical collapse over four-way roadway intersections. The calculation simply is for what height of roof rock must collapse and expand to fill an underlying mined void applying three types of collapse geometry. Once the void and chimney are filled with caved rock (gob), it is assumed that further roof collapse will be prevented by the broken rock fill. Gray, Bruhn and Turka (1977) tabulated data on 126 chimney collapses above room- and-pillar workings in the nominally 6-foot thick Pittsburgh Seam to the overlying ground surface. The relative cumulative frequency curve, Figure 11. Cumulative Percent of Chimney Collapse Height, suggests that there is a very small probability, 0.8 percent, that a collapse chimney of any type will progress upward through 200 feet of Pennsylvanian formation coal overburden to the ground surface, irrespective either mining geometry or collapse geometry. Gray et al. (1977) recorded the elapsed time after mining that chimneys, sinkholes, breached the ground surface and pillar collapse troughs dropped the ground surface, shown on Figure 12. Time Interval Between Mining and Surface Breached or Dropped. They indicate that the time interval can be as much as 100 years. The McClane Canyon Mine has extracted approximately 36% of Cameo Seam coal by advance room-and-pillar mining at approximately 225 feet of depth, apparently without any chimney collapse to the overlying ground surface. This can be seen on Figure 6. McClane Canyon Mine Workings. This is common practice for operating coal mines because the roof is reinforced as it is exposed and can be re- supported as required during the operating life of the mine to prevent progressive chimney collapse. After a mine is closed progressive deterioration of the roof can result in chimney failures, which at shallow depths can and frequently do breach the ground surface. Areas where the overburden thickness is less than 200 feet above the Cameo Seam in the Proposed Coal Lease area should be considered at risk for long-term chimney collapse to the surface. The 200-foot overburden contour is shown on Figure 13. Overburden and Outcrop Map for the Project Area. The 200-foot overburden contour extends approximately 360 feet upstream from the outcrop line in Big Salt Wash and approximately 550 feet upstream from the outcrop line in Garvey Canyon. Long- term protection from chimney subsidence to the overlying ground surface can be provided in such shallow overburden by partially backfilling the entries in these two areas upon final closure of the Red Cliff Mine. C-14 DBMS 306