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the reservoir (sec. 8). The outlet discharge at maximum reservoir level, elevation 10,041.0, is about 875 cubic feet per second (fig. 6).
The outlet works, as shown in figure 7, includes a low bear-cage trashrack structure which is set out into the reservoir away from a rock slide and talus slope area. The outlet conduit extends from the trashrack to the inlet tunnel portal and has an inside diameter of 6 feet 6 inches, the same as the tunnel to which it connects. At station 7+24 the tunnel connects with the gate chamber which houses a 4- by 5-foot high-pressure emergency gate. The emergency gate connects to a 56-inch-diameter steel pipe which extends to the discharge valves. The steel pipe is protected by an 8-foot 6-inch horseshoe tunnel which also provides access to the gate chamber from the valve house. The 56-inch steel pipe bifurcates into 48-inch sections at station 11+37 and terminates at two 48-inch butterfly valves which regulate the flow into the existing river channel.
Various studies 'made during the outlet works design include the following:
(1) The possible use of a conduit rather than a tunnel.
(2) The use of jet-flow gates for regulation at the gate chamber and a free-flow downstream tunnel.
(3); Hollow-jet valves for regulation. (4): The possibility of diverting through the installed emergency gate. Some aspects and conclusions of each of the above considerations are listed below:
(a) Consideration was given early to the possibility of using a conduit rather than a tunnel; in fact, the field data included a statement that a conduit should be used. Layouts were made with a conduit on each abutment near the river channel. It was impossible to get a suitable alinement without excessive cuts near the downstream toe of the dam. A comparative estimate showed that the conduit would cost about $50,000 more than the adopted tunnel layout. Dam foundation stripping later revealed a poor foundation condition in the vicinity of the proposed conduit alinement. If the conduit layout had been adopted the construction complexity and cost would have materially increased.
(b) The tunnel was excavated with little difficulty from one heading at the downstream portal. Steel liner plates were not required and a relatively few 6- by 6-inch H-20 structural steel supports were used. Some supports were placed at the portals and some through a fault zone section near the gate chamber.
(c) Information in the Denver office to the effect that access to a control house located near the toe of the dam would be very difficult made it desirable to consider an outlet gate which could be controlled from the crest of the dam.
(d) A jet-flow gate was suggested; in fact, a model study was made to observe the flow pattern and air requirements for the free-flow downstream tunnel. After considerable model testing and discussion of available time for design and delivery of the new jet-flow gate, it was decided to adopt a more conventional-type gate. Additional information indicated rather easy access to a valve house at the downstream toe of the dam, and the standard layout shown in specifications No. 2594 was adopted. It should be noted that the valve house access road crosses the spillway discharge channel and may require maintenance and repair following the infrequent use of the spillway.
(e) Just prior to issuance of the specifications it was decided to substitute the hollowjet regulation valves with butterfly valves. With the invert of the tunnel placed at streambed a certain amount of sand, mud, and debris may be carried into the outlet pipe and operating mechanism of the hollow-jet valves. A butterfly valve would not be damaged seriously by mud and debris collecting against it. The discharge flow pattern is somewhat erratic but believed to be satisfactory.
(f) When specification drawings were being prepared it was believed that the 4- by 5 foot high-pressure emergency gate would not be at the dam site when the contractor wished
SPILLWAY DISCHARGE IN THOUSANDS OF C.F. S.
* 56" Dio. pipe not installed
Figure 6. --Area-capacity-discharge curves for Platoro reservoir, spillway, and
outlets. From drawing 253-D-294
to place concrete in the gate chamber. Provision was therefore made to install the gates in "second-stage" concrete. However, the gates were available and were installed in the first-stage construction.
21. Hydraulic Design of Outlet Works. - Several schemes, trials, and computations were made to arrive at the outlet works layout finally adopted. Based on an outlet requirement of 600 cubic feet per second with reservoir water surface at elevation 9963.0, the principal head losses were computed as follows:
This loss checks closely the total 70-foot head available between reservoir elevation 9963.0 and the centerline of the outlet valve, elevation 9893.0. The ratio
was used to determine water surface elevations for various discharges. For very low heads the control is at the throat of the trashrack structure.
22. Structural Design of Outlet. - The trashrack structure (fig. 8) was designed for a 40-foot hydrostatic head and checked against a 24-inch ice slab loading. Working stresses for concrete were based on a 28-day strength of 3,000 pounds per square inch. The allowable steel stress for the conduit and upstream tunnel was 12,000 pounds per square inch; and for all other structures it was 20,000 pounds per square inch. The conduit just downstream from the trashrack was designed for an external load equal to the full reservoir head of 140 feet. The 6.5-foot-diameter pressure tunnel was designed for an internal load equal to the full head (140 feet) at the gate chamber and decreasing uniformly to 40 feet at the inlet portal, and for an external load decreasing from 140 feet at the inlet portal to zero at the gate chamber. Some elliptical hoop reinforcement was specified to utilize the steel to the best advantage. The downstream tunnel was not reinforced except for a 40-foot length adjacent to the gate chamber and a 40-foot length at the outlet portal.
The anchor block was designed for hydrostatic and dynamic forces and checked against sliding. The valve house floor was designed for an H 15-44 truck loading plus a 10 percent impact load. When the discharge valves are repaired or replaced, it is desirable and provision is made to load or unload them directly onto or from a truck in the valve house. The roof slab is designed for a live load of 100 pounds per square foot and a dead load of 125 pounds per square foot.
Three separate designs of a retaining wall adjacent to the valve house were made. A design based on field data available in the Denver office was made. A suggestion for realinement was made by the construction engineer, who submitted additional information. Later, when the redesign was reviewed by the construction engineer, it was agreed to compromise the two previous layouts and a third design was made. Design data for the final design are as follows: