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West Elk Mine <br />• The cracks along the southern edge of Section 17 had influence from both MCC's F Seam <br />mining and Bear's B and C Seam mining and the barrier pillars between them. <br />• The cracks located between 5NW and 6NW longwall panels were most likely caused by the <br />reinitiation of an old landslide. <br />The cracks above 8NW longwall panel that CDMG had earlier attributed to subsidence were in <br />areas of low overburden and within a large old landslide mass. Other cracks have been <br />observed within the old landslide mass that is completely outside the mining area of influence. <br />Therefore, when assessing the potential for surface cracks due to longwall mining subsidence, the <br />presence of mitigating factors must be considered, such as existing landslide activity, adjacent to or <br />overlying mine workings, and very low overburden. <br />An additional factor in predicting the probability of surface cracking is the location with respect to <br />the longwall panel layout. With the exception of two of the 8NW longwall panel cracks, all of the <br />cracks were located above chain or barrier pillars, where stresses are maximized. The only cracks to <br />have formed within the internal portions of the longwall panels were very minor and had begun to <br />heal shortly after mining. These cracks are consistent with those formed by dynamic subsidence as <br />discussed within the section entitled Effects of Mining on Surficial Geologic Features. Also <br />discussed in that section were Mr. Dunrud's observations of additional cracks in the Apache <br />Rocks mining area (longwall Panels 12, 13, and 13A). Each of these cracks was located on <br />ridges or bluffs in zones of tension, and observed depths were only a few feet. <br />The observed dimensions of these cracks, widths of three to four inches, lengths of 10 to 20 feet, <br />and depths of less than one foot, indicate that the probable hydrologic consequences are negligible. <br />If one of these cracks were to intercept surface flow during the short time that the crack was open, <br />the volume of water that could be stored is less than 0.0002 acre-feet. This finding is very important <br />in that it indicates that the potential loss of surface flows via cracks may be a problem only where <br />the channels cross the pillars. It is in these locations where monitoring is most important and, if <br />necessary, mitigation might be needed. <br />WWE and Richard Dumud have determined that, except under precipitous slopes, the maximum <br />depth of surface cracks that will develop in the current permit area is 50 feet, and that typical crack <br />depths will rarely be greater than 25 to 35 feet. In the locations where surface fracturing does <br />occur, temporaiy loss of surface flow to more permeable zones in the overburden may be observed. <br />Cracks also become "sealed" with time as sediment transported by water washes into them. Tension <br />or surface cracks, which occur over longwall panels, will tend to "heal" themselves through <br />compression as the long -wall progresses and adjacent overburden subsides. Tension cracks over <br />chain pillars may stay open even after adjacent overburden has subsided, but will eventually <br />heal/seal. . - <br />In those locations where surface flow occurs across tension cracks at chain or barrier pillars. <br />temporary loss of flow may occur to more permeable zones in the overburden. Given the <br />discontinuous nature of these sand units within the Mesaverde Fonnation, however, these more <br />permeable uiuts will likely become saturated quickly, thereby reducing the introduction of <br />2.05-190 Revised June 2005 PR10. Januaiy 2006, March 2006; Rev. Alai 2006 PR10. Nov. 2006 TR107;Sep. 2007 PR12;Feb 2008 PR12 <br />