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Fall 2007 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• cracks, construction cracks occur in a rather continuous zone where weathered and~'or fractured <br />bedrock has been cut through, or where brush and trees have been pulled out of soil and,'or <br />culluvium. <br />• <br />• <br />For diagnostic purposes, the largest ditterence between subsidence cracks and construction <br />cracks is that the construction cracks: 1) have a less regular patter, ?) arc related to the material <br />they occur in, and 3) lack any spatial relationship to the long~~-a11 mining geometry. <br />_ ~M ~~. <br />:~,.; - <br />.i~~ / .rte i <br />_ ~ ~;. _ _ . <br />. v ,~-. _ <br />.'~'~ ~ ti ~r"C <br />t <br />Figure 2. This cut bank in bedrock is located above mined longwall panel <br />17 in an excavation area at well site MDW 17-02. Cracks on either side of <br />the dominant, irregular crack (right middle) are natural or the result of <br />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 Vlesaverdc and Wasatch <br />Formations in the West Elk Mine area where the rocks are weathered to clays and silts (Figure ~). <br />The process of desiccation involves the shrinking of previously wetted clays and silts during the <br />drying process. <br />831-032.780 Wright Water Engineers, Inc. Page 6 <br />November 2007 <br />