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<br /> <br />Soils <br /> <br />The soils investigations were performed by Materials Testing Service, <br />Inc. of Denver, Colorado. The complete soils report is included in the <br />appendix of this report. <br /> <br />A total of eleven (11) electrical resistivity stations, nine (9) drill <br />holes and two (2) triaxial shear tests were made on the dam structure along <br />the centerline. The soils classifications of the core material remained re- <br />latively constant and consisted of GP, SM, ML, GM. The ML soils are fine <br />grained soils and the others classified as coarse grained soils. <br /> <br />The GP soils were predominantIy in the top few feet of the embankment. <br />The GM soils were generally in the top layers except for test bore hoIe 7, 8, <br />and 9. A pIot of the dry density versus the liquid limit, see Figure <br />indicates the embankment has a swelling and settling tendency dependent on <br />reservoir level. This is explained further by the fact that many of the drill <br />holes were saturated and water Ievels in the drill holes moved rapidly in the <br />same direction as the reservoir level, increasing or decreasing. <br /> <br />Furthermore, the plasticity index in all samples was very low or <br />"none plastic". This is not surprising since a very low percentage of material <br />was found to pass the 200 sieve for all samples. It is to be noted.that most <br />of the samples in the main portion of the dam were totally saturated. <br /> <br />Comparison of this data with recommended design practices of zoned <br />embankments on impervious foundations (2) was made to determine re- <br />commended upstream and downstream slopes. Using the U. S. Bureau of <br />RecIamation, Design of Small Dams, Chapter VI, Tables 18 and 19 as a guide <br />for comparison based on soils types, the dam should have a 3: 1 upstream slope <br />and a 2-1/2: 1 downstream slope for conditions not affected by rapid draw down. <br /> <br />Further, it is recommended that transitions be designed to avoid <br />"piping" of the fine materials from one zone to another zone. For rock fill <br />outer zones, the impervious core should have a sand gravel or rock fines <br />blanket of a few feet designed as a filter to stop piping. On the Idaho Springs <br />Reservoir there is no indication of a filter blanket or transition zone between <br />the rockfill and the impervious core material. <br /> <br />A Iook at the triaxial shear properties shown'in Figure 19 and 20 of <br />the soils report indicates very low cohesion properties and a low angIe of <br />friction in the materials. The lack of cohesion would aIso indicate a rather <br />high permeability of the core material. For soils of this type under optimum <br />compaction, the permeability is less than 10 feet per year. <br /> <br />1II-3 <br /> <br />