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Suluidence Evaluation For Exhibrl 608 South of Divide Mining Area Page 3 that area. However, in other areas they may be difficult to distinguish, such as in areas that have been mined, but where conditions are also favorable for construction, desiccation, and/or gravity-induced tension cracks to occur. 4.1 Subsidence Cracks and Compression Features Subsidence cracks are open cracks that most likely occur in areas where the ground surface has undergone extension during subsidence processes. Cracks as much as 3.5 inches wide, for example, have been observed in sandstone outcrops at Apache Rocks where zones of maximum extension (or tension in rack mechanics temvnology) occur. As discussed in Section 5.3.2, cracks close-and the underlying rocks become compressive~elow the neutral surface (the boundary between tensile and compressive strain) of the rocks downwarping as a single unit Therefore, any water located in cracks above the neutral surface is blocked from traveling downward into rocks in compression below the neutral surface. Cracks in the zone of maximum tension occur approximately perpendiculaz to the orientation of the longwall mining faces (transverse cracks) and parallel to the orientation of the longwall mining panels (longitudinal cracks). The cracks commonly do not conform to such a precise pattern. As with other deformational processes in nature, crack orientation may be quite variable. The transverse tension cracks that locally occur above the longwall mining face often have a dynamic history. They open when the longwall face moves beneath a particulaz area, and they close again when the longwall face moves out of the area of mining influence of the area. Longitudinal cracks occur above and roughly parallel to the edges of the longwall mining panel above the gate road pillars and the haulageway (or beltway) pillars. Longitudinal tension cracks commonly remain open, particulazly in areas above gate roads with arigid-pillaz configuration. The cracks may stay open or close in areas above gate roads with a combination rigid- pillaz/yield-pillaz conSguration. However, as discussed in Section 5.3.2, it is unlikely that cracks will occur in colluvium and alluvium in the stream valleys of the South of Divide mining area. Compression features (bulges and warps) also occur above the longwall mining panels in areas where the ground surface undergoes compression in the subsidence process. The compression features, which occur toward the center of the mining panel in zones of maximum compression, aze usually more difficult to recognize. They often are masked, or absorbed, by soil and colluvium, or are hidden in the bnrslt and grass. They also may be indistinguishable from natural humps and mounds in the soil and colluvium. 4.2 Construction Cracks Cracks caused by eonstmction activities are common on the banks of newly constructed roads and drill pads. These cracks are caused by the bulldozer and related differential compaction during construction activities. The cracks are most noticeable where fractured and weathered bedrock is encountered. However, this type of cracking also occurs in soil and colluvium where . roots of brush and trees are pulled out of the road cut by the bulldozer. In contrast to subsidence 831-032690 Wright Water Engineers, Inc.