GENERAL32163

• Permit No: M1998014
• IBM Index Class Name: General Documents
• Doc Date: 7/31/2003
• Doc Name: Over Burden Stockpile Stability
• From: DMG - ACS
• To: DMG - SSS
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1313 Sherman St., Room 215 C O ]- O RA I~ O
Denver, Colorado 80203 D I v i s t o N o r
Phone: [phone] M 1 N &RA L

FAX: [phone]
GEOLOGY
REC LAMATION•MWING
DATE: July 31, 2003 SAFETY•SCIEN CE
TO: Steve Shuey Bill Owens
Governor

FROM: Allen Sorenson
5 Greg E Watcher
Executive Director
Ronald W. Canany
RE: Over Burden Stockpile Stability, Lafarge Gypsum Ranch Pit, Division Director
Permit No. M--1998-014 Natural Resource Trustee
The Gypsum Ranch Pit sand and gravel deposit is on a terrace located approximately 60 feet above the Eagle
River. At the north end of the terrace there is a sharp bend in the river channel where the river is actively
undercutting and over-steepening the terrace escarpment. The location of the river bend and the terrace
escarpment are shown in Sgure 1 attached to this memo. The erosion has left a 60-foot cliff dropping down to
the river, and the Operator has placed an elongate over burden stockpile along the edge of the cliff The
geotechnical stability study prepared by HP Geotechnical and submitted with the original permit application in
1998 calls for "special remedial actions" if "occupied or critical structures are located near the top of the
escarpment." No analyses or remedial actions were undertaken prior to the stockpiling of overburden along the
edge of the cliff The Division of Minerals and Geology (DMG) is concerned that given the inherent instability
of the escarpment, the stockpiled overburden may slough into the river. An inspection of the site was conducted
on March 14, 2003 to take measurements of the escarpment. The report of that inspection will be provided
under separate cover. The information collected during the inspection has been used to evaluate the stability of
the escarpment and estimate the potential for the overburden stockpile to fail into the river.
The critical, or least stable cross section of the escarpment is illustrated in figure 2. This is referred to as the
west profile because it is the westernmost of the two profiles measured during the March 14, 2003 site
inspection. The shear strength parameters for the soil and rock materials within the profile are also shown on
figure 2 and are estimated from values for similar materials found in geotechnical literature and DMG's
experience with similar materials at other sites. The results of slope stability analyses of the west profile are
attachment 1 to this memo. Analysis 1 of the west profile yields a safety factor of 0.99 for a circular failure
surface initiating near the back of the overburden stockpile and terminating at the toe of the sloughed gravel.
Any safety factor less than one is an indication of the potential for failure. Since the west profile has not failed
in the period of time since the stockpile was put up, the acting safety factor for the west profile must be greater
than one. The west profile was back analyzed to ascertain what the shear strength of the sand and gravel deposit
would have to be for the safety factor of the analysis 1 failure surface to be 1.5. The results of the back analysis
(see attachments) show that the gravel deposit would have to exhibit a friction angle of 51 degrees at zero
cohesion to hold a safety factor of 1.5. The results of extensive testing of similar deposits may be found in the
geotechnical literature, and these results lead to the conclusion that the Gypsum Ranch deposit does not have a
friction angle as high as 51 degrees, which means that the safety factor for the analysis 1 failure surface is less
than I.S.
A second back analysis of the analysis 1 failure surface was conducted (see attachments). The results of this
back analysis show that a friction angle of 41 degrees for the gravel deposit at zero cohesion would provide a
safety factor of 1.1 for the analysis 1 failure surface. It is reasonable to think that the Gypsum Ranch gravel
deposit may have a friction angle as high as 41 degrees. However, it is also possible that the gravel deposit is
exhibiting some cohesive strength. For example, the analysis 1 failure surface would have a safety factor of 1.1
Office of Office of Colorado
Mined Land Reclamation Active and Inactive Mines Geological Survey