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Spring 2009 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• For diagnostic purposes, the largest difference between subsidence cracks and construction <br />cracks is that the construction cracks: 1) have a less regular pattern, 2) are related to the material <br />they occur in, and 3) lack any spatial relationship to the longwall mining geometry. <br />• ... ?:! .. �'.'ff 'Fi,,.i:� <br />Figure 2. This cuthank m bedrock is located above mined Longwell Panel 17 In an excavation <br />area at well site MDW 17 -02. Cracks on either side of the dominant, irregular crack (right middle) <br />are natural or the result of construction. The most prominent irregular crack has an estimated 75 <br />percent likelihood of being mine related. <br />2.3 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 where the rocks we weathered to clays and silts (Figure 3). <br />The process of desiccation involves the shrinking of previously wetted clays and silts during the <br />drying process. <br />• <br />831- 032.791 Wright Water Engineers, Inc. Page 7 <br />November 2009 <br />