2008-03-21_PERMIT FILE - C1980007A (8)

Home Browse Search

very low permeability and, because the clay matrix is armored by the interstitial gravel and boulders, are resistant to erosion (the Deep Creek Ditch locally flows in this material at steep gradients). Based on the above-mentioned observations, no effects are expected when ponds in the South of Divide or Dry Fork mining areas are undermined. The clay-rich material that lines these ponds is expected to provide a seal against any subsidence effects. No stock watering ponds in the Apache Rocks and Box Canyon mining areas have been noticeably affected when longwall mining occurred beneath them. The ponds above mined longwall panels 13-A, 15, 16, and 22 in Horse Gulch, Sylvester Gulch (stock ponds P27-2 and P27-3), and West Flatiron (stock pond P23-4) respectively, showed no noticeable water level change when compared to pre-mining water levels (after taking seasonal precipitation levels into account). These ponds, which are located in clay-rich (Wasatch clay) material in order to minimize leakage, occur in overburden that ranges in depth from about 800 feet in Horse Gulch to 2,200 feet in the stock pond P23-4 above the eastern end of mined longwall panel 22. 11.3 Streams and Ditches A greater potential exists for longwall mine subsidence to impact streams in the South of Divide and Dry Fork mining areas than in the Apache Rocks and Box Canyon mining areas. This is primarily because a) the overburden depth is less than in the Apache Rocks and Box Canyon mining area, particularly in the northwestern part of the area, and b) the coal extraction thickness is projected to be greater for the E Seam than was mined in the B Seam (14 feet versus 12 feet). Although no subsidence cracks are expected to affect streamflow in the South of Divide and Dry Fork mining areas, based on prior observations in the West Elk and Somerset mine areas, it is recommended that detailed subsidence and surface flow measurements be done in order to verify this concept. The measurements should be conducted particularly in the northwestern part of the mine area where coal extraction thickness is projected to be a maximum of 14 feet and overburden depth ranges from 375 to 400 feet. The primary streams in the South of Divide mining area are Dry Fork of Minnesota Creek, Deer Creek, Poison Creek, and Lick Creek. The primary streams in the Dry Fork mining area are the upper regions of Dry Fork of Minnesota Creek and Deep Creek. A primary source of water to Minnesota Reservoir comes from the Deep Creek Ditch, wherein water is diverted from the upper drainage of Deep Creek and transmitted to Dry Fork. The Deep Creek ditch was constructed in debris flows or colluvium and alluvium derived from the debris flow, as described in Section 11.2, this debris flow material is not expected to be impacted by longwall mining. There is some area of the Deep Creek Ditch that has a hard rock bottom. This area is limited to the lower gradient areas above the first landslide in the Dry Fork mining area (Refer to Map 1). The ditch at that location lies over the bleeder entries of panel E5. There is a minor possibility that cracks may develop at this location. This area should be monitored and appropriate action taken to seal cracks if they occur. As discussed in Section 5.3.2, no cracks were observed in the alluvium and colluvium of Sylvester Gulch and Deep Creek during periodic field observations in the Apache Rocks and Box Canyon mining areas. The near-surface alluvial material consists of primarily sand, silt, clay, and soil that range in estimated thickness from 25 to 150 feet. In the two areas mentioned the drainages were located above rigid pillars and panel boundaries where the overburden depth ranges from 800 to 1,050 feet. This is expected to also be the case in the upper region of Deep Creek within the Dry Fork mining area. The alluvium and colluvium in Dry Fork and Lick Creek, which also has an estimated thickness range of 25 to 75 feet, contains more clay than does the Deep Creek alluvium. Therefore, it is even less likely that cracks will occur in colluvium and alluvium in the stream valleys of the South of Divide mining area despite the shallow overburden. This should also be the case for the streamed valleys in the Dry Fork area, where the overburden is 800 to 1400 feet thick. In the South of Divide mining area the overburden depth to the E Seam ranges from approximately 375 feet above the western edge of longwall panel E2 to 1,300 feet above the eastern limit of Tetra Tech - 0907171P 25