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Subsidence Evaluation for the <br /> Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 6 <br /> 4.0 FIELD RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE <br /> FEATURES ABOVE THE WEST ELK MINE <br /> Four different types of features are observed in the area above the West Elk Mine: 1) subsidence <br /> cracks and bulges, 2) construction cracks, 3) desiccation cracks, and 4) gravity-induced tension <br /> cracks. They can be distinguished easily in some areaswhere, for example, no mining has <br /> occurred in that area. However, in other areas they may be difficult to distinguish, such as in areas <br /> that have been mined,but where conditions are also favorable for construction, desiccation, and/or <br /> gravity-induced tension cracks to occur. <br /> 4.1 Subsidence Cracks and Compression Features <br /> Subsidence cracks are open cracks that most likely occur in areas where the ground surface has <br /> undergone extension during subsidence processes. Cracks as much as 3.5 inches wide, for <br /> example, have been observed in sandstone outcrops at Apache Rocks where zones of maximum <br /> extension(or tension in rock mechanics terminology)occur. As discussed in Section 5.3.2, cracks <br /> close—and the underlying rocks become compressive—below the neutral surface (the boundary <br /> between tensile and compressive strain)of the rocks downwarping as a single unit. Therefore, any <br /> water located in cracks above the neutral surface is blocked from traveling downward into rocks <br /> in compression below the neutral surface. <br /> Cracks in the zone of maximum tension occur approximately perpendicular to the orientation of <br /> the longwall mining faces(transverse cracks)and parallel to the orientation of the longwall mining <br /> panels (longitudinal cracks). The cracks commonly do not conform to such a precise pattern. As <br /> with other deformational processes in nature, crack orientation may be quite variable. <br /> The transverse tension cracks that locally occur above the longwall mining face often have a <br /> dynamic history. They open when the longwall face moves beneath a particular area, and they <br /> close again 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 gateroad pillars and the haulageway (or beltway) pillars. Longitudinal tension cracks <br /> commonly remain open, particularly in areas above gate roads with a rigid-pillar configuration. <br /> The cracks may stay open or close in areas above gate roads with a combination rigid-pillar/yield- <br /> pillar configuration. However, as discussed in Section 5.3.2, it is unlikely that cracks will occur <br /> in colluvium and alluvium in the stream valleys of the Southern Panels and Sunset Trail mining <br /> area. <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 colluvium, <br /> or are hidden in the brush and grass. They also may be indistinguishable from natural humps and <br /> mounds in the soil and colluvium. <br /> 831-032.912 Wright Water Engineers, Inc. <br /> December 2020 <br />