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<br />Subsidence Over Time <br />According to Syd Peng's Coal Mine Ground Control, significant <br />subsidence can occur months or years after mining only if the <br />critical width of cave was not achieved during mining. The <br />critical width of cave is defined as that width of excavation <br />that causes the maximum surface subsldence, and is most <br />dependent upon the depth of the coal seam. As shown in <br />Chapter 9 in Peng, mining at a depth of 1000 feet results in <br />a critical width of 1200 feet. Mining at 500 feet depth results <br />in a critical width of 600 feet. Although a single panel is <br />only 350 feet wide, consecutive mining of ad,~acent panels will <br />produce a width of cave far in excess of the critical width. <br />When this is done, residual subsidence is expected to be only <br />2 or 3 percent of the total ,initial subsidence. For the'RetonCreelcNo.IMine <br />this would result in a residual subsidence of approximately <br />1.5 inches. The weak, relatively soft strata above the coal <br />will also aid in reducing residual subsidence. <br />Effect on Stream Channels <br />As can be seen from Map No. 6 , the only significant <br />stream channel that could be affected by subsidence is the <br />South Starkville Gulch channel located in Section 6, T34S, <br />R63W and Section 1, T34S, R64W. <br />Complete extraction of panels up to the western permit boundary <br />will cause changes in the profile of the stream bed. but it is <br />not thought that these changes will significantly affect flow. <br /> <br /> <br />567 <br />Revised 5/93 <br />D. H. EMLING COMPANY <br />