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Fall 2007 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• Longitudinal cracks occur above, and roughly parallel to, the edges of the longwall mining panel <br />,shove the gate road pillars (Figure 1). Longitudinal cracks commonly remain open, particularly <br />in areas above gate roads with arigid-pillar configuration. The cracks may be open or closed in <br />areas above gate roads with a combination rigid-pillar'yield-pillar configuration. <br />Compression features (bulges and warps) also occur above the longwall mining panels in area: <br />~vhcre the ground surface undergoes compression in the subsidence process. The compressiot <br />features, which occur toward 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 anc <br />colluvium, or arc hidden in the brush and grass. They also may be indistinguishable from natura <br />humps and nu~unds in the soil anti colluviwn. <br />• <br />2.2 Construction Cracks <br />Cracks caused by construction activities are common on the banks of newly constructed roads <br />and drill pads (Figure 2). 'these cracks are often caused by the bulldozer during construction <br />activities. They arc most noticeable where fractured and weathered bedrock is ena~untercd <br />during excavation. However, this type of cracking also occurs in soil and colluvtum where roots <br />of brush and tries are pulled out of the road cut by the bulldozer. In contrast to subsidence <br />• <br />831-032.780 Wright Water Engineers, Inc. Page 5 <br />November 2007 <br />Figure 1. Examples of subsidence cracks located at Apache Rocks in hard sandstone (left) and claystone (right) <br />above the head gate entry pillars of mined longwall panel 14. <br />