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SPRING 2007 Subsidence and Geologic Field Observations <br />Box Canyon, and South of Divide Mining Areas <br />example, have been observed in sandstone outcrops at Apache Rocks where zones of maximum <br />extension (or "tension" in rock mechanics terminology) occur. Cracks close-and the <br />underlying rocks become compressive-below the neutral surface of rock behaving as a beam or <br />plate. Therefore, any water located in surface cracks is blocked from traveling downward into <br />rocks in compression. <br />Cracks in the zone of maximum tension are located roughly perpendicular to (transverse cracks) <br />and parallel to (longitudinal cracks) the orientation of the longwall mining panels. The cracks <br />commonly do not conform to such a precise pattern. As with other deformational processes in <br />nature, crack orientation may be quite variable. <br />The transverse cracks that ace located above the longwall mining face have a dynamic history. <br />They open when the longwall face moves beneath a particular crack area, and then close again <br />when the longwall face moves out of the area of mining influence. <br />Longitudinal cracks occur above, and roughly parallel to, the edges of the longwall mining panel <br />above 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 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 />humps and mounds in the soil and colluvium. <br />831-032.780 <br />July 2007 <br />Wright Water Engineers, Inc. <br />Page 3 <br />