DWR_3391463 - Laserfiche WebLink

relating to the shear strength of a gray, sandy silt. Accord- ingly, the firm of Dames $ Moore was selected to provide a third party analysis of the stability, subsequent to construc- tion but prior to the dam's use as a tailings pond. They reviewed the initial data developed during design and construc- tion and further did their own field investigation, laboratory testing, engineering analysis, and report preparation. It was their opinion that "the existing embankment is stable and will remain stable as tailings are deposited in the reservoir." The Dames & Moore investigation did not center on problems of seepage, but rather centered on the problem of stability re- sulting from low shear strength material. Analysis of the drilling logs contained in the Lincoln- DeVoro, Francis-Meador-Gellhaus, and Dames $ Moore reports provide valuable information on the geologic material supporting the tailings dam. Bedrock in the area consists of the sandstones, siltstones, and shales of the Creede formation, which is frac- tured and locally faulted within the area. While some of the reports indicate this material is "impermeable", the highly frac- tured nature of the bedrock renders this a questionable conclusion. The Creede formation is overlain with clays, silts, sands, and gravels, derived from erosion of the Creede formation and pre -Cambrian material located upgradient from the Creede forma- tion. This is a very heterogeneous material, and was deposited in a high energy environment. The old stream channels, consist- ing of sands and gravels, cross the site north to south. For example, Lincoln-Devore Testhole 4, located on the axis of the -2-