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Fall 2008 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• <br />L <br />• <br />'1 ? r ?id? <br />Y? <br />:cam';: }• p i? <br />Figure 42. October 2008 view of Crack 5 zone at Apache Rocks. The crack, which has a <br />maximum width of 3.0 inches (foreground), is locally covered with colluvium and soil. Another <br />crack, out of view and as much 0.5-inch wide, is located in the sandstone (upper left) about 12 feet <br />south of this crack, <br />The total horizontal strain (maximum extension) over the 28046ot area during the last four field <br />visits (i.e., spring and tall 2007, and spring and fall 2008) is 16.0 inches, or 133 feet. Assurning <br />that extension only Occurs at crack locations (a valid assumption in these brittle sandstones), the <br />maximum calculated cumulative strain over the area is therefore 133 teet divided by 280 feet. <br />which equals 0.00475 (or 0.48 percent). The ranee of' maximum horizontal strain measured in <br />2000 was 0.75 to 1.5 feet across a crack area totaling 320 feet (or 0.23 to 0.47 percent). <br />The maximunl horizontal strain value calculated during the last four field observations is yerv <br />close to the maximum yaluc calculated in 2000. (only the crack area and number of cracks has <br />decreased. Some of the smaller cracks observed to the north in the summer of 2000 have closed. <br />and the Southern cracks have become wider over the last seven years. <br />831-032 790 Wright Water Engineers, Inc. Page 55 <br />December 2008