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~~~~~LJII[~il~n~ILIn~. <br />Envlrnnmcntal & Englnaering <br />July 8, 2005 <br />Tom Leidich <br />Lafarge NA <br />10170 Church Ranch Way, Suite 200 <br />Westminster, CO 80021 <br />Envlranmentsl Support tor: <br />• Tr~nspvrtat/vn <br />• Land Development <br />• M/ning <br />Industry <br />Re: Responses to memorandums from Kathleeh L. Sullivan, P.E. to Gregg R <br />Squire dated June 23, 2005 and June 28,.2005 <br />Responses to the Memo dated June 23, 2005 from Kathleen L. Sullivan, P.E. to <br />Gregg R. Squire are as follows; <br />1. The last sentence on page 2 is admittedly, somewhat ambiguous. The <br />intent of this statement is, in other words, that the critical surface for the <br />0.5H:1 V slope returns a factor of safety of less than 1.0 indicating that the <br />slope itself (with no setback) is likely to fail. This result is actually <br />testament to the conservative nature of the physical soil strength <br />parameters used in the analysis due to the fact that pit highwall slopes in <br />the vicinity of the proposed Riverbend Operation do occur at 0.5H:1V or <br />steeper. <br />2. For the purpose of identifying soil types, each boring log was reviewed for <br />overburden, sand and gravel and bedrock soil type. The overburden soil <br />types varied from Clay (CL) to silt (ML). Clay layers were almost always <br />interbedded with silt layers. Instead of separating out each clay layer from <br />the silt, the conservative approach of assuming all overburden is silt was <br />assumed (clay. is a cohesive soil silt is not). For the gravel layer the <br />deposit varies from sandy graves to gravely sand. Since'sand and Gravel <br />have similar phi values and sand has some cohesion, the conservative <br />approach of using gravel (GW) for all sections with. cohesion set to zero <br />was taken. Bedrock was in all cases observed in the test hole logs as <br />weathered shale. Avery conservative value of 0 was used for the <br />cohesion of the weathered shale in the analysis. <br />It should be..noted that, for a given setback, the failure surface identified is in all <br />cases the critical failure surface. This is due to the fact that the bottom layer is <br />weathered. shale with zero cohesion. <br />Responses to the Memo dated June 28, 2005 from Kathleen L. Sullivan, P.E, to <br />Gregg R. Squire are as follows; <br />525 3'° Avenue, Suite 211, Longmont, CD 8DSO7 <br />ph 3D3-532-0951 fax 3D3-532-D953 <br />