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• 3.0 FACTORS INFLUENCING SUBSIDENCE <br />Subsidence may be influenced by the local geology in the following ways: <br />1. Geologic structure. Attitude of the bedrock, faulting, and jointing may control mine <br />layout and mining method. In steeply dipping, faulted coal beds, for example, a <br />certain mine layout and method, such as room-and-pillar or limited panel-pillar may <br />be required. Joints often control the way in which the roof rocks break, cave, and <br />fracture, both underground and at the surface during mining and subsidence. In <br />relatively flat-lying, unfaulted coal seams like the South of Divide and Dry Fork <br />mining areas, there is latitude to develop the most efficient layout and method to <br />recover a maximum amount of the coal resource with a minimum of impact. <br />2. Strength and behavioral properties of the rocks. These properties control the amount <br />and rate of subsidence. Strong, brittle sandstones and siltstones tend to break and <br />cave in large blocks on the mine floor. The bulking factor is greater for strong rocks <br />than it is for soft, weak rocks. The greater bulking factor of strong, caved material <br />commonly reduces the height of caving and the subsidence factor over soft, weak <br />rocks. Conversely, the height of fracturing often is greater for strong, brittle rocks <br />than it is for soft, weak rocks. <br />3. Stratigraphic sequence. The stratigraphic distribution of rock units (stratigraphic <br />sequence) influences the effects of mining and subsidence. For example, strong and <br />brittle sandstones in the mine roof, as discussed above, can reduce the height of <br />caving compared to shales, whereas sandstones in the fractured zone above the <br />caved zone may increase the height of fracturing compared to shales. Conversely, <br />the height of caving may be increased and the height of fracturing decreased where <br />weaker shale and claystones occur in the fractured zone above the coal seam to be <br />mined. <br />In addition, the lithology of the overburden rock may control the subsidence factor. <br />The subsidence factor may be less where the overburden contains a greater <br />proportion of thick, strong sandstones, and greater where the overburden contains <br />thin, weak shales. In the South of Divide and Dry Fork mining areas, the first 200 to <br />300 feet of rocks above the E Seam consist primarily of siltstones, shales, <br />claystones, local lenticular sandstones, and coal seams. <br />4. Moisture content. Wet or saturated conditions in the mine roof and overburden tend <br />to reduce the bulking factor of the caved roof rocks. Therefore, the subsidence factor <br />commonly is greater under wet conditions than it is in dry conditions. In general, the <br />greater the saturation of the mine roof and overburden rocks, the greater the <br />subsidence factor. <br /> <br />Tetra Tech - 0907171P