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Observation of the West Plt Pond embanKment showed no signs of movement or <br />distress Indicating Instablllty. At the time of our visit the pond level was Just below the <br />outlet, at the lowest operating level. Mr. tiger wkh 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 eras, Including a larger slide <br />to the south shown on Flg. 1. The bedrock generally consists of alternating layers of <br />sandstones and shales or clayatone. The regional dip of the bedrock Is 4 to 5 degrees, <br />with a striko of N 70° W. Localized varlatlone are common. Observation of the bedrock <br />In the West Pk 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 />clayatone seems was observed in our previous eke 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 wkh <br />the base of the slide Hoar the top surface of a clayatone layer. The clayatone 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 />Tactors which singly, or in combination, cou-d have triggered the movement Including: <br />Seepage water from the south. A redudlon In strength and Increase In <br />drlving forces due to seepage of water from the south. Water would soften <br />and lubricate the clayatone 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 (n weight, Increasing drlving 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 clayatone layer, <br />reducing the resisting forces, and also increased the slope height. <br />3. Water In the sedimentation pond. The water In the sedlmentatlon 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 n Blight 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 />A11 of the above, oombined with the unfavorable orientation o} the weak clayatone <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 />ll#:fiStttLAfi~fi «N~SdW~H1111J WdLll:li Sfi-A -L N~SdWOHl/11~:1~9 1N3S <br />