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have been ripped out of soil and/or colluvium during the construction process. The most diagnostic <br />features of construction cracks are that they 1) have a less regular pattern, 2) are related to the <br />material they occur in, and 3) they lack of any spatial relationship to the underlying longwall mine <br />geometry. <br />4.3 Desiccation Cracks <br />Desiccation cracks tend to occur in claystones and siltstones of the Mesaverde and Wasatch <br />Formations in the West Elk Mine area, particularly where the rocks are weathered to clays and silts. <br />The process of desiccation involves the shrinking of the clays and silts after a dry period that follows <br />a wet period, when the material swells (the shrink/swell process). <br />Desiccation cracks can often be recognized by their irregular, branching and diverging pattern- <br />less regular than typical subsidence cracks. Some of the largest desiccation cracks in the West Elk <br />Mine area were observed in clays of the Barren Member of the Mesaverde Formation in the Horse <br />Gulch-Minnesota Reservoir area and in the weathered claystones of the Wasatch Formation on <br />West Flatiron, where there has been no mining. The larger, more regular desiccation cracks and <br />construction cracks may be confused with subsidence cracks in areas where mining has occurred. <br />However, transverse and longitudinal subsidence cracks have a definite spatial relationship to the <br />longwall mining panel causing the cracks. <br />4.4 Pseudo Subsidence Features (Gravity-Induced Tension Cracks) <br />Cracks have been observed on high, steep ridges, near cliffs, and in landslides, in the Box Canyon <br />and Apache Rocks mining areas. These cracks look very much like subsidence cracks, but cannot <br />be, because no mining had been done when they were observed. A good example of a gravity- <br />induced crack is the extensive crack that Dunrud observed on the narrow ridge of West Flatiron in <br />August 2002. This crack was as much as 3.5 in wide and 150 feet long. This was not a mining- <br />related crack because no mining had occurred in the area. The possibility of gravity-induced <br />cracking in the rugged country above planned mining activities at the West Elk mining areas is a <br />good reason to perform baseline studies of the area prior to mining so that these features can be <br />documented prior to any mining. <br />Cracks and bulges caused by landslides are other types of gravity-induced features that may <br />appear to be related to subsidence, particularly in areas that have been, or are being, undermined. <br />However, landslide-induced features are related to the geometry of the landslide rather than the <br />mine geometry. For example, cracks are most common in the upper area of a landslide, whereas, <br />bulges are most common in the lower area of the slide. This spatial and geometric relationship to a <br />landslide footprint on steep, unstable slopes, rather than the mine geometry can usually be used to <br />differentiate between gravity-induced and mine-induced surface features. <br /> <br />Tetra Tech - 0907171P