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

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West Elk Mine • A) Subsidence Monitoring Until 1999, subsidence monitoring at West Elk Mine was accomplished using conventional survey methods of a monument grid. The grid was laid out over the first three longwall panels to be mined (panels 1-3NW), and successfully verified MCC's prediction about the amount of subsidence. The grid is shown on Map 29. As subsidence of the three longwall panels was completed and the surveys no longer showed movement, monitoring of the grid was discontinued in 1999. However, the monuments have remained in place to be surveyed in the event that any unanticipated ground surface displacement occurs in the vicinity of the monuments. In the Jumbo Mountain permit revision application (PROS), MCC proposed measuring subsidence using aerial photogrammetric methods. MCC had been collecting data from aerial flights since 1995. Areas were flown prior to, during, and following mining. MCC has evaluated this data and determined that measuring subsidence using aerial methods is not feasible for a number of reasons. The first is the rugged topography of the areas that are undermined makes it difficult to place survey panels and interpret the data. In addition, placing an adequate number of survey panels has also been difficult due to limited access and thick vegetation. Another difficulty, particularly on Jumbo Mountain, is that it is difficult to distinguish between actual subsidence and ground movement due to landsliding. For Panel 25, MCC will install six (6) reflectors or monuments placed on Sylvester Gulch to determine subsidence. These stations are depicted on Map 53B. These stations will be surveyed prior to commencement of longwall mining in Panel 25, and then when longwall mining commences they will be surveyed monthly and reported quarterly and semi-annually. The monthly surveys will continue for two months following completion of longwall mining in panel 25. The monitoring of MCC's subsidence grid established the amount of subsidence that occurs over a longwall panel, when it occurs, where it occurs, and when it is complete; therefore, there is no longer a need for additional grids. Instead, MCC will visually inspect the ground over the areas that have been undermined to document any disturbance that may have occurred. MCC will also visit new mine areas prior to any subsidence occurrence to document pre-existing conditions, and will also visit locations where cracks have previously been documented to verify that the cracks are healing. MCC will utilize traditional survey methods, as necessary, to evaluate structures of concern, for example, Dry Fork. Also, MCC will conduct subsidence monitoring of the following: Dry Fork thalwegs, roads, well casings, inverts of culverts, flumes, monuments at stock ponds, and buildings, Monument Dam, and Minnesota Reservoir. Specific subsidence monitoring measures and plans will include the verification of the subsidence angle-of-draw in the SOD and Dry Fork Lease area. The monuments established as part of the baseline survey will be resurveyed at least three months but no more than nine months after the Ion-wall face has moved past the end of the longitudinal survey line to determine the amount of subsidence that has occurred and the angle of draw of subsidence. A report detailing the angle of draw observed during the aforementioned survey will be submitted to the Division with the semi-annual subsidence report. To verify the subsidence angle-of-draw in the SOD area, MCC will perform a baseline survey over at least 5,000 feet, or the entire length of the panel :.71 ichever is shorter) of tl.e list sec rd E-seam panel, prior to the start of long'"-all mining, with survey-grade GPS equipment. The determination of which E-seam panel is surveyed first is dependant upon access conditions and mine plan timing. The survey will consist of using a survey-grade GPS system to locate stations on approximately 300 foot intervals on the longitudinal axis of each panel. Each station, or point, will be measured for location and elevation. Two Survey lines per panel will also be established perpendicular to the 2.03-163 Revised June 2003 PR10, Rev. March 1006; Alay 2006 PRIO, Nov. 2006TR107,Apri1 2007TR108;Sep. 2007 PR12; Feb. 2008 PR-12