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Evaluation of Potential Subsidence Impacts of Longwall Mining in the Spruce Stomp Lease Area <br />to Aquatic Life and Water Supply <br />in the stream bed. However, Mr. Dunrud observed no cracks in the stream bed on his first <br />visit to the area about a day after the visit by U.S. Steel personnel. Mr. Dunrud did observe <br />small, irregular cracks a fraction of an inch to an inch or two wide, 10 to 25 feet long, and a <br />few inches to perhaps 1 -foot deep, in colluvium an estimated 10 to 20 feet thick and located 15 <br />to 30 feet above the stream. He observed no cracks in saturated alluvium underlying the Bear <br />Creek stream. There was no observed or reported loss of flow downstream in Bear Creek <br />from this area, and no inflows to the mine were reported from this area. <br />Two possibilities (or a combination of both) provide an explanation for these observations: <br />1) The alluvium stretched without rupturing when mine subsidence occurred in the stream <br />channel. <br />2) Cracks in the alluvium healed and sealed by sloughing prior to observation by <br />Mr. Dunrud. Healing and sealing of any cracks present in the stream alluvium is a <br />viable alternative, because the vertical limit of cracks would only be about one -half the <br />alluvium thickness. The alluvium would be in compression below its neutral surface <br />(see Figure 1 in Appendix 4). <br />It is important to note that there was no observed or reported reduction in stream flow in Bear <br />Creek or inflow to the mine due to the subsidence from the time of the observation in the mid - <br />1970s to the spring of 1999. <br />5.2.4 Characterization of Pre- and Post - Mining Geologic Hazards <br />The surface geology in the West Fork Terror Creek mining area consists of primarily <br />unconsolidated deposits of clays and silts of the Wasatch Formation intermixed with basalt <br />boulders derived from extrusive rocks capping Grand Mesa. This material, mapped as debris <br />flows (Dunrud 1989), is commonly stable and resistant to stream erosion and to mass - gravity <br />movements. No movement was observed on this type of material (which is Wasatch clays and <br />silts mixed with intrusive igneous rocks) in the West Elk mining area during a 17 -year period <br />of subsidence observations (1996 — 2012, inclusive) by Mr. Dunrud. <br />121 - 014.000 Wright Water Engineers, Inc. Page 20 <br />January 2013 <br />