1982-06-01_REVISION - M1977215 - Laserfiche WebLink

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Mr. Thomas M. Robertson 3 June 17, 1981 • Boring logs prepared by Envirologic Systems, Inc., as part of a piezome[er installation program in 1979. C f ~! L F L Ii. l_ L L • Well installation logs for the dewatering wells constructed in 1979. • Geologic information on the area published in the literature. • Observations made during a site visit on March 3, 1981. The Lower Tailings Dam and Pond are situated on a south slope of a relatively steep hillside in a shallow valley. The bedrock in this area is part of the Creeds Formation which is composed of stream, lake and pyroclastic deposits, the result of volcanic activity in the San Juan region. The Creeds Formation consists of sandstones and siltstones which are fractured and faulted in the area. The hillside on which the Lower Tailings Dam is located is reported as being extensively faulted, however, the faulting in the area has been inactive for some time and no faults in the immediate dam area have been discussed in existing formation. Faults mapped in the area north of the site generally run north-south or northwest to southeast. Since the dam axis runs east-west, the downstream seepage is in the general direction of the identified faults. It is possible that unidentified faults exist beneath the pond and/or the dam. The soils which overlie the Creeds Formation consist of interbedded alluvial sands, silts, and clays, with some gravel. The investigation of the surface and subsurface soil conditions in the pond area conducted by Lincoln-Devore in 1974 identified three general soil types of significant quantity; a silty sand which was classified as "pervious" and "extremely permeable" in its native state, a clayey sand of low permeability, and a silty clay of low permeability. The boring logs show that the soil profile consists of alternating layers of pervious (silty sand) and impervious (clayey sand and/or silty clay) soil layers. The pervious. soil layers may be continuous from the reservoir area under [he dam and downstream of the dam. The geologic cross section along the cutoff trench identifies several pockets of coarse gravel. These coarse gravel deposits, which probably have high permeabilities, are 0 to 5 feet below the surface. In some locations, they are intersected by the cutoff trench; in other locations, they appear [o pass beneath the cutoff trench. The Creeds Formation is directly below these coarse gravels at several points along this cross section. Alluvial deposits along the stream channel in the reservoir area, in the vicinity of the dam, and downstream of the dam have been identified from aerial photographs and the topography of the area. The interpreted areal extent of the alluvial deposits is shown in Figure 2. The alluvial deposits along the stream channels are estimated to range from 10 to 30 feet deep. Although all of the soils throughout the dam and reservoir fiJ~0.~5~ `~Fla'~l3'/l~l` 1111