Laserfiche WebLink
Subsidence Field Observations <br />West Elk Mine <br />July 22-24, 2003 <br />silts. The process of desiccation involves the shrinking of the clays and silts after a dry period <br />that follows a wet period, when the material swells. <br />Desiccation cracks can often be recognized by their irregulaz, branching, and diverging pattern- <br />less regular than typical subsidence cracks. Some of the lazgest desiccation cracks in the West <br />Elk Mine area were observed in clays of the Barren Member of the Mesaverde Formation in the <br />Horse Gulch-Minnesota Reservoir area where there had been no mining. The lazger, more <br />regular desiccation cracks and construction cracks may be confused with subsidence cracks in <br />areas where mining has occurred. However, transverse and longitudinal tension cracks have a <br />definite spatial relationship to the longwall mining panel causing the cracks. <br />' 2.4 Gravity-Induced Tension Cracks (Pseudo Subsidence Cracks) <br />These cracks have been observed on high, steep ridges and near cliffs in Box Canyon Lease Tract <br />and in the Apache Rocks Lease Tract (for example, see pages 11-12 in the 2002 report and pages <br />11-2 of the 1999 report). These cracks look very much like subsidence cracks, but cannot be <br />because no mining had occurred. The extensive crack on a narrow ridge on West Flatiron, which <br />was as much as 3.5 inches wide and 150 feet long on August 27, 2002, is a good example of a <br />gravity-induced crack. It was easy to determine that this was not amining-related crack because <br />no mining had occurred in the area. The possibility of gravity-induced cracking in the rugged <br />country above planned mining activities at the West Elk Mine is a good reason to perform <br />baseline studies of the area prior to mining so that these features can be documented prior to any <br />mining. <br />831-032.610 Wright Water Engineers, Inc. Page 5 <br />' September 2003 <br />