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in a 650 ft buffer around longwall panels (maximum extent area of expanded mine plan; see maps <br />Appendix E). Overburden varies in potentially subsided areas from 500 to 1800 ft, so the average <br />affected areas outside longwall panels would be less than 650 ft (for 1200 ft of cover buffer would <br />be 436 ft, for 500 ft buffer is 180 ft). Maximum subsidence (drop) is approximately 6ft along the <br />center of a panel (between two gate roads) and gradually decreases as you near gate road pillars <br />(1.2% change in slope over 500 ft - very subtle [panels 1000 ft wide]). The deeper the cover the less <br />the subsidence and broader angle of draw. There is virtually no subsidence along entries, gate roads <br />and barrier pillars. Subsidence can be calculated in specific areas based on this type of information <br />once the detailed mine plan is finalized. <br />At this time there are no known potential effects to any eligible or need data sites that would <br />result from any proposed surface facility. In the event that a potential effect were identified, <br />avoidance or treatment of the specific site is recommended. <br />. Potential effects from subsidence may occur. One site, a standing historic granary, is in <br />deteriorating condition and it is recommended that this site be treated whether or not it will be <br />underlain by subsidence. This site, 5RT519, is a previously recorded grain storage facility of an <br />unusual character and it has been evaluated as eligible for the NRHP. Natural deterioration has <br />degraded the site's physical integrity to the point where it may collapse at any time. It is <br />recommended that this site be documented utilizing the BLM Level II recording detail (Athearn <br />1990). This work should be.done in the first year the mine is permitted and prior to any subsidence <br />or upgrading of roads for heavy truck traffic. <br />At the present time, the potential effects on any specific prehistoric site in the mine permit <br />area are not known. Based upon what is known about the potential effects of subsidence on open <br />non-architectural sites, we propose that a treatment plan be designed with some recommended steps. <br />Our recommendation is based on the assumption that at some point early in the operation of the <br />mine, subsidence from a longwall panel will occur in an area containing an eligible or need data site. <br />Since the effects of subsidence on open camps of the kind documented in the Sage Creek area are <br />not known, we propose a pilot program to characterize the nature of a few selected sites, and to <br />subsequently conduct condition monitoring on these sites after the ground under the sites has been <br />subsided. The proposed assessment and monitoring program should consist of the following steps: <br />1. Identification of two.to three eligible or need data sites in proposed subsidence areas; <br />2. Take site condition photographs at designated and marked locations; <br />3. Conduct test excavations at each site to characterize the site's content and stratigraphic detail <br />in at least one master stratigraphic profile for each site; <br />4. Placement of several elevation datums over the site's area to measure subsidence; <br />5. Await subsidence of the sites during the normal sequence of mining activities; <br />6. Re-open and analyze the master stratigraphic profile for each site; <br />7. Shoot elevations of the elevation datums at annual intervals for a period of three years or <br />until no annual elevation changes are occurring; <br />8. Through elevation measurements and annual photographic documentation, measure the <br />effects of terrain lowering and any subsequent increase in erosion on the sample of sites for <br />a period of 10 years, and; <br />9. If sites are found to be deteriorating, design and implement a treatment plan for all eligible <br />or need data sites that might be effected by subsidence. <br />45