2005-06-24_REVISION - M2000016 ‎(6)‎

• Permit No: M2000016
• IBM Index Class Name: Revision
• Doc Date: 6/24/2005
• Doc Name: third adequacy review
• From: dmg
• To: lafarge west inc
• Type & Sequence: AM1
• Media Type: D
• Archive: No

INTEROFFICE MEMORANDUM
COLORADO DIVISION OF MINERALS AND GEOLOGY
1373 Sherman S[., Rm. 215 -
Denver, CO 80203
To: Gregg R. Squire c <J
FROM: Kathleen L. Sullivan, P.E.
Su17JECT: M-2000-016, Lafarge West, Inc.: Riverbend Operation, AM-01
Geotechnical Adequacy Review
DATE; June 23, 2005
CC: Carl Mount, DMG (via e-mail); Allen Sorenson, DMG (via a-mail)
This memo contains adequacy review comments regarding the Division's slope stability
review of the Lafarge West, Inc.: Riverbend Operation 112(c) Amendment (AM-01)
Application, File No. M-2000-016. 1 have directed my comments to the applicant, who may
contalct me with any questions at [phone].
1. Please explain the meaning of the last sentence on p. 2 of 5 of the' "Ft. Lupton Pit Slope
Stability Study' (Study) by Weiland, Inc., "I! should be noted that minimum factors of
safety were determined for each slope analyzed and in all cases, the proposed slope
with assumed stratigraphy and soil characteristic parameters returned a value for FS of
less than 1.0, indicating that the proposed slope would tail under the existing conditions."
2. In the February 11, 2005 adequacy review, the Division requested drill hole records. The
data provided contained borehole locations and overburden and sand and gravel
thicknesses, but not ground water Levels or information on soil characterization. Please
provide well logs and completion diagrams so the Division may verify how the soil was
classified and depths to ground water.
Based upon the Division's analyses of the data provided to this point, the proposed setbacks
appear acceptable (analyses attached}. A final determination will be made upon receipt of
the items requested above.
To increase confidence in future submittals from the applicant and its consultant, the Division
is providing the following feedback and guidance:
• Page 1 of 5 of the Study states that when a water feature was not present, "the boring
data was averaged for all of the borings within the cell ° Averaging all boring data within
a cell is not an acceptable practice because to allow a minimum factor of safety of 1.00,
all inpuls must be conservative. This means maximum total depths must be input into
the slope stability analyses.
• There is significant variability in the cell pumping rates used to predict drawdown and
resultant phreatic surfaces for input into the slope stability model. In this situation, the
outcome was conservative, so the phreatic surfaces applied in the model are acceptable.