PERMFILE102461

• Permit No: M2004078
• IBM Index Class Name: Permit File
• Doc Date: 3/12/2005
• Doc Name: Mining Setbacks from Permanent Valuable Structures
• From: DMG-acs
• To: DMG-esc
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
pepartment of Natural Resources
1313 Sherman St„ Room 215
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]
DATE: March 12, 2005
TO: Erica Crosby
FROM: Allen Sorenson ~~
I
COLORADO
DIVISION OF
MIN &RALS
GEOLOGY
RECLAMATION•MINING
SAFETY•SCIENCE
Bill Owens
Governor
Russell George
Executive Director
Ronald W, Cattany
Division Director
Natural Resource Trustee
RE: Mining Setbacks from Permanent Valuable Structures, SW TKO Joint Venture, LLC,
Lupton Lakes Pit, File No. M-2004-078
The Division of Minerals and Geology (DMG) engineering staff have completed a review of stability
analyses and proposed mining setbacks contained in the Lupton Lakes permit application. Geotechnical
borings were completed at the site, as well as certain field and laboratory testing to ascertain subsurface
conditions. The depth to weathered bedrock ranges from 35 to 55 feet throughout the site. The application
contains stability analyses of five critical cross sections located at the proposed perimeter of mining. Four
of the five analyses yielded safety factors just over one; the single analysis with a higher safety factor was
for a cross section with a gentle out slope. The Applicant did not analyze any of the soil strata for shear
strength; the application states that the sheaz strength inputs to the analyses were inferred from engineering
judgment and the results of the other geotechnical tests that were conducted. The DMG agrees that the
shear strength values proposed in the application are reasonable for the soil strata at this location. However,
if no shear strength testing is done, and if safety factors of just over one are to be acceptable, more
conservative strength inputs must be used. The maximum strength values that will be acceptable to the
DMG aze shown in the following table.
Dry Unit Weight Effective Cohesion Effective Friction Angle
Overburden 103 cf 0 33 de rees
Sand and Gravel 119 cf 0 35 degrees
Weathered Claystone 116 pcf 0 14 de rees
Claystone Bedrock 116 pcf 500 psf 26 degrees
Using these shear strength inputs, the DMG analyzed three setback distances from the edge of highway 85
in the vicinity of boring MW-03. The thickness of the soil strata and the depth to bedrock used in the DMG
model are taken from the well log for this boring. The Applicant has proposed a 25-foot setback at the
location analyzed by DMG. It is DMG's understanding (although this is not clear from review of the text or
maps in the application) that the 25-foot mining setback is to be measured from the edge of the highway
right-of--way. DMG further understands that there is 20 feet between the edge of the right-of--way and the
shoulder of the highway. If this were the case, a total setback distance of 45 feet would be provided.
DMG's analysis of a 45-foot setback distance yields an unacceptably low safety factor of 0.67. A 65 foot
setback yields a safety factor of 0.97, while the safety factor of 1.06 is provided when the setback is 70-feet.
Copies of the DMG's analytical results are attached to this memo.
Office of Office of Colorado
Mined Land Reclamation Active and Inactive Mines Geological Survey