PERMFILE123707

• Permit No: M2001085
• IBM Index Class Name: Permit File
• Doc Date: 7/8/2002
• Doc Name: Clay Liner Design and Relationship to the Amount of Reclamation Bond
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1313 Sherman SL, Room 215
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]
DATE: July 8, 2002
TO: Erica Crosby
DIVISION OF
MINERALS
Sc
GEOLOGY
RECrA MA710N
MINING•SAFETY
Bill Ovrens
Governor
FROM: Allen Sorenson Greg E. Walther
Executive Director
Ronald W. Canany
RE: Clay Liner Design and Relationship to the Amount of Reclamation Bond, Acting Division Director
Setbacks to Offsite Structures, 124rh Avenue Estates, File No. M-2001-085
The Division of Minerals and Geology (DMG) has completed a review of the Ground Engineering
Consultants report dated May 31, 2002 (Ground report). With submittal of the report, 124'h Avenue Estates
Partners requests a reduction in the amount of reclamation bond required for permit issuance and requests a
reduction in the excavation setbacks from valuable manmade structures on surrounding properties.
Item No. I -Setbacks to Valuable Structures
The Applicant has proposed a 10-foot setback from the Fulton Ditch easement line, and a 25-foot setback
from the U.S. Highway 85 right-of-way. The setbacks proposed for the north and south pit perimeters are 25
feet from the property lines and existing fences. At the point of intersection with cross section A-A
"Proposed Water Storage Plan" map dated June 18, 2002 and prepared by Jehn and Associates, the edge of
the Fulton Ditch is between 40 and 50 feet from the proposed edge of the excavation. The depth of
excavation along the west side of the pit parallel to the Fulton Ditch is shown on the "Proposed Water
Storage Plan" map to be 46 feet.
Historically, the DMG has rarely approved setbacks as tight as those proposed for this project. In support of
the proposed setbacks, the Applicant has provided quantitative slope stability analyses included in the
Ground report. The clay stone bedrock component of the slope cross sections that were modeled in the
stability analyses is assumed to be a high shear strength material with cohesion of 1,000 pounds per square
foot. By setting the strength of the bedrock so high, only very high safety factors will result for any modeled
failure surfaces that would pass, even slightly, through the bedrock zone in the cross sections analyzed. The
Applicant recognizes this affect on page 5 of the Ground report where it is stated that "bedrock parameters
do not affect the results strongly." Given that the upper few feet of the clay stone bedrock is "highly
weathered locally" (page 4 of the Ground report), the DMG does not agree that the strength of the bedrock is
not going to be a factor in the stability of the excavation back cut prior to placement of the clay liner. In fact,
the geotechnical literature describes cases where the residual strength of weathered clay stone can exhibit
zero cohesive strength. However, the DMG is in agreement that the slopes proposed in the Ground report
will be sufficiently stable to protect structures, if in addition to the 10 foot setback to the Fulton Ditch
easement line, the Applicant will further commit to a minimum 40 foot setback from the top edge of the
excavation back cut to the east edge of the top of the bank of the Fulton Ditch. This requirement should not
impose much, if any hardship on the Applicant depending on how close the Fulton Ditch encroaches on the
easement line over the reach in question.