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Subsidence Evaluation for the <br />Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 6 <br /> <br />831-032.923 Wright Water Engineers, Inc. <br />December 2021 <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 areas—where, 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.