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The trashrack is a boxlike structure 26. 5 feet long, 15 feet wide, and 10. 5 feet high (fig. 10). Seven openings each 8.25 feet square are located two on each side, two on the top, and one on the upstream end of the structure. The trashracks are square and interchangeable. The downstream end consists of a transition from a square interior of the trashrack to the circular tunnel section. Stoplog guides are provided upstream from the transition.

The tunnel has an overall length of 725 feet, an inside diameter of 7. 5 feet, and is lined with a minimum thickness of 10 inches of concrete. The inlet and outlet portals are set at elevation 940.00 and 930.00, respectively.

The gate chamber is constructed monolithic with the tunnel lining at a location slightly upstream from the dam axis. The intake for the 22-inch pipe is located at the upstream end of the gate chamber (fig. 10) in the right side of the tunnel lining. This intake is provided with trash bars. From the intake the 22-inch pipe is extended to an 18-inch wedge valve located in a small side compartment in the gate chamber. The wedge valve is for emergency use only. From this valve the 22-inch pipe is extended to a location under the tunnel invert. The 4-by 4-foot high-pressure gate is located in the central portion of the gate chamber. It has a capacity of 1150 cubic feet per second at maximum water surface elevation 1055.0 (fig. 12). It is designed to be operated only fully closed or fully opened.

A 6-foot-diameter concrete-lined access shaft extends upward from the gate chamber to the crest of the dam. This shaft is equipped with steel ladders and is covered with a metal cover. The controls for the high-pressure gate are located on top of this shaft.

The valve house is located on the right side of the tunnel and forms a short exit channel from the outlet portal. The 22-inch outlet pipe (sec. 18) is extended to the 18inch pivot valve in the valve house. The centerline of this valve is set at elevation 933.77 (fig. 11). This outlet pipe with the valve 100 percent open has a discharge capacity of approximately 67 cubic feet per second at maximum water surface elevation 1055. 0 («g. 13).

Immediately downstream from the valve house is the stilling basin, excavated in rock. No concrete lining is provided. The 18-inch valve is so located that the maximum trajectory of the discharge falls at the downstream end of the stilling basin floor. The stilling basin floor is 7 feet lower than the tunnel invert, which provides a sufficient depth of water to form a hydraulic jump when the tunnel is discharging. A short outlet channel connects the stilling basin to the natural stream channel.

18. Outlet Pipe . - A 22-inch-diameter steel outlet pipe is provided for diverting small quantities of water, as discussed in section 14. This pipe is concreted in the tunnel lining under the invert downstream from the gate chamber to the valve house. The discharge is controlled by an 18-inch pivot valve which is bolted to the steel flange of the pipe in the valve house at elevation 933. 77. For practical reasons the pipe was made one-fourth inch thick for its entire length. To minimize cavitation pipe bends were designed with large radii. The pipe was shop fabricated in the erection length as shown on the drawings with butt straps and flanges mounted. The field girth joints are of the be 11-and-spigot type and so designed that all field welding can be done from the outside of the pipe. All shop joints in the pipe were butt welded. Steel plates were required to have a minimum tensile strength of 50, 000 pounds per square inch, 20 percent minimum elongation in 2 inches, and a maximum carbon content of 0. 30 percent, and to be suitable for fusion welding. Flanges were the standard 150-pound design, forged steel, plain faced, and drilled to the American Standards Association 125-pound template.

19. Design Assumptions. - Assumptions used for designing the different outlet works structures were as follows:

(1) The trashrack structure was designed to resist a load, caused by a partial clogging of the trashracks, in an amount of 40 feet of hydrostatic head from the outside.

(2) The transition section downstream from the stoplogs was designed to withstand pressures at the maximum reservoir water level. The allowable tensile stress in the reinforcement was 20, 000 pounds per square inch.

(3) The thickness of the tunnel lining was determined by a rule of thumb providing 1 inch of thickness for each foot of internal diameter. In this instance slightly more thickness was provided. The tunnel lining was reinforced for a length of 120 feet upstream from the gate chamber. At the upstream end the hoop reinforcement and longitudinal reinforcement consisted of 1-inch-round bars spaced at 15 inches and 3/4-inch-round bars spaced at 19 inches, respectively. The amount of hoop reinforcement was increased in the downstream direction, while the longitudinal reinforcement remained constant. The spacing of the hoop reinforcement bars was uniformly decreased in steps from 15 inches to 6 inches at the end of the reinforced length of the tunnel lining. Also the tunnel was reinforced for a distance of 30 feet near the inlet and outlet portals. One-inch-round bars spaced at 12 inches and 3/4inch-round bars spaced at 12 inches were used for the hoop reinforcement and longitudinal reinforcement, respectively.

(4) The gate chamber and shaft were designed to withstand outside hydrostatic pressures at the maximum reservoir water level. The gate chamber plug was reinforced to resist water loads equivalent to 113 feet of head. An allowable stress of 13, 000 pounds per square inch was used for the reinforcement around the gate frame, lining, and transition. Immediately upstream from the gate chamber, the allowable stress in the reinforcement was 18,000 pounds per square inch.

(5) The valve house roof was designed to carry the dead weight and a snow load of 30 pounds per square foot. A spacing of 12 inches for the 1-inch-round bars was used in the valve house walls for the transverse reinforcement as well as the longitudinal reinforcement.

