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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />-26- <br /> <br />cobble and boulders is 2.7 to 2.85 and their durability and abrasion <br />resistance is estimated to be good to excellent. These materials would be <br />excellent for use as riprap. <br /> <br />The sand and gravel fraction of the materials excavated from the test <br />pits is generally of mixed composition and includes basalts and fragments of <br />shales and sandstones. The sands and gravels are angular to subrounded. <br />Gradation analyses completed on the minus 3/4-inch materials taken from test <br />pits TP-1 and TP-2 are given in Appendix E. Results of specific gravity ana- <br />lyses of the minus No.4 materials from test pits TP-1 and TP-2 ranged from <br />2.71 to 2.73. The results of Atterberg limits tests on the materials from <br />these test pits indicated plasticity indices ranging from 5 to 6 and liquid <br />limits ranging from 25 to 26. <br /> <br />Published geologic data indicate that surficial materials at the site <br />explored by the test pits are relatively thick and consist of Holocene and <br />Pleistocene landslide deposits and slopewash derived from Mancos shale and the <br />Basalt caprock forming the nearby Flat Tops. In general, these materials are <br />not distinguishable from one another and are similar in character throughout <br />the area. Materials found in the test pits were similar to materials observed <br />in the right abutment area, in scarps located in the stream channel downstream <br />of the toe of the dam, and in the lower portion of the existing spillway chan- <br />nel. Bedrock outcrops were not observed in the left or right abutment, or toe <br />area of the dam. We do not anticipate that bedrock will be encountered in <br />open-cut excavation in the toe area of the dam required for construction of <br />the soil-cement overtopping protection. <br /> <br />In general, the materials found in the test pits are good for use in <br />constructing the required modifications to Sheriff Reservoir Dam. Cobble and <br />boulders could be removed for use as riprap. The remaining silty and clayey <br />sands and gravels would be a good base material for use in soil-cement and for <br />raising the dam crest. The overall average gradation anticipated for soil <br />materials excavated at the site, including the toe of the dam, is as follows: <br /> <br />Fraction <br /> <br />% by Total <br />Dry Weight <br /> <br />20 <br />20-25 <br />30-35 <br />20-30 <br /> <br />Comment <br /> <br />Cobble and boulder <br />Gravel <br />Sand <br />Minus No. 200 <br /> <br />Widely graded <br />Widely graded <br />Widely to poorly graded <br />Low plasticity <br /> <br />The above average gradation was assumed in our quantity and unit cost esti- <br />mates to determine the volume of riprap generated from the excavated <br />materials, as well as materials available for use -in soil-cement, raising the <br />dam crest, and for disposal in the right abutment area. <br /> <br />Structural Plans and Details - Constructing overtopping protection on a <br />dam with inadequate spillway capacity has been demonstrated to be an accep- <br />table, cost-effective means of providing required flood routing protec- <br />tion. The concept has been approved by the Colorado SEO on two recently <br />constructed projects. One of the SEO-approved and constructed projects used <br />