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INTEROFFICE MEMORANDUM <br />COLORADO DIVISION OF MINERALS AND GEOLOGY <br />7313 Sherman St., Rm. 215 <br />Denver, CO 80203 <br />To: Gregg R. Squire c c <br />FROM: Kathleen L. Sullivan, P.E. I <br />SuRJECT: ~M-2000-016, Lafarge West, Inc.: Riverbend Operation, AM-01 <br />,iGeotechnical Adequacy Review / <br />DATE: ~ June 23, 2005 <br />CC: Carl Mount, DMG (via e-mail); Allen Sorenson, DMG (via a-mail) <br />This memo contains adequacy review comments regarding the Division's slope stability <br />review of the Lafarge West, Inc.: Riverbend Operation 112(c) Amendment (AM-01) <br />Application, File No. M-2000-016. I have directed my comments to the applicant, who may <br />contact me with any questions at (303) 866-4060. <br />1. Please explain the meaning of the last sentence on p. 2 of 5 of the "Ft. Lupton Pit Slope <br />Stability Study" (Study) by Weiland, Inc., "It should be noted that minimum factors of <br />safety were determined for each slope analyzed and in all cases, the proposed slope <br />with assumed stratigraphy and soil characteristic parameters returned a value for FS of <br />less than 1.0, indicating that the proposed slope would fail under the existing conditions." <br />2. In the February 11, 2005 adequacy review, the Division requested drill hole records. The <br />data provided contained borehole locations and overburden and sand and gravel <br />thicknesses, but not ground water levels or information on soil characterization. Please <br />provide well logs and completion diagrams so the Division may verify how the soil was <br />classified and depths to ground water. <br />Based upon the Division's analyses of the data provided to this point, the proposed setbacks <br />appear acceptable (analyses attached). A final determination will be made upon receipt of <br />the items requested above. <br />To increase confidence in future submittals from the applicant and its consultant, the Division <br />is providing the following feedback and guidance: <br />• Page 1 of 5 of the Study states that when a water feature was not present, "the boring <br />data was averaged for all of the borings within the cell." Averaging all boring data within <br />a cell is not an acceptable practice because to allow a minimum factor of safety of 1.00, <br />all inputs must be conservative. This means maximum total depths must be input into <br />the slope stability analyses. <br />• There is significant variability in the cell pumping rates used to predict drawdown and <br />resultant phreatic surfaces for input into the slope stability model. In this situation, the <br />outcome was conservative, so the phreatic surfaces applied in the model are acceptable. <br />