Laserfiche WebLink
Evaluation of Potential Subsidence Impacts of Longwall Mining in the Spruce Stomp Lease Area <br />to Aquatic Life and Water Supply <br />Characterization of Post - Project slopes is based on the existing slope characterization in <br />conjunction with Mr. Dunrud's subsidence model. As with the channel profile characterization, <br />the greatest magnitude of change is expected to occur at the transitions between longwall panels <br />and barrier pillars or unmined areas. These changes could lead to accelerated erosion rates for <br />short periods of time until stable slopes are reestablished similar to those present in the area <br />today. <br />5.2.3 Characterization of Surface Fractures <br />Surface subsidence fractures have been observed above mined longwall panels at several area <br />coal mines, including, for example, the West Elk Mine, for which WWE has conducted annual <br />to semiannual subsidence surveys for the past 17 years. Observed fractures have typically <br />occurred on consolidated areas such as roadways, drilling pads or other disturbed and <br />compacted areas. These subsidence fractures tend to disappear within several months as <br />erosion and weathering fill in the areas. No such fractures have been observed to date in <br />saturated alluvial deposits. In several cases, fractures have been observed where brittle <br />sandstone outcrops or subcrops with little or no lateral support and where they overlie portions <br />of longwall panels with the greatest tensile strain. <br />Mr. Dunrud reports an observation of cracks along Bear Creek above the U.S. Steel mine at <br />Somerset during the summer and early fall of 1976. The subject area was located above a B- <br />seam room - and - pillar mine where the coal extraction thickness was 10 feet and the overburden <br />depth ranged from only 250 to 500 feet. <br />Irregular, en- echelon (offset) cracks as much as 1 -foot wide, and 25 to 50 feet long were <br />observed in weathered bedrock and thin colluvium (a few feet thick) on the east side of the <br />valley 75 to 100 feet above Bear Creek. The crack depth ranged between 3 and 10 feet deep <br />before sloughing began. Cracks 4 to 8 inches wide and 10 to 20 feet long were also mapped <br />on the west side of Bear Creek. These cracks occurred on either side of the extraction panel, <br />crossed the road and extended eastward upslope and westward towards the stream. <br />U.S. Steel personnel reported that the cracks were located on either side of the panel and also <br />121 - 014.000 Wright Water Engineers, Inc. Page 19 <br />January 2013 <br />