2008-04-02_REVISION - C1981044 (5)

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Evaluation of Subsidence Impacts Subsidence monitoring began in the permit area in the early 1980s after mining had been largely completed in the bond release area. No structures were undermined in the bond release area. Renewable resource lands that were undermined in the bond release area are aquifers (T'wentymile Sandstone, Middle Sandstone, Yampa River alluvium) and pasturelands. Impacts to aquifers were discussed in the Evaluation of Hydrologic Impacts. The Division':> policy regarding liability releases for underground workings is set forth in a Policy Memorandum titled "Release from Liability of Areas Overlying Underground Coal Mine Workings", dated April 13, 1987. This policy allows a release of liability for underground workings three years after mining has ceased, if the major subsidence effects have occurred, and if no revegetation of the land surface is necessary. The three- year timeframe may be adjusted based on site-specific information. The underground workings in the bond release area satisfy the three-year liability period, based on mining in the bond release area having ceased more than 20 years ago. No revegetation of the land surface is necessary in the bond release area. The major subsidence effects in the bond release area appear to have occurred based on comparison of'the subsidence effects found during the bond release inspection with the predicted effects that are likely to occur in the future. The inspection found cracks in sandstone outs;rops over the 1St West Mains and the 1St Left Panel. The cracks are several inches wide, a:re at least 10 feet deep, and are up to 150 feet in length. These cracks may have resulted i:rom tensile stress acting on these competent layers of sandstone where they overlie barrier pillars along the margins of the workings. Additional cracks in the outcrops may :form in the future, but probably at a lower rate than before due to the stress relief provided) by the previous deformation. The mine operator has backfilled the cracks for hazard mitigation. (Subsidence troughs found during the bond release inspection over the 1St West Mains were most likely created by older, shallower mining.) In addition to surface cracks, a subsidence effect that can be expected to occur in the future is the formation of depressions on the land surface that will result if the roof or pillars fail in tlhe workings. Depressions probably can form on the land surface only where the width of the underlying mine void is less than about 1.0 times the overburden thickness (subcritical areas) because the areas where the width is more than 1.0 times the overburden thickness (critical and supercritical areas) would have reached maximum subsidence shortly after mining, as usually occurs in critical and supercritical areas. Other than subsidence cracks, subsidence features were not found during the bond release inspection in tlhe two critical/supercritical areas of the site, the 1St Left Panel and an area 1,000 feet norl:hwest of the No. 5 portal. These areas probably have subsided a few feet, similar to F-seam room-and-pillar mining in the 3`d East area one mile southeast of the bond release area where 4.8 feet of subsidence was measured by the operator (permit page 2.05-57). Visual evidence of subsidence in the 3`d East area was limited to surface cracks.