Upper Stillwater Dam near Bonneville, Utah, will be the first dam in the United States to be constructed with a combination of such unconventional materials and techniques as: slipformed fly ash concrete facings made with slipform paving equipment, and a roller compacted concrete base slab. The dam interior also will be a roller compacted mix. A typical concrete mix would have been too costly for so large a structure. Tests of various mix designs resulted in selection of a mix with 50:50 percent by volume of cement and fly ash with 1/4-inch slump. This was used for both faces and the base slab.
Slipforming was specified for the starting block and facings because it allowed for continuous placement of concrete and eliminated hand labor that would be needed if forms were used. Slipforming also eliminated the need for hand finishing. Facing elements were slipformed on the concrete base in five layers, the first layer against a 25 1/2-inch wide by 12-inch-high starting block. Each construction stage called for one layer of upstream and downstream facings. Right behind this facing operation, two 12-inch lifts of roller compacted fly ash concrete were placed by vibratory roller between the upstream and downstream walls. The slipformer traveled on the core to slipform the next layer of facings. A 1-foot height of the facing elements protruded above the compacted concrete to permit slipforming of the next layer of facings.
Guidance of the slipforming equipment for grade and alignment was controlled by laser because there was no room for conventional stringline guidance on the upstream face. A specified tolerance of 1/4 inch both horizontally and vertically was achieved using this system.