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<br /> <br />Sanborn Creek Mine T.R. No. 29 -Supplemental Response <br />Geotechnical Adequacy <br />Subsidence Impact Prediction <br />The response states that an additional analysis of the projected impacts of subsidence <br />is being prepared by Agapito & Associates. Please forward that analysis for my <br />review upon its arrival. I suggest the approval reference the applicant's commitment <br />to completion and submittal of the analysis in a timely fashion. <br />Appendix L - "West Valley Fill and Haul Road Design" <br />(1) Coal refuse fill sediment pond design <br />OXBOW did verify the depth to the Rollins Sandstone bedrock beneath the <br />proposed concrete sediment pond wall in September of 1996. The contact <br />between alluvium fill and the Rollins Sandstone bedrock was determined to be <br />in excess of several tens of feet. As such, I continue to be concerned <br />regarding the potentially detrimental impacts seepage beneath the concrete wall <br />might have upon the adjacent railroad embankment. In a report prepared for <br />OXBOW, Lincoln-Devore, Inc., recommends over excavation and compaction to <br />preclude the infiltration of waters impounded within the sediment pond. In a <br />report dated June 18, 1997, Lincoln-Devore states that it believes that <br />recompaction of the excavated material to 90% of the maximum density (ASTM <br />D-1557) will obtain a minimum Soil Coefficient of Permeability of 1.0x10-' <br />cm/sec. <br />This may or may not be the case. Therefore, I believe it prudent, because of <br />the potential risk to the adjacent railroad embankment, to require OXBOW to <br />verify that a minimum Soil Coefficient of Permeability of 1.0x10-' cm/sec is <br />achieved following the over excavation and recompaction. This verification <br />could be accomplished by either laboratory analysis of the material or by in-situ <br />field permeability determinations. I recommend that we stipulate the <br />performance of verifying tests in our approval documents. If OXBOW fails to <br />attain a sufficiently low permeability through over excavation and recompaction <br />an alternative methodology, such as installation of an impermeable liner, should <br />be required. <br />(2) Stability of the proposed West Valley Haul Road, Sediment <br />Pond Road, Cemetery Road and Elk Creek Section Road <br />Balaz & Associates, permitting consultant to Oxbow Carbon & Minerals, Inc., <br />commissioned a report by Lincoln-Devore, Inc. To analyze the geotechnical <br />stability of the West Valley Fill and the Haul Road. In response to our earlier <br />adequacy comments, Balaz & Associates directed Lincoln-Devore, Inc. to <br />