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West Elk Mine <br />> Apache Rocks mining area due to the topography and rock outcrops, the South of Divide <br />mining area will be more analogous to the northwest panels. <br />These calculated risk factors aze important because they enable a determination of the frequency <br />with which any given channel is likely to experience a crack. For example, there aze eight stream <br />channels in the Apache Rocks mining pemut area representing 18 crossings of the chain barrier <br />pillars. At any given crossing, the probability of encountering a crack is 0.2 percent. Therefore, <br />throughout the entire Apache Rocks Honing area, the combined probability of encountering a <br />surface crack is only 3.6 percent. Within the Box Canyon rnining azea, there are seven stream <br />channels representing four stream systems (i.e., Sylvester Gulch, Box Canyon, the unnamed <br />tributary west of Box Canyon, and an unnamed tributary to Raven Gulch). These seven stream <br />channels cross chain barrier pillazs seventeen times. The resulting combined probability of <br />encountering a surface crack in this area is 3.4 percent. <br />There are eleven stream channels in the South of Divide mining area representing <br />approximatly 24 crossings of the chain/barrier pillars. At any given crossing, the probability of <br />encountering a crack is 0.2 percent. Therefore, throughout the entire South of Divide mining area, <br />the combined probability of encountering a surface crack is only 4.8 percent. These risk <br />factors help to put the loss of surface water due to surface cracking into perspective. Based on his <br />annual observations at the mine, Mr. Dunrud fmds it highly unlikely that a surface crack will <br />develop within the alluvium and/or colluvium of the stream channels in the South of Divide <br />permit revision area. <br />In the unlikely event that a channel does encounter a surface crack, the next questions are: (1) How <br />much water will be lost?, and (2) What is the significance of the water "loss"? As for question (1), <br />it is conservative to assume that a crack over a pillar will be 100 feet long, 30 feet deep and <br />conservatively, an average of 0.25 feet wide, for a total volume of 0.02 acre-feet. A loss of this <br />magnitude, even if it were to occur a number of times per yeaz, is not significant. Furthermore, and <br />of perhaps greater importance, surface flows lost to cracks will still eventually return to the North <br />Fork (on the north side of the drainage divide) and to the Dry Fork (on the south side of the divide), <br />so the water will not be lost to the system. <br />This analysis conservatively assumes that surface cracks, which develop will remain open and that <br />they will not close with time. However, crack closure is, indeed, very likely. An important <br />mitigating factor regarding surface water loss into subsidence cracks is "healing" due to expansion <br />of the materials in the crack. To investigate this subject relative to West Elk Mine, Mr. Rold met <br />with Mr. Pat Rogers and Mr. Jeff Hynes of the Colorado Geological Survey in January, 1995. Both <br />men are experts in the subject of swelling soils and Mr. Hynes is the Colorado Geological Survey's <br />principal expert on subsidence reseazch and evaluation. <br />Both Mr. Rogers and Mr. Hynes stated that, if the expansive shales and claystones of the Mesaverde <br />Formation in the subsurface were not at their highest attainable water saturation, the addition of <br />water in a subsidence crack would produce additional swell. More importantly, the introduction of <br />waters will increase the plasticity index of the shales or claystones. The higher the plasticity index <br />of a shale or claystone, the more easily it deforms under the overburden pressure assisting in crack <br />~ closure. To put it simply, "the wetter a mud ball gets, the easier it is to mold." WWE agrees with <br />1.05-161 RevisedJw+e 1005 PR70 <br />