(6) The channel lining walls and floor were reinforced with two layers of reinforcement bars. A spacing of 12 inches was used for these bars. Each layer provided for transverse and longitudinal reinforcement. In the walls the transverse reinforcement was provided by 5/8-inch-round bars and 3/4-inch-round bars placed in the near and far face layers, respectively. The longitudinal reinforcement consisted of 5/8-inch-round bars placed in both the near and far face layers. The left wall of the channel lining was anchored to the rock with six 1-inch anchor bars. In the floor 3/4-inch-round bars were used in the top and bottom face layers for the transverse reinforcement. For the longitudinal reinforcement 3/4-inchround and 5/8-inch-round bars were used in the top and bottom face layers, respectively.

D. Modifications

20. General. - During construction it became necessary to change the requirements of the specifications and alter plans because of the revised flood hydrograph and because the excavated rock from the spillway would not meet riprap size requirements. Order for changes No. 1 (sec. 23) issued June 20, 1947, directed that the crest elevation of the dam and the line and grade of the spillway be changed from the original drawings and specifications. This change in grade reduced the quantity of suitable rock for dam construction. Accordingly, three sites, designated borrow areas 4, 5, and 6 (sec. 10 and fig. 4), were located that contained suitable rock and were within an economic haul distance. A price per cubic yard for stripping these borrow areas and a price for excavating the rock and transporting it to the dam were stipulated in the order for changes.

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CHAPTER m—DESIGN

Dam

11. Embankment. - O'Sullivan Dam, called Potholes Dam until the name was officially changed on September 28, 1948, is a zoned earth-fill structure with a length

of approximately 19,000 feet and a height ranging up to 160 feet above streambed (fig. 6). The crest elevation is 1061.0. The total embankment volume including riprap and gravel blanket approximates 8, 770, 000 cubic yards of material. The dam is protected by an open-channel spillway 500 feet wide, excavated in rock with a concrete crest. A 30-foot gravel roadway is constructed along the crest of the dam embankment and through adjoining rock cuts. That portion of the roadway which extends across the concrete spillway crest was designed for use as both a spillway and road.

The dam embankment was designed as a zoned-earth fill consisting of three zones. The central impervious zone (zone 1) is made up of selected sand, silt, and clay, spread and compacted by rolling to 6-inch layers. The slopes of this zone were allowed to vary somewhat but in general are 1-1/4 to 1 upstream and 3/4 to 1 downstream. Zone 2 is a transition zone of selected silt, sand, and gravel placed to serve as an inverted filter between the impervious inner zone and the pervious outer zone. Zone 2 slopes are in the order of 1-1/2 to 1 upstream and 1 to 1 downstream. The upstream slope of the pervious zone 3 consisting of selected sand and gravel is 3 to 1 between elevations 960 and 1047.5, 4 to 1 below elevation 960, and 2-1/2 to 1 above elevation 1047. 5. The downstream slope of zone 3 is 2 to 1 down to elevation 960, 4 to 1 between elevations 960 and 930, and 2 to 1 below elevation 930. Section 34 (d) discusses modifications made in top portion of the dam.

Protection for the slopes of the dam is provided by a blanket of rock 2 feet thick on the downstream face of the dam and 3 feet thick on the upstream face. The thickness was specified as the depth normal to the slope. The rock was placed against the zone 3 material and was required to follow the same slope as the outer slope of zone 3.

12. Cutoff Trench. - A cutoff trench in the foundation of the dam was used to act as a barrier to the flow of water through original ground under the dam. The trench was designed with a maximum width of 50 feet and with side slopes of 1-1/2 to 1. When foundation conditions became known, the resident engineer prescribed the exact location and the variation in bottom width with elevation. The bottom width was set at 15 feet from elevation 1060 (1 foot below the crest of the dam) to elevation 1045; from elevation 1045 to elevation 1010 the trench was widened 1 foot for each foot that the bottom of the trench was below elevation 1045; and below elevation 1010 the trench was 50 feet wide. It was also established that the top of the 1-1/2 to 1 slope of the upstream side of the trench and the upstream toe of the zone 1 material would be located the same distance at any one point from the dam axis.

Originally a concrete grout cap 3 feet wide and 3 feet in minimum depth below the surface along the centerline of the cutoff trench was designed to facilitate grouting. However, the character of the rock in the bottom of the trench was such that a grout cap was required only at each end of the closure section, where there was a layer of spongy interflow material. The grout caps were placed across these areas and keyed into good rock at the ends of each grout cap. Grouting of the rock foundation under pressure to insure against leakage was required as shown on the drawings or as directed by the contracting officer.

13. Crest Details. - The freeboard for the dam was established at 6 feet above maximum water surface elevation 1055.0, giving a crest elevation of 1061.0 as discussed in section 14. Camber was provided along the crest to ensure that the freeboard would not be diminished by foundation settlement or embankment consolidation. The selection of the amount of camber was somewhat arbitrary but also was based on the results of laboratory tests which were made to determine the amount of foundation settlement and embankment consolidation. The camber diagram is shown in figure 7.

The roadway constructed on the crest of the dam consisted of 6 inches of selected road surfacing material placed on top of 1 2 inches of selected gravelly material and meets local county road standards. Concrete guard rail posts conforming to state highway requirements were adopted to provide safe travel over the dam crest.

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Figure 6. --General plan and sections. (Sheet 1 of 2). --From Drawing No. 222-D-10120

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