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Fall 2008 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 colltiguration. The cracks nlay he open or closed in <br />areas above gate roads with a combination rigid-pillariyield-pillar configuration. <br />Compression features (bulges and warps) also occur above the longwall raining 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 maxinuim compression. <br />are usually more difficult to recognize. They often are masked. or absorbed. by soil and <br />colluyiunl, or are hidden in the brush and ?_,rass. They also may he indistinguishable from natural <br />humps and hounds in the soil and colluyiunl. <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> Figure 1 Examples of subsidence cracks located at Apache i ):KS in hard sand stone /left) and <br /> claystone (right) above the head gate entry p illars of mined Longwall Panel 14. <br />2.2 Construction Cracks <br />Cracks caused by construction activities are common oil the hanks of* ne\\ lv 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. I lowcyer. this type of cracking also occurs in soil and colluviuia \\ here 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 ions \\hcre \\cathered and or fraClured <br />bedrock has been cut through. or \\-here brush and trees have been pulled out of soil and/or <br />?ullu? iunt. <br />831-032 790 Wright Water Engineers, Inc. <br />December 2008