the intake with the tunnel portal (fig. 131). A reinforced-concrete bridge provides access from the shore to the intake.

Fishscreens were provided at the intake structure to protect the game fish in the reservoir. The low velocity required through the screens made it necessary to use a large area of screen. The necessary area was furnished by a circular arrangement about a morning-glory-type spillway which permits full peripheral flow into the intake and results in an economical structure with uniform flow through the fishscreens.

The intake structure has 20 sides or bays, each provided with 2 fishscreens. Inside the 20 bays are 10 motor-operated cast-iron slide gates; each slide gate controls the flow through 2 bays. The fishscreens on any two bays may be removed, cleaned, and maintained when their corresponding gate is closed.

The screen cloth is No. 2-1/2 mesh with No. 12-gage wire. For the design flow of 3, 200 secondfeet, the average velocity through the gross opening at the fishscreens will be 1 foot per second. The distance between the fishscreens and the gate is approximately 18 feet. The water will be accelerated to an average velocity of 8.9 feet per second in this 18-foot length. A minimum submergence of 5 feet is provided at the top of the fishscreens when the reservoir water surface is at minimum operating elevation 1898.0.

Facilities for washing the screens consist of a rotating gantry crane equipped with a grappling device to lift the screen to the top of the guides where it can be manually washed with a portable hose and nozzle connected to an electrically operated pump. The pump is housed below the operating deck, and it is capable of delivering 50 gallons per minute at a pressure of 100 pounds per square inch.

The section of cut-and-cover conduit is 17 feet 6 inches in diameter and has a shell thickness of 2 feet 2 inches. The pipe was designed to withstand the external earth and water loads with the pipe empty and the reservoir at maximum operating water surface. A shaft (fig. 131) is provided in the top of the conduit to give access to the tunnel for inspection and maintenance purposes. In order to locate the tunnel construction favorably with respect to flood stages in the river, a vertical reverse bend was introduced in the cut-and-cover conduit between the intake structure and the tunnel. This permits use of the morning-glory type of intake structure.

The access bridge has a width of 12 feet between curbs and an overall span length of 66 feet. The design for the bridge as well as the deck and supporting members of the intake structure was based on an

H-20 loading without impact in accordance with "Standard Specifications for Highway Bridges," of the American Association of State Officials, 1953 edition.

170. Hydraulic Properties. Hydraulic properties for the 17-foot 6-inch diameter tunnel, described in section 165, are as follows:

(a) Piezometer Taps. --Piezometer taps were designed to be installed at tunnel stations 8+50 and 558+85. These taps will be used when the tunnel is put into service to determine actual hydraulic losses in the tunnel. Details of the piezometer installation are shown on figure 132.

171. Tunnel Design. (a) Tunnel Section. --Consideration of the various alternative tunnel sections that might be constructed led to the selection of a circular concrete-lined section. The advantages for the circular tunnel section were that it would offer the greatest strength to external loads, could be easily reinforced against bursting where necessary, and would provide the most favorable hydraulic section. A 17-foot 6-inch diameter finished circular section was adequate to facilitate construction and, it was believed, would cause little more difficulty in excavating and cleaning up the bottom than a horseshoe-shaped tunnel.

Typical sections for the Clear Creek Tunnel specified a minimum thickness of concrete lining (A line thickness) of 9 inches. Payment line for excavation (B line) for the unsupported or rock-bolted section was specified to be 15 inches from the inside finished diameter of the tunnel. Details of these typical sections, as well as details for typical supported sections, are shown on figure 126.

Near the lower end of the tunnel where rock cover over the tunnel was low and internal head was high, special sections were specified to accommodate the installation of a heavy plate steel tunnel liner. Details of these special sections, including specific requirements for pressure grouting, are shown on figure 133, and details of the tunnel liner are shown on figure 134. Concrete for the tunnel lining was specified to develop a strength of 3,000 pounds per square inch.1/

(b) Lined Versus Unlined Tunnel. --Studies were made to determine the feasibility of constructing an unlined tunnel. To develop a 130-megawatt powerplant capacity it was determined that the actual diameter of an unlined tunnel would have to be 25 feet 4 inches, assuming an n value of 0.035 in Manning's formula.

Cost comparisons between an unlined tunnel of such diameter and a 17-foot 6-inch concrete-lined tunnel indicated that the lined tunnel would be somewhat less costly for this particular power conduit. Consideration was also given to a tunnel that would be lined throughout portions of its length and unlined for the remaining portions of length, but this type of design did not prove feasible from a planning or cost standpoint. In view of the disadvantage of higher estimated cost for the unlined tunnel and the uncertainties associated with maintaining an unlined tunnel, it was decided to construct a tunnel that would be fully lined throughout its length.

Figures 135 and 136 show various stages of tunnel construction.

(c) Tunnel Supports. --The amount of structural-steel rib support estimated to be required was determined after careful consideration of the surface geology and knowledge of the rock gained by test drilling and inspection of the core samples. Provision was also made in the specifications for the use of rock bolts for support of the tunnel excavation. No permanent timber supports were permitted in the tunnel, and all timber lagging except that essential to construction was specified to be removed prior to placement of concrete in the lining.

(d) Reinforcement of Tunnel Lining. --Some reaches of the tunnel lining were reinforced with circular steel hoops to resist the bursting pressure due to internal head. Usually these reaches involved locations where the rock cover over or about the tunnel was shallow, but there were also a few locations in badly faulted or weak rock where the lining was reinforced (fig. 137).

Where hoop reinforcement was used, the clear distance from the inside of the concrete lining to the hoop steel was 4 inches. Longitudinal reinforcement bars were placed on the inner side of the hoop bars for ease of placing. Hoop bars varied in size from 7/8 inch in diameter to nearly 2-3/8 inches in diameter depending upon the head and other conditions in the tunnel.

Extensive weathering of the rock in the vicinity of the outlet portal led to a decision to use a plate steel liner in the tunnel near the outlet. Maximum internal head in the tunnel at the outlet portal will be 250 feet, and the plate steel liner was extended into the tunnel from the portal for a distance of about 385 feet, where, at the point of termination, the ratio of internal head to cover over the tunnel was between. about 1.5 and 2.0. The plate steel liner was designed to resist the maximum internal bursting head without

1/Design considerations during construction raised the concrete strength to 5,000 pounds per square inch Tsee sec. 175).