2009-06-19_REVISION - M1987049

• Permit No: M1987049
• IBM Index Class Name: REVISION
• Doc Date: 6/19/2009
• Doc Name: Adequacy Review #3, Geotechnical Stability Review
• From: DRMS
• To: Ready Mixed Concrete Company
• Type & Sequence: AM2
• Email Name: JLE
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF RECLAMATION, MINING AND SAFETY
Department of Natural Resources
1313 Sherman St., Room 215
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]
COLORADO
D I V I S ION OF
RECLAMATION
MINING
SAFETY
Bill Ritter, Jr.
DATE: June 18, 2009 Governor
Harris D. Sherman
TO: Jared Ebert Executive Director
/f Ronald W. Cattany
Division Director
or
FROM: Allen Sorenson Natural Resource Trustee
RE: Review of Proposed Geotechnical Setbacks, Ready Mixed Concrete Company,
Bromley Lakes Pit, Amendment AM-02, M-1987-049
The Division of Reclamation, Mining, and Safety (DRMS) engineering staff has reviewed the above
referenced permit amendment application (AM-02) and has determined that the following issues remain to
be addressed relative to the proposed geotechnical setbacks.
1. Subsurface utilities along Brighton Road are mentioned in Exhibit D, but are not described in Exhibit S;
a description of these utilities must be provided. Critical subsurface utilities such as sewage lines and
high pressure gas lines may require more intensive analysis than the general global stability analysis
provided in AM-02.
2. The geotechnical stability exhibit included in AM-02 describes a maximum pit depth of 25 feet.
Geotechnical borings are mentioned in Exhibit D, but locations and logs of the borings are not provided.
The Applicant must describe the basis for the determination of a 25 foot maximum depth, as excavation
depth is critical to evaluation of the sufficiency of proposed setbacks.
It is stated in Exhibit D of AM-02 that temporary excavation slopes will be 1 H:1 V or steeper. The
general stability analysis provided in AM-02 considers 1H:1V excavation slopes. The DRMS will
require evaluation of vertical slopes for establishment of setbacks.
4. It is stated in Exhibit D that overburden and topsoil stockpiles will be constructed in the setback from
Brighton Road. The stockpiles and illustrated on Exhibit C maps, but no vertical dimension or side slope
angles are provided. The general stability analyses provided in AM-02 do not consider the surcharge
load imposed on excavated slopes by these stockpiles. The DRMS will require evaluation of surcharge
loads for establishment of setbacks.
5. Shear strength parameters and stability analyses:
It is stated in the geotechnical stability exhibit that the shear strength parameters for sand and gravel and
overburden input to the general stability analysis provided were derived by back calculation in
consideration of observed stability of near vertical excavations in the existing pits. The results of the
back calculation for shear strength of the sand and gravel produce a cohesion value: of 200 pounds per
square foot and an angle of internal friction of 45 degrees. These are extremely high values. For
example Design of Small Dams published by the U.S. Bureau of Reclamation (3rd :Edition) includes
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