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physically and economically, and therefore it never will be recovered. Most of the Fish Creek Seam has been <br />previously mined by surface techniques at the Energy No. 2 Mine. No impacts due to subsidence affect the Wolf <br />Creek Seam or several "lower series" coal seams which underlie the Wadge Seam. <br />Subsidence related phenomena was restricted to areas above the proposed room and pillar and longwall panels, and <br />within the expected angle -of -draw. No subsidence occurred in association with the mine development entries. <br />The extraction of pillars on retreat in the production panels and retreat mining in the longwall panels resulted in <br />trough subsidence at the surface. The effect of this subsidence was to lower the ground surface by a proportion of <br />the vertical height extracted underground. The surface influence created by underground extraction was observed <br />ahead of the retreating face, at an angle known as the angle -of -draw. The subsidence took place in a transition zone <br />between the subsidence limits (defined by the angle -of -draw) and a position over the previously extracted area. <br />The effect of this transition zone was to induce slight changes in the surface slope angle and produce horizontal <br />strains associated with the slope change. <br />The extraction of coal by these methods resulted in effects on the underground strata, which will be mainly related <br />to the roof directly above the seam. The roof strata caved, with fracturing and collapse of strata to a maximum <br />height of ten times the extraction height. Above this point there may be significant fracturing and bedding plane <br />separation to a maximum of sixty times the extraction height. Above the fractured zone the strata will flex with <br />limited bed separation, but extensive fracturing or fracture dilation due to tensile and compressive strains in a zone <br />extending from the ground surface to a depth of approximately 50 feet. The potential surface and near surface <br />subsidence effects were not significant due to the existence within the mine area of interbedded sandstones, <br />siltstones, and mudstones overlain by relatively fine- grained alluvial/colluvial materials. Fracturing of the roof <br />strata increased the permeability and dewatered the immediate overburden aquifer, as described in Section <br />2.05.6(3), but these effects did not extend to the Twentymile Sandstone Aquifer. The surficial geologic units are by <br />nature not subject to significant fracturing and when fractures do develop, they tend to collapse, fill and heal <br />relatively quickly. <br />As no underground mining activities will be conducted at the Fish Creek Tipple, surveying, monitoring, and <br />controlling subsidence were not be necessary in the tipple area. <br />SOUTHWEST MINING DISTRICT <br />A survey of the area overlying the planned mining activity has identified the following structures, resource features <br />and facilities that could be affected by potential subsidence. <br />Land Use <br />The lands overlying the area undermined were comprised of cropland, pastureland, rangeland and disturbed areas. <br />These land uses are shown on Map 3, Premining Land Use. <br />From TCC's prior experience undermining and subsiding these types of areas, excluding rangeland, no use impacts <br />resulted to these areas. Likewise, for the rangeland areas no use impacts resulted as a result of subsiding the area. <br />Existine Structures <br />There are three dwellings which lie within the subsided area in the Southwest Mining District. Twentymile's <br />affiliate owns two of the three dwellings, and has an agreement with the owner of the other dwelling which <br />Permit Renewal No. 3 2.05 -175 �L�^ £; �Y(1tl11/1/99 <br />