2008-03-21_PERMIT FILE - C1980007A (8)

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4.0 FIELD RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE FEATURES IN THE WEST ELK MINING AREA Four different types of features are observed in the West Elk mining area: 1) subsidence cracks and bulges, 2) construction cracks, 3) desiccation cracks, and 4) gravity-induced tension cracks. They can be distinguished easily in some areas-where, for example, no mining has occurred in 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 rock mechanics terminology) occur. As discussed in Section 5.3.2, cracks close-and the underlying rocks become compressive-below 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 perpendicular 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 particular 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, particularly in areas above gate roads with a rigid-pillar configuration. The cracks may stay open or close in areas above gate roads with a combination rigidpillar/ yield-pillar configuration. 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, are usually more difficult to recognize. They often are masked, or absorbed, by soil and colluvium, or are hidden in the brush and grass. They also may be indistinguishable from natural humps and mounds in the soil and colluvium. 4.2 Construction Cracks Cracks caused by construction 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 cracks, construction cracks occur in a continuous zone where weathered and/or fractured bedrock is encountered during road construction. Construction cracks may be confused with subsidence cracks, particularly where mining has occurred in the area, and where local bedrock is weathered and fractured, or where brush and trees Tetra Tech - 0907171P