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<br />iiiiiiiiiiiiiiiiii <br />STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Depanmenl of Natural Resources <br />1313 Sherman A., Room 215 <br />Denver, Colorado 8020] <br />Phone: 13031 866-3567 <br />FA%:0031 /332 6106 <br />March 18, 1996 <br />Mr. Allen Weaver <br />Mine Engineer <br />Southfield Mine <br />P.O. Box 449 <br />Florence, CO 81226 <br />J Re: Preliminary Adequacy Review <br />Technical Revision No. TR-24 <br />Southfield Miae (C-81-014) <br />Dear Mr. Weaver: <br />~I <br />I~~~~ <br />DEPARTMENT OF <br />NATURAL <br />RESOURCES <br />Roy Romer <br />Govermr <br />lames 5. Lochhead <br />Eaecmive Director <br />Michael g. LOng <br />Division Director <br />The Division has conducted a preliminary adequacy review of the <br />above referenced application for a technical revision to the <br />Southfield Mine permit. The technical revision application <br />proposed a reduction in subsidence monitoring for the mine. The <br />Division's comments are given below: <br />1) Pace 2.05.6-56: The last sentence of the first paragraph <br />under Probable and Worst-Case Subsidence, although not a <br />part of the revised text, is confusing. From the <br />statement made, "RMG predicted probable maximum vertical <br />subsidence ranging from 1.46 feet to 5.85 feet and <br />projected a worst-case vertical subsidence range from <br />2.60 to 4.36 feet," it would appear that the 'probable <br />maximum vertical subsidence' is greater than the 'worst- <br />case vertical subsidence,' which is counterintuitive. <br />Upon inspection of Tables 1.0 and 2.0 in Exhibit 23, <br />Subsidence Information, it becomes evident that the <br />'probable maximum vertical subsidence,' or 'maximum <br />expected subsidence' as it is called in Table 1.0, is <br />predicted for retreat extraction, whereas the 'worst-case <br />expected subsidence' predicted for some structures is for <br />development extraction only. It appears that the worst- <br />case expected subsidence predicted for structures is less <br />than maximum vertical subsidence, because retreat <br />extraction is not permitted under the structures. Please <br />clarify this in the permit text. <br />