2011-12-07_REVISION - M1986015 ‎(8)‎

• Permit No: M1986015
• IBM Index Class Name: REVISION
• Doc Date: 12/7/2011
• Doc Name: Adequacy Response
• From: Tuttle & Associates
• To: DRMS
• Type & Sequence: AM4
• Email Name: TC1
• Media Type: D
• Archive: No

MEMORANDUM
December 5, 2011
File No. 111057 -000
A DIVISION OF BRIERLEY ASSOCIATES, LLC
Engineering Solutions from the Ground Down
TO: Mr. Gary Tuttle , Tuttle & Associates, Inc.
FROM: Rebecca Brock, P.E. & Nathan Soule, P.E., P.G.
SUBJECT: Pueblo East Am -04 Adequacy Letter
Lyman Henn would like to provide the following response to Section 6.5 of the November 28, 2011 letter titled
"Pueblo East Pit, File No. M -1986 -015 AM -04; Adequacy Review ":
1. The minimum factor of safety requirement of 1.01 for the temporary slopes specified is based on:
a. Email communication with Peter Hayes with the DRMS on April 29, 2011;
b. Allen Sorenson's official Division of Reclamation, Mining and Safety memo regarding typical
soil strength parameter dated March 12, 2003 (attached);
c. The conservative values used in the slope stability analysis; and
d. Lyman Henn's experience with similar projects using the same parameters.
2. Lyman Henn was provided dimensions and locations of proposed stockpiles around the gravel pit. The
dimensions of the stockpiles are proposed to be 10 ft high and 12 ft wide at the top with 1:1 side slopes.
Stockpile material was assumed to be composed of Overburden soil. The slopepiles were conservately
placed at the property line such that the weight of the soil would have the maximum effect on the driving
force of the failure circle. Conservatively, a unit weight of 114 pcf was used for the stockpiled material,
which is the same unit weight used for the in situ Overburden soil. The stockpiles were added to the
slope stability model sections previously evaluated in our letter referenced above to evaluate their effect
on the recommended setbacks provided in that letter.
As shown in the attached figures, the stockpiles do not further destabilize the temporary highwalls mined
at a near vertical slope. Therefore, the setback limits recommended in the above referenced letter remain
adequate for these cross sections. The stockpile has a minimal effect on the cross section with the 1:1
slope. In order to maintain the 1.01 FOS, the limit of mining should be moved away from the property
line by two feet such that the setback from the crest of the slope to the property line is 57 ft.
As discussed in the above referenced letter, failures originating in front of the recommended setback point
may have a FOS less than 1. The stability analyses provide recommended minimum setbacks to prevent
damage to significant, valuable and permanent man -made structures. Further analysis should be
performed prior to construction to evaluate the global stability of the proposed highwalls themselves.
3. The phreatic surface used in the analyses are based on the following considerations:
a. Groundwater levels during drilling were measured at 8.3 feet and 11.6 feet below ground surface;
b. A conservative value of 1 foot below the ground surface was assumed for the natural static
groundwater; and
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