2008-03-21_PERMIT FILE - C1980007A (5)

Home Browse Search

West Elk Mine • 4. A limited set of cracks occurred above the barrier pillar on the east boundary of 1NW longwall panel, along the east line of Section 20. Again, this area was also affected by room-and-pillar miring in the F Seam. While mining was occurring in 8NW longwall panel, MCC received a complaint from the landowner. regarding cracks on his property. As a result of the complaint, CDMG conducted an inspection of the surface cracks on the owner's property and wrote an inspection report summarizing their observations (CDMG, 1996). Since WWE was denied access to the site by the land owner, and therefore limited to low-elevation aerial reconnaissance, the discussion of the mechanism behind the formation of these cracks relies mostly on the CDMG inspection report. These cracks were observed in the vicinity of a relatively large, historic, episodically active landslide (Dames and Moore 1993). Extensive recent landsliding was observed in the SW1/4 of Section 24 in the spring of 1996. The landslide activity created numerous cracks and "graben-like extensional troughs up to ten feet wide and five feet deep." These cracks were parallel to the fall line, which is typical of the translation of the sliding debris as shown in Figure 19. According to Dr. Pendleton of DMG, the observed features are typical of large landslide masses in the Williams Fork Formation (geological equivalent of the Mesaverde Formation) and they occur prolifically throughout the North Fork Valley on slopes of varying gradient and aspect. Based on his experience in the Forth Fork Valley, Dr. Pendleton concluded that subsidence does not appear to be a significant determinant in the reactivation or initiation of landslide activity. CDMG concluded that "there is no evidence with which to definitively verify or discount a connection between subsidence of the MCC mine workings and this active landslide." Eight crack locations were visited during the CDMG inspection. While most of the surface cracks were attributable to rejuvenated landslide movement, three minor cracks were reported to be the result of mining subsidence (Nos. 2, 3, and 8 on Figure 19A). These cracks were three to four inches wide and less than one foot deep. Field observations by MCC personnel indicated that these cracks were already healing shortly after mining had occurred. Two of these cracks (Nos. 3 and 8) are typical of the dynamic subsidence process. As mining occurs, the overburden above the mined portion subsides, and differential movement results between the mined and ummined areas. Surface 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. 6. Within the Apache Rocks mining area Mr. Duiirud has observed several additional surface cracks (see report titled "Subsidence Observations, West Elk Mine, July 22-24, 2003. These include the following: ® Location 13 - located in the NW 1/4. Section 28, T 13 S, R 90 W o Location 3 -bluff located above the eastern part of Ion-wall panel 14 above the eastern end of longwall panel 14, in NE 'ia. Section 27. ® Location 1 - located in massive sandstones of the Ohio Creek Formation, above the approximate middle part of longwall panel 14, in the NE '/4, Section 27. 2.05-111 Revised Juw 200.1 PRIO, Rev. Alarch 2006: Rev. April 2006 PRIO; Alai, 2006 PR10, Nov. 2006 TR107, Sep. 2007 PR-12; Feb. 2008PR-12