PERMFILE41947 - Laserfiche WebLink

Home Browse Search

West Elk Mine cracking can occur at the location of the differential subsidence. As mining continues, the ~ adjacent overburden subsides and the surface cracks will usually close completely (DeGraff and Romesburg 1981). These cracks healed significantly as evident in the late summer of 1997. Additionally, the overburden thickness under the areas where the cracks occurred was less than 500 feet-one of the few locations within the permit area where the overburden is this shallow. Given the long and extensive history of mining in the area, it is surprising that orily four azeas of subsidence related features have been identified It cannot be proved that these represent the only cracks associated with the mine, but given the regular survey activity, inspection of the subsidence profiles, and seasonal landowner, hunter, and U.S. Forest Service (USFS) personnel activity, additional cracks would have been noted if they existed. This lack of evidence of surface cracking would lend strong validation to the premise that longwall mining has had minimal surface impacts at the West Elk Mine. Landslides - It could be expected that the changes in stress and strain of the neaz-surface strata and possible neaz-surface fractures associated with the predicted subsidence could reactivate or initiate landslides. Presently, there aze four known locations within the pemut area where the reactivation of a landslide could be potentially linked to past or current mining. The first azea, in Lone Pine Gulch (Section 20, T13S, R90W, 6a' P.M.), contains numerous old landslide features, including steep, hummocky topography with many smaller surficial slumps. Cracking and slumping occurred on one section of a jeep trail in this area in early October 1994. The cracks appeared during mining • of the SNW longwall panel in the B-Seam. Location of the crocks coincides with the boundary of eazlier F-Seam room-and-pillar panels. The second area, discussed previously, is above the 8NW longwall panel. This landslide activity, which could not be definitively linked to longwall mining, is described in considerable detail in CDMG's inspection report (CDMG 1996). The third area is above the 9NW longwall panel. Another known landslide azea is south of Highway 133 near Box Canyon within the Box Canyon Permit revision area. These landslides are outside the projected longwall mining subsidence effects (i.e., 16 degree angle of draw) by more than 600 feet. Amore thorough description of these slides can be found in Section 2.04.6, Geology Description. A small portion of the development mining area along the northwest side longwall panel 18A underlies a known area of landslide disturbance south of Highway 133 as shown on Map 1 of Exhibit 60C. As a result, the worst possible consequence associated with mining-induced subsidence in the West Flatiron lease area is that this landslide will be reactivated during mining activities causing a potential crossing of Highway 133 and an obstruction to traffic. Based on observations made in the field both of active and inactive landslide areas before and after mining activities, MCC and it's consultant (WWE) concludes that the greatest potential for the worst possible consequence would occur during a very wet precipitation period. Such periods have been observed in the North Fork Valley as recently as the mid-1990s causing initiation or reactivation of numerous natural landslides. With few exceptions, most landslides along the North Fork Valley aze relatively shallow in depth and move relatively slowly downhill (gravity creep) providing an opportunity for monitoring and • evaluation. Monitoring of the landslide toe south of Highway 133 will provide an opportunity to assess whether reactivation of the landslide has occurred and to what extent this reactivation may 2.05-I04 Revised Jun. 1995 PR06;1/96 RT'03; Revised May l999 TR89;Revised Jan 1998 PR08; March 1005 PR11 ,~,~'~~