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0 Three subsidence phases are associated with trough subsidence. These are shown in <br />Figure 3. <br />1. The subcritical phase occurs immediately at the beginning when movement is in a <br />small area at the center of the basin. <br />2. The critical phase occurs as the basin area expands when the maximum value of <br />the downward movement is reached at the center. The critical excavation width is <br />generally larger than 1.4 times the overburden thickness and is influenced by position and <br />strength of competent layers within the overburden. <br />3. The supercritical phase occurs as the basin develops a flat bottom. In this phase, <br />the basin area continues to increase with the cave area, but subsidence will remain at the <br />maximum value attained in the critical phase. <br />Thus, the surface response of longwall mining activity, shown in Figure 3, begins <br />with the subcritical phase, then progresses to the critical phase, and finally, to the <br />supercritical phase. The subsidence process first shows effects on the surface as the upper <br />strata bend, including tension (expansion), which causes near-surface fractures to open up <br />and new ones to be created. Figure 3 shows how the middle portion of the excavation <br />expands as subsidence continues, going through a cycle of, first, tension and then <br />compression, which heals tension cracks. The tighter and more uniform the cave, the <br />better the fracture and healing process. Final subsidence shows an excavation with the <br />middle portions lower in elevation, but back to a near-original state. Areas on the edge of <br />the excavation basin are subjected to tensile strains. <br />Considering panel width to average overburden depth ratios for the project area (1.0 <br />to 2.4), most longwall panels are considered to have supercritical widths, particularly to <br />the east, and thus the great majority of subsidence is expected during the mining of <br />individual longwall panels. Additional subsidence occurs particularly over the gate <br />• <br />Maleki Technologies, Inc. Page I I