« PreviousContinue »
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 (fig. 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 bas in 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 bell-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.
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.
Figure 10. --Gate chamber, shaft, and trashrack structure concrete outlines. (Sheet 1 of 2). -- From Drawings Nos, 222-D-10953 and 10397
Figure 10.--Gate chamber, shaft, and trashrack structure concrete outlines. (Sheet 2 of 2). --From Drawings Nos. 222-D-10953 and 10397