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October 24, 1991 <br />Mr. Randall P. Price <br />Page 2 <br />magnitudes throughout the general mined area; a series of high resolution grids <br />located over selected pillared areas designed to provide site specific data with <br />regard to subsidence magnitude and areal extent under differing geologic and <br />topographic conditions; and periodic visual walkovers of areas undermined by <br />pillaring techniques. Modifications to the subsidence monitoring program are to <br />be presented in the Annual Subsidence Report submitted to the Division. <br />Subsidence monitoring is to continue until cessation is approved by the Division <br />in writing. <br />In the 1983 subsidence study conducted by Geo-Hydro Consulting Inc., predictions <br />as to the nature and extent of subsidence in the permit area were formulated. <br />Page 32 of the subsidence study projects the maximiun possible subsidence over the <br />Orchan9 Valley mine to be approximately 7 feet. Page 37 projects a conservative <br />angle of draw of 21 deg3rees. A subsidence control plan detailing specific <br />mitigation measures for potentially impacted structures and resources was <br />developed based upon the projection of worst case impacts stemming from the <br />aforementioned subsidence developments. All survey data gathered to date support <br />the conservative nature of the of the initial predictions for both draw angles <br />and subsidence magnitudes. Survey data has consistently shaan that subsidence <br />magnitudes are in fact much less than predicted (see 1990 Annual Subsidence <br />Report, Station Summary Table, Column C) and that draw angles may be more <br />typically on the order of 16 degrees although a draw angle of as high as 20 <br />degrees was calculated in a 1986 analyses of the high resolution transects <br />located over the East mine (see 1986 Annual Hydrology and Subsidence Report). <br />(UVCC believes that adequate subsidence data has been collected to verify that <br />the initial subsidence predictions are reasonably conservative. NVCC therefore <br />requests the Division to consider the following changes to the subsidence <br />monitoring program in effect at Orchard Valley as specified in Permit No. C-81- <br />038. Fage 2.05-146 has been changed to indicate that wide-spaced monument <br />stations will be surveyed once per year as opposed to "typically twice per year." <br />Page 2.05-147 has been cha~xled to specify the commitments pertaining to the <br />continuation of the wide-spaced monitoring grid network. <br />wide-spaced subsidence monitoring monuments located within or adjacent to current <br />active workings will continue to be surveyed annually per the approved subsidence <br />monitoring plan as set forth in Permit No. C-81-038. Specifically, wide-spaced <br />monitoring grid stations 6, 7, 14, 15, 16, and 88 as depicted on Map No. 3-1 will <br />be surveyed during 1991. Data collection will continue annually until sites are <br />stabilized. Additional wide-spaced monitoring grid monuments have and will <br />continue to be installed and surveyed prior to the commencement of mining in any <br />given area. COVCC will propose specific stations for surveying in each future <br />Annual Subsidence Report. <br />The visual walkover requirements as set forth on page 2.05-145 of the permit <br />document and the monitoring of surface water hydrology features to detect <br />subsidence impacts will remain unchanged with this revision. Specific surveying <br />