2011-06-15_PERMIT FILE - M2010049 ‎(2)‎

• Permit No: M2010049
• IBM Index Class Name: PERMIT FILE
• Doc Date: 6/15/2011
• Doc Name: Slope Stability Review
• From: CGRS Environmental Services
• To: DRMS
• Email Name: MAC
• Media Type: D
• Archive: No

CG
June 11, 2011
Division of Reclamation, Mining and Safety (Division)
1313 Sherman Street, Room 215
7ttn ver , Colorado 80203
: Mr. Michael Cunningham
RE: Varra Great Western Sugar Project
Response to Division Letter?
DRMS File No. M-2010-049 - Slope Stability Review
CGRS No. 1-135-12539aa
Dear Mr. Cunningham:
The following provides responses to Mr. Cazier's comments.
ENVIRONMENTAL SERVICES
RECEI
kkN 15 2011
+, Division
y,Division of Reclamation,
and Safety
Item 1- It is our understanding that mining will not exceeded 45 feet in the northern tracts due
to engineering issues and less valuable aggregate.
Item 2(a) - The last sample obtained from boring 2 was in the transition zone between
unconsolidated deposits and the Pierre shale. After conversations with Terracon, the
geotechnical contractor, the upper portion of the last sample was used for the grain size analysis
and the lower portion was used for strength testing. The lower portion apparently had a much
higher clay/weathered shale content. We used the values reported by Terracon which are
reasonable for weathered shale.
Item 2(b) - The values used were either reported by Terracon or in the absence of measured
values we used table values provided by your agency. Across the study area the second soil layer
varied between silty sandy gravel to lean clay. To be conservative in our analysis lean clay
strength and weight values were used. The table provided by the Division is presented as
Attachment A as is the input table for the PCStabl model.
Item 2(b) - The model can run multiple simulations; however only the most conservative result
is reported on the graphic output. One method can also be selected and I have provided the
Spencer method output for an example (Attachment B). As can be seen for this scenario the
Safety Factor is less conservative than the Bishop method.
Item 3 - As we modified a previous report the reference was inadvertently left in.
Item 4 - We have labeled the three Plates for reference and have provided color reproductions
in Attachment C.
P.O. Box 1489 Fort Collins, CO 80522 T 800-288-2657 F 970-493-7986 www.cgrs.com