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Observation of the West Pit Pond embankment showed no signs of movement or <br />distress indicating instability. At the time of our visit the pond level was just below the <br />outlet, at the lowest operating level. Mr. Kiger with Colowyo stated that pond levels were <br />not elevated above this level in the recent past. <br />Our past observations and review of aerial photographs indicate that several <br />landslides have occurred in the native slopes in the general area, including a larger slide <br />to the south shown on Fig, t. The bedrock generally consists of alternating layers of <br />sandstones and shales or claystone. The regional dip of the bedrock is 4 to 5 degrees, <br />with a strike of N 70° W. Localized variations are common. Observation of the bedrock <br />in the West Pit show undulations or folds in the bedrock surface in localized areas. The <br />development of perched water within sandstone seams, flowing above less permeable <br />claystone seams was observed in our previous site visit and is presumed to be occurring <br />to the east of the slide area. <br />CONCLUSIONS <br />In our opinion, the slide is a translational, sliding block type earth movement with <br />the base of the slide near the top surface of a claystone layer. The claystone layer has <br />an unfavorable orientation with respect to stability and visually appears to be highly <br />plastic and, when wetted, would have a very low residual strength. There are several <br />factors which singly, or in combination, could have triggered the movement including: <br />Seepage water from the south. A reduction in strength and increase in <br />driving forces due to seepage of water from the south. Water would soften <br />and lubricate the claystone surface and also reduce effective stresses. <br />The topsoil pile could have covered the area where water previously <br />seeped out of the slope, and caused pore pressures to rise. The <br />saturated soil mass would increase in weight, increasing driving forces. <br />2. Changes in slope geometry. The weight of the topsoil increased the upper <br />slope angle and height, increasing driving forces. The excavation of the <br />detention pond removed overburden soils above the claystone layer, <br />reducing the resisting forces, and also increased the slope height. <br />3. Water in the sedimentation pond. The water in the sedimentation pond <br />can also enter the lower portions of the slide area causing similar effects <br />as the seepage water. The effects of the pond water would be limited to <br />the top elevation of the pond plus a slight increase due to capillary action. <br />Elevated water levels in the pond were not present prior to the slide <br />according to mine personnel. <br />All of the above, combined with the unfavorable orientation of the weak claystone <br />layer contributed to the conditions which created unstable conditions. In our opinion, the <br />water in the detention pond probably was the least critical factor of the three. We believe <br />2 <br />