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The strata above the upper caving limits will subside, and this will <br />result in trough subsidence at the surface. The ground surface above <br />production panels in the five year mine plan is mildly undulating, and <br />dips toward the northwest at approximately 10%. The maximum possible <br />subsidence has been estimated from guidelines in the Subsidence <br />Engineers' Handbook. (Ref. 1). <br />Smax = m x a <br />where: Smax = maximum possible subsidence <br />m = extraction height <br />a = Subsidence Factor <br />For the longwall panel widths of 600 ft. at a depth of 1,100 ft., the <br />Subsidence Factor is estimated to be: a = 0.54 (Ref. 1). <br />The maximum extraction height expected is 10 ft., and this will produce a <br />maximum predicted subsidence of 5.4 ft. This prediction is based on full <br />• extraction, and therefore can be considered a worst case situation. <br />It is expected Room and Pillar mining will result in a maximum predicted <br />subsidence of 4.0 feet for the same depths (1100 ft.), panel widths of <br />480 ft. and a Subsidence Factor of 0.40. Again, this is a worst case <br />condition assuming full extraction. <br />The general overburden stratigraphy is depicted in Figure D, with the <br />predominant members being approximately 600 ft. of marine shales and the <br />150 ft. thick Twentymile Sandstone. <br />The predicted angle of draw is obtained from the percentage of sandstone <br />found in the overburden (Ref. 2 and 4) using the following formula: <br />Angle of draw = 30.2° - 0.22 X % sandstone. At 1,100 ft. of depth, <br />there is approximately 35% of sandstone in the overburden. This <br />results in a predicted angle of draw of 22.5°. <br />. An angle of draw prediction of 25° has been presented in the calculations <br />to be slightly conservative. <br />Ex. 7-5 <br />