Laserfiche WebLink
2004 Geologic Hazard Field Observations <br />South of Divide Mining Area <br />9.3 Streams and Ditches <br />The primary streams in the South of Divide mining azea are Dry Fork of Minnesota Creek, Deer <br />Creek, Poison Creek, and Lick Creek. The primary source of water to Minnesota Reservoir <br />comes from the Deep Creek Ditch, where water is diverted from the upper Deep Creek drainage <br />and transmitted to Dry Fork. The Deep Creek ditch was constructed in debris flows, and as <br />discussed above, this debris flow material is not expected to be impacted by longwall mining. <br />No cracks were observed in the alluvium and colluvium of Sylvester Gulch and Deep Creek <br />during periodic field observations in the Apache Rocks and Box Canyon mining areas. The neaz- <br />surface alluvial material consists of primazily heterogeneous to stratified sand, silt, clay, and soil <br />that range in estimated thickness from 25 to 150 feet. hi the two aeeas mentioned the drainages <br />were located above rigid pillars and pane] boundaries where the overburden depth ranges from <br />800 to 1,600 feet. The alluvium in Dry Fork and Lick Creek, which also has an estimated <br />thickness range of 25 to 150 Feet, contains more clay than does the Deep Creek alluvium. <br />Therefore, it is even less likely that cracks will occur in colluvium and alluvium in the stream <br />valleys of the South of Divide mining area. <br />In the South of Divide mining azea the overburden depth to the E Seam ranges from <br />approximately 350 feet above the east eastern edge of longwall panel E2, to 1,300 feet above the <br />western limit of longwall panels E6 and E7 (350 to 400 feet is the minimum overburden depth in <br />the northwestern part of the mine area). However, based on observations above the Somerset <br />Mine in the Bear Creek area, subsidence cracks are not expected to occur in the Dry Fork <br />alluvium where the overburden depth ranges from 350 to 800 feet. <br />The probable reason for the lack of cracking in alluvium is that the fine sand- to clay-sized <br />material and overlying soil yields without cracking or bulging as it deforms as a discrete unit, or <br />as discrete units, in the subsidence process. This same reasoning also applies to the colluvium in <br />the azea. Although subsidence cracks were locally observed in colluvium less than one foot to a <br />few feet thick, no cracks were observed in colluvium more than about ten feet thick. No cracks <br />L J <br />831-032.621 Wright Water Engineers, Inc. Page 34 <br />November 2004 <br />