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Memo to Steve Shuey 2 July 31, 2003 Gypsum Ranch Pit Permit No. M-1998-014 if the gravel deposit had a 38 degree friction angle and 160 psf cohesion. Problematically, the cohesive strength shown by sand and gravel deposits may not be relied upon for slope stability as it is not true cohesive strength but rather is a pore water presswe effect termed apparent cohesion. A second slope stability analysis of the west profile was conducted. The analysis 2 failwe surface initiates more toward the front of the over burden stockpile, and is still significant but is a much smaller failwe zone than that analyzed as analysis 1. Using the shear strength parameters shown in figwe 2, the analysis 2 failwe surface shows a safety factor of 0.53. Obviously, if 0.53 were the true safety factor, the slope would already have failed. In order to better understand the physics of the analysis 2 failure surface, two sets ofback-analyses were conducted. These back analyses show that for the analysis 2 failwe surface to have a safety factor of 1.5, the gravel deposit would need a friction angle of 72 at zero cohesion. For a safety factor of l.l, the gravel deposit would need a friction angle of 64 at zero cohesion. Both of these friction angles are well above the range of feasible friction angles for a sand and gravel deposit. Then why has the slope not yet failed? Again, apparent cohesion is a likely factor. For example, the analysis 2 failwe swface has a safety factor of 1.0, meaning it is on the cusp of failwe, if the friction angle in the gravel deposit were 45 degrees and the cohesion were 450 psf. The analyses documented in the attachments and discussed above consider circular failure swfaces. Whereas observations at the location of the west profile indicate that the operating system of slope failwes at the edge of the cliff is undercutting of the evaporite bedrock by the sharp outside bend in the Eagle River, which causes infrequent but occasionally large scale calving of bedrock slabs into the river. The mass wasting of the bedrock undercuts the terrace gravel deposit leading to progressive calving of the gravel and leaving a vertical gravel cliff face, but this mechanism of failwe does not preclude the possibility that circular failwes may occur in the futwe or that they have occurred in the past. These observations demonstrate that failwe along the escarpment is progressive, continuing, and occasionally massive. Based on these observations, it is certain that if the over burden stockpile were to remain where it is for a long enough period of time, there would be a failwe that would result in overburden being dumped into the Eagle River. Such an occurrence would be a violation of the Land Reclamation Act for the Extraction of Construction Materials both as an unacceptable off-site impact and as a failure to minimize impacts to the hydrologic balance. There is no way to predict when such a failure would occur, nor is it possible to predict the magnitude of the next or any subsequent failures. Slope failures most frequently occw as a result of some triggering action; in the case of the escarpment, the trigger could be a large calving of bedrock, a period of persistent rainfall, or an earthquake. Failure could also occur without an apparent triggering mechanism. Resultingly, DMG has determined that the placement of an over burden stockpile at the location of the west profile poses an unacceptable risk to the Eagle River. As discussed above, the west profile is located at the least stable spot along the rim of the escarpment. Other areas of the escarpment are less steep, are not being as aggressively undercut by the river, and have more space between the edge of the over burden stockpile and the edge of the cliff. During the March 14, 2003 site inspection, measurements were taken of a second, apparently more stable profile called the east profile. Across section of the east profile with annotations of the shear strength parameters is presented in figure 3 attached to this memo. The stability analyses of the east profile are included as attachment 2. The east profile was analyzed to determine if the entire over burden stockpile along the escarpment is unstable or if only the critical areas, such as the west profile location, are a problem. Analysis 1 of the east profile looks at a failwe swface that initiates at the toe of the over burden stockpile and terminates at the contact between the loose gravel and the bedrock. If a failure were to occw on this surface, none of the overbwden pile would slough. The safety factor for this failwe surface is 1.00, indicating that the slope is metastable at this location. The results of analysis 1 further show that locating the stockpile back from the edge of the cliff greatly reduces the risk that any of the stockpile will fail into the river.