The thickness of lining of the 18-foot-diameter tunnel was set at 15 inches and 6 inches was allowed between the "A" and "B" lines for limits of excavation. This resulted in an average thickness of 18 inches, which agrees with the popular criterion of 1 inch thickness of lining for each foot of tunnel diameter.

Owing to severe climatic conditions, it was decided to reinforce the tunnel lining so that cracking due to temperature change would be held to a minimum. Reinforcement was arbitrarily set at 0.2 percent for transverse reinforcement bars and 0.1 percent for longitudinal reinforcement bars.

Three tiers of drainage holes were drilled in the upper portion of the tunnel at approximate elevations 5860, 5900, and 5940. Each tier consisted of three 3-inch-diameter drill holes, 50 feet in depth, radiating upstream on the centerline of the tunnel and 30° to each side of the centerline. These drainage holes are intended to relieve any hydrostatic pressure due to reservoir percolation.

The rock-enclosed tunnel portion of the spillway terminates at station 6+72.5, from which point a cut-and-cover portal structure extends for 104 feet. The upstream 72.5 feet of the structure has a circular arched top, 18 inches minimum thickness, which supports the road fill of the powerplant access road. The downstream 39 feet is the flip portion of the outlet structure having a 145-foot bucket radius. The flip bucket lip is at elevation 5610.34 which is 4.56 feet above the lowest invert elevation in the tunnel. The tunnel will, therefore, always be full of water to the above elevation and will have to be pumped out whenever inspection is desired.

The following design criteria were used:

(1) Arch barrel. --Obtain stresses from arch tables for:

a.

Weight of road fill, 125 pounds per square foot.

b. A 60-ton trailer hauling unit placed to give maximurn stress.

(2) Flip bucket. -- Use nominal reinforcement consisting of 1-inch-diameter reinforcement bars on 12-inch centers each way.

53. Intake Structure. Details of the intake structure are shown on figures 52, 53, and 54. Various sizes of gates were considered, with the final selection consisting of two 16.75- by 34-foot fixed-wheel gates. Radial gates were also considered. However, the fixed location of the roadway crossing would have forced the intake structure farther upstream, resulting in higher costs. The spillway crest, station 1+00, is at elevation 6006.0. There is a gradual transition from the rectangular section at the crest, including a center pier, to a 26.5-foot-diameter circular section at the tunnel portal at approximately station 1+75. At this point of tangency, the tunnel slopes 55° with the horizontal. The 55° slope was considered to be the most desirable angle for the inclined portion of the tunnel to facilitate removal of tunnel muck. The crest portion of the centerline of the spillway has a parabolic shape defined by the equation X2 = -105Y.

The left wall of the intake structure abuts the rock abutment, the lower portion of which is shale zone 9L (10 to 12 feet thick.) Three lines of 3-inch-diameter weep holes, four per line, have been provided to relieve any hydrostatic pressure that may occur in the vicinity of the shale zone. A split-sewer pipe drainage system has been placed on the foundation of the crest structure for uplift pressure relief.

The right wall of the intake structure is approximately 8 feet thick in the area adjoining the arch dam. A 1/2-inch corkboard joint filler in the portion of the joint above the spillway crest prevents any transfer of stress from the arch dam to the spillway wall. The remainder of the wall between the downstream face of the dam and the transverse contraction joint through the spillway at station 1+50 is cantilevered from the thicker upstream portion. The wall portions of the intake structure downstream from station 1+50 to the tunnel portal is anchored to the rock with 1-3/8-inch-diameter anchor bars on 5-foot centers each way. A portion of the right wall without rock support was designed as a cantilever supported from the base.

The depth of the gate slots required an 8-foot-thick center pier which reduced rapidly in thickness downstream from the gate to 4 feet at station 1+15.75 and terminated with a

width of 27 inches at station 1+50. The upstream nose of the pier extends into the reservoir, beginning at the sharp crest at approximately elevation 6001, and projects upstream on about a 1/2 to 1 slope to elevation 6043. Model studies were responsible for this unusual shape which produced the smoothest entrance conditions through the crest structure. The following design criteria or procedures were established:

(1) Crest.-- Check the section from centerline of center pier to the joint between the spillway and the thrust block as a unit, using the following two loading conditions to determine foundation pressures under crest:

Drains operative:

Full reservoir head, H, at upstream face varying to 1/3 (H+20) at upstream end of drains thence to 10-foot head at downstream end, station 1+50.

Drains inoperative:

Full reservoir head, H, at upstream face varying uniformly to 10-foot
head at downstream end, station 1+50 (allowable stresses may be increased
by 1/3).

Nominal reinforcement to consist of 1-inch-diameter reinforcement bars on 12-inch centers each way.

(2) Center pier. -

Investigate the pier with one gate open- - unbalanced head.

Apply 10 percent earthquake (not in resonance zone--increase allowable
stresses by 1/3).

Check upstream portion of pier for vertical steel required to fasten stem to
base for total waterload.

Nominal reinforcement:

Upstream of station 1+10--1-3/8-inch-diameter bars on 12-inch centers,
vertical bars.

Downstream of station 1+10--1-inch-diameter bars on 12-inch centers,
vertical bars.

Horizontal bars-- 3/4-inch-diameter bars on 12-inch centers except use
1-inch-diameter bars on 6-inch centers for first 5 feet above crest.

(3) Left wall. --Design anchor bars for the following loading conditions:

Drains operative:

Full reservoir head, H, upstream varying to 1/3 (H+20) at upstream drain thence to 10-foot head downstream.

Drains inoperative:

Full reservoir head, H, upstream varying to 10-foot head downstream (allowable stresses may be increased by 1/3.)