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Subsidence Evaluation For Exhibit 608 South of Divide Mining Area Page 4 • Construction cracks may be confused with subsidence cracks, particularly where mining has occurred in the azea, and where local bedrock is weathered and fractured, or where brash and trees have been ripped out of soil and/or colluvium during the constmction process. The most diagnostic features of construction cracks aze that they 1) have a less regulaz pattern, 2) aze related to the material they occur in, and 3) they lack of any spatial relationship to the underlying longwall mine geometry. 4.3 Desiccation Cracks Desiccation cracks tend to occur in claystones and siltstones of the Mesaverde and Wasatch Formations in the West Elk Mine azea, particulazly where the rocks aze weathered to clays and silts. The process of desiccation involves the shrinking of the clays and silts after a dry period that follows a wet period, when the material swells (the shrink/swell process). Desiccation cracks can often be recognized by their irregulaz, branching and diverging pattern- less regulaz than typical subsidence cracks. Some of the lazgest desiccation cracks in the West Elk Mine azea were observed in clays of the Barren Member of the Mesaverde Formation in the Horse Gulch-Minnesota Reservoir area and in the weathered claystones of the Wasatch Formation on West Flatiron, where there has been no mining. The lazger, more regulaz desiccation cracks and construction cracks may be confused with subsidence cracks in azeas where mining has occurred. However, transverse and longitudinal subsidence cracks have a definite spatial relationship to the longwall mining panel causing the cracks. • 4.4 Pseudo Subsidence Features (Gravity-Induced Tension Cracks) Cracks have been observed on high, steep ridges, neaz cliffs, and in landslides, in the Box Canyon and Apache Rocks mining azeas. These cracks look very much like subsidence cracks, but cannot be, because no mining had been done when they were observed. A good example of a gravity-induced crack is the extensive crack that the author observed on the narrow ridge of West Flatiron in August 2002. This crack was as much as 3.5 in wide and 150 feet long. This was not a mining-related crack because no mining had occurred in the azea. The possibility of gravity- induced cracking in the rugged country above planned mining activities at the West Elk mining azeas is a good reason to perform baseline studies of the azea prior to mining so that these features can be documented prior to any mining. Cracks and bulges caused by landslides aze other types of gravity-induced features that may appeaz to be related to subsidence, particulazly in azeas that have been, or aze being, undemuned. However, landslide-induced features aze related to the geometry of the landslide rather than the mine geometry. For example, cracks are most common in the upper azea of a landslide, whereas, bulges aze most common in the lower azea of the slide. Tlus spatial and geometric relationship to a landslide footprint on steep, unstable slopes, rather than the mine geometry can usually be used to differentiate between gravity-induced and mine-induced surface features. 831-032.620 Wright Water Engineers, lnc.