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t U CHAPTER 4 PRELIMINARY DESIGN OF DAM AND SPILLWAYS 1 DAM TYPE EVALUATION AND SELECTION The most significant characteristics of the existing Elkhead Dam with respect to the method of enlargement are embankment geometry, construction materials, and current condition. The dam is a homogeneous earthfill structure which has performed well for twenty years and it is in good condition. Since the dam was originally conceived, and through the many subsequent engineering evaluations, there has been a consistent consensus that the dam type be earthfill. The Pilot Knob Dam, which was proposed several miles upstream at a similar site on Elkhead Creek, was also proposed as an earthfill structure. While there has never been a suggestion that another dam type is more appropriate, it is meaningful to objectively revisit the qualitative issues on the type and configuration of the dam enlargement. The most significant issues can be summarized as follows: 1. The existing dam is an earthfill structure. Switching to another type of dam does I not permit maximum utilization of the facility which already exists. 2. A topographic saddle exists adjacent to the dam which can be used as an emergency spillway, thereby eliminating the need for a dam which can be overtopped or for an expensive structural emergency spillway. 3. Clay for the dam core and material for the shell/random fill zones are economically available at the site in adequate quantities. 4. Suitable rock must be hauled to the site from a distance, thereby effectively eliminating a rockfill dam. 5. An earthfill dam provides the highest aesthetic conformity with the natural rolling topography of the reservoir area. 6. The dam site is not narrow nor does it have exposed or shallow sound bedrock, which are the characteristics best suited to a concrete gravity or arch dam. 7. The dam width-to-height ratio is greater than 10:1, effectively eliminating thin section dam types. 8. The wide trapezoidal cross-section of the site favors an earthfill dam. 9. The lack of strong abutment and foundation rock needed for concrete gravity dams over 50 feet high favors an earthfill dam at this site. 1 4-1