PERMFILE42013 - Laserfiche WebLink

Home Browse Search

Subsidence Evaluation For Exhibit 608 South of Divide Mining Area Page 3 that area. However, in other areas they maybe difficult to distinguish, such as in aeeas that have been mined, but where conditions aze also favorable for construction, desiccation, and/or gravity- induced tension cracks to occur. 4.7 Subsidence Cracks and Compression Features Subsidence cracks aze 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 downwazd 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 pazallel 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 maybe quite variable. The transverse tension cracks that locally occur above the Iongwall 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 azea of mining influence of the azea. 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 aeeas above gate roads with arigid-pillaz configuration. The cracks may stay open or close in aeeas above gate roads with a combination rigid- pillaz/yield-pillaz 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 azea. 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 towazd 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 maybe indistinguishable from natural humps and mounds in the soil and colluvium. 4.2 Construction Cracks Cracks caused by construction activities aze common on the banks of newly constructed roads and drill pads. These cracks aze 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 aze pulled out of the road cut by the bulldozer. In contrast to subsidence 831-032.620 Wright Water Engineers, lnc.