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16. Abutment Structures. -The two abutment structures, located one on each side of the spillway, connect the spillway gate structure with the adjacent dam embankment. Each abutment structure (fig. 12) consists of an inner wall panel and an outer wall panel, each of a modified slab and buttress design. Each inner panel has a base length of about 60 feet and a base width of about 75 feet. The outer panels are similar and are about 84 feet 6 inches long, tapering uniformly from the width of the inner panel to 44 feet at the outer edge. Buttresses, spaced at 20-foot centers, provide support for the face wall from elevation 2709 to elevation 2789. Below the embankment line, the buttresses and face wall are of monolithic construction. Above the embankment line, these structures are articulated and the face wall consists of 8-foot-high sections made watertight with rubber waterstops. Two waterstops are provided for vertical joints and one for horizontal joints. The upstream and downstream buttresses have 0.4 to 1 slopes.

A beam and column forms a bent with the upstream portion of each buttress to support a highway bridge.

In the design of the abutment face walls the following loadings were assumed:

(1) Water in reservoir at elevation 2785.0; a base uplift corresponding to water surface elevation 2773.0; uplift diminishing uniformly from full at upstream edge to zero at downstream edge.

(2) Water in reservoir and base uplift for water at elevations indicated in (1) above; uplift over base diminishing uniformly but reduced 50 percent at the centerline of the first heel slab drain.

(3) No water in reservoir and no uplift; dry earth fill on both sides of the walls.

For stability analyses, the abutment structures were investigated for overturning, base pressures and sliding. With reservoir water surface at elevation 2785.0, and full upstream horizontal saturated earth and water loads, the following additional conditions were assumed in investigating stability:

(1) No downstream horizontal loads; uplift loads as indicated in (1) above and assuming all base drains are inoperative. (For sliding only.)

(2) Downstream horizontal earth loads for dry embankment; full uplift over entire base area.

The bridge columns were designed for supporting a bridge subjected to an H20S16-44 loading and 30 percent impact, and an ice load of 1,000 pounds per foot of width of column applied at midheight.

17. Gate and Bridge Supporting Structure. - The gate and bridge structure (figs . 12 and 13) provides support for a spillway bridge and three spillway gates with controls. The bridge has a length of about 438 feet and a width of 26 feet. Support for the bridge is provided by buttresses spaced 20 feet apart near the abutments and by four piers spaced 50 feet over the gate structure. The four 8-foot-wide piers extend to elevation 2785, and contain stoplog slots and float wells with floats for use with the hydraulic gate operating system. To provide a minimum of contraction of the flow of water, the outer piers were tapered from elevation 2754 to elevation 2785.

On top of the left abutment pier and downstream of the spillway bridge is an access and compressor house 21 feet 6 inches long and 8 feet wide. A walkway is provided on the right edge of the pier. On top of the right abutment pier is a river outlet control house of the same positioning and dimensions as the access and compressor house. A steel platform, spanning the piers beneath the highway bridge, is provided for gate hoisting machinery.

River outlets are located at the base of the intermediate piers and pass through the crest structure. To keep hydraulic losses low, these outlets have bellmouth entrances. The outlet gate chambers are located directly below these piers and are connected by an 8- by 8-foot gallery in the crest.

Trashrack structures are located at the entrance of each of the river outlets and extend from the inlet walls to the gate structure. Each trashrack structure has two

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