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Subsidence and Geologic Field Observations <br />Apache Rocks and Box Canyon Mining Areas <br />July 12-15, 2004 <br />The transverse tension cracks that aze located above the longwall mining face have a dynamic <br />history. They open when the longwall face moves beneath a particulaz crack azea, and then close <br />again when the longwall face moves out of the area of mining influence in this area. <br />Longitudinal cracks occur above, and roughly parallel to the edges of the longwall mining panel <br />above the gate road pillazs. Longitudinal tension cracks commonly remain open, particulazly in <br />areas above gate roads with arigid-pillar configuration. The cracks may stay open or close in <br />areas above gate roads with a combination rigid-pillar/yield-pillaz configuration. <br />Compression features (bulges and warps) also occur above the longwall mining panels in areas <br />where the ground surface undergoes compression in the subsidence process. The compression <br />features, which occur towazd the center of the mining panel in zones of maximum compression, <br />are usually more difficult to recognize. They often are masked, or absorbed, by soil and <br />colluvium, or are hidden in the brush and grass. They also may be indistinguishable from natural <br />humps and mounds in the soil and colluvium. <br />2.2 Construction Cracks <br />Cracks caused by construction activities are common on the banks of newly constructed roads <br />and drill pads, These cracks are often caused by the bulldozer during construction acdvides. The <br />cracks aze most noticeable where fractured and weathered bedrock is encountered. However, this <br />type of cracking also occurs in soil and colluvium where roots of brush and trees aze pulled out of <br />the road cut by the bulldozer. In contras[ to subsidence cracks, construction cracks occur in a <br />rather continuous zone where weathered and/or fractured bedrock has been cut through, or where <br />brush and trees have been pulled out of soil and/or colluvium. <br />The most diagnostic difference between subsidence cracks and constmction cracks is that the <br />construction cracks 1) have a less regulaz pattern, 2) are related to the material they occur in, and <br />3) lack any spatial crack relationship to the longwall mining geometry. <br />831-032.640 Wright Water Engineers, Inc. Page 5 <br />September 2004 <br />