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Spring 2009 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• in areas above gate roads with a rigid -pillar configuration. The cracks may be open or closed in <br />areas above gate roads with combination rigid - pillar /yield- pillar 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 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 and <br />colluvium, or are hidden in the brush and grass. They also may be indistinguishable from natural <br />• <br />Figure 1. Examples of subsidence cracks located at Apache Rocks in hard sandstone (left) and <br />claystone (right) above the head gate entry pillars of mined Longwall Panel 14. <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 are most noticeable where fractured and weathered bedrock is encountered <br />during excavation. However, this type of cracking also occurs in soil and colluvium where roots <br />of brush and trees are pulled out of the road cut by the bulldozer. In contrast to subsidence <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 />• colluvium. <br />831- 032.791 Wright Water Engineers, Inc. Page 6 <br />November 2009 <br />humps and mounds in the soil and colluvium. <br />