2005-06-28_REVISION - M2000016 (2)

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INTEROFFICE MEMORANDUM COLORADO DIVISION OF MINERALS AND GEOLOGY 13]3 Sherman St., Rm. 215 Denver, CO 50203 TO: Gregg R. Squire p n FROM: Kathleen L. Sullivan, P.E. ~e~'li~x-Q~.~ susJECr: M-2000-016, Lafarge West, Inc.: Riverbend Operation, AM-01 Geotechnical Adequacy Review -Water Level Drill Hole List DATE: June 28, 2005 CC: Carl Mount, DMG (via a-mail) The following comments are directed to the applicant. Pursuant to discussions during our meeting on Friday, June 25, 2005, I am providing the following list of drill hole numbers for which the Division requires well logs and/or completion diagrams to verify ground water levels utilized for the geotechnical slope stability analyses. 1. N0000-BH107 (Phase 2, Cell 2 SW -Figure 8) 2. N000-BH116 (Phase 2, Cell 1 W -Figure 4) 3. N000-BH117 (Phase 2, Cell 1 W -Figure 4) 4. MY03-MW01 [per map] (or BH01?) (Phase 5, Cell 5 N, E, S -Figure 18) 5. CH00-BH11 (Phase 5, Cell 5 N, E, S -Figure 18) 6. CH00-BH12 (Phase 5, Cell 5 W -Figure 19) 7. RI01-BH09 (Plant Area N, W -Figure 29) Please also identify the corresponding drill hole and water level data used to define the phreatic surfaces for the referenced figures, if different than those drill holes identified above, and provide their well logs andlor completion diagrams. The Geotechnical Adequacy Review Memo dated June 23, 2005, stated that averaging all boring data within a cell is not an acceptable practice because to allow a minimum factor of safety of 1.00, all inputs must be conservative, noting that maximum total depths must be input into the slope stability analyses. In other words, the Division requires worst-vase-scenario analyses. Since our meeting, I detected that as a result of using averages instead of using maximum total depths, the setback calculated for Phase 5, Cell 5 - N, E, S (Figure 18) is not characteristic of the stratigraphy for the east side of the cell, per drill hole CH00-BH11. I reanalyzed the slope stability for this east wall using the material depths from the representative drill hole. This analysis reveals that the 94 feet from the toe of the slope and 69 feet from the top of the slope setbacks are inadequate to protect the significant man-made structures along this slope (e.g., gas well, gas and power lines). Depending upon the location of the phreatic surface, the setbacks need to be increased between 5 and 30 feet for the east slope of this cell.