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Fall 2009 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining ,Areas <br />• 5.23 Traverse M-M' <br />This traverse begins at the fork in Colorado State Highway 133 and the Box Canyon drill road <br />and ends at MDWs 21-25.5 and 21-28. This route lies above mined Longwall Panels 18, 19, 20 <br />and 21. <br />Materials encountered along this traverse include landslide deposits near M, colluvium, and some <br />alluvium mixed with colluvium (see Maps 1 and 3). Other MDW sites located along this <br />traverse include 19-07, 19-08, and 20-21.5 and 20-25. Overburden depth to the B-seam along <br />this traverse ranges from about 400 feet near M to 1,200 feet at M'. <br />Subsidence related cracking was observed along a 200-foot reach of the Box Canyon access road <br />during the fall 2008 field visit. This location (Location 18 of this report) was visited again <br />during the fall 2009 field visit. Relatively little evidence of these cracks was observed (Figure <br />60). <br />• During the spring 2008 field visit, Photographic Observation Point 10 was established in the <br />lower end of Box Canyon to observe potential effects on a series of outcropping sandstone units <br />as the mining of Longwall Panel 18 occurred beneath that location. This location was visited <br />again during the fall 2009 field visit (i.e., approximately 12 months after longwall mining had <br />occurred beneath the area). A photographic comparison of the sandstone outcrop from before <br />and after mining is provided in Section 5.23.2. <br />5.23.1 Location 18 <br />Location 18 is an area of the Box Canyon access road covering a distance of approximately 200 <br />feet, where several subsidence-related cracks were observed for the first time during the fall 2008 <br />field visit. This location is above the west end of mined Longwall Panel 18 in an area <br />approximately 500 feet above the B-seam (Map 1). This location is above the western boundary <br />of the longwall panel and in an area of maximum tensile strain. <br />Most of the observed cracks were discontinuous and oriented in a x110-30°E direction. Crack <br />• lengths were typically 2-3 feet with widths ranging from 1-6 inches (Figure 60). It should be <br />831-032.791 Wright Water Engineers, Inc. Page 67 <br />April 2010