Figure 134. --Clear Creek Power Conduit--Tunnel liner.

4-10-80

ADDED BOʻLINER TO UPSTREAM ENG

UNITED STATE

considering assistance from the rock. It was also designed to resist buckling from external waterload. The steel liner was fabricated from grade B plate, firebox quality in accordance with ASTM Designation A 201. Both shop- and field-welded joints in the plate liner were specified to be double-welded butt joints. Stress relieving was not required. Details for the installation and grouting behind the plate liner are shown on figure 133, and details of the steel liner are shown on figure 134.

(e) Outlet. --At the outlet of the tunnel it was planned that the conduit would be bifurcated into two penstocks that would carry the water to the Clear Creek Powerplant. As the penstocks were to be constructed under another contract, it was necessary to make provision in the tunnel contract for excavation of a portal cut that would accommodate installation of the branching pipes and penstocks. The base width of the portal cut for this purpose was made 32 feet. Cut slopes for the open excavation at the portal were set at three-fourths horizontal to one vertical. The height of the portal cut from top of cut to invert elevation was 75 feet.

(f) Disposal Areas. --The Government specifications provided four areas for the disposal of materials excavated from the tunnel (figs. 138 and 139). Any one of the four areas was capable of holding all of the muck from the tunnel. The disposal areas were located, respectively, near the inlet and outlet of the tunnel, near draw 1906, and near Crystal Creek, the two latter locations being along the tunnel alinement where construction adits or shafts might be constructed.

172. Surge Tank. (a) Hydraulic Design. --The following data and/or assumptions were used in the design of the surge tank (figs. 140 and 141):

Maximum forebay water surface elevation (Lewiston Reservoir), 1902
Minimum forebay water surface elevation (Lewiston Reservoir), 1898
Maximum tailwater elevation (Whiskeytown Reservoir), 1210

Minimum tailwater elevation, 1190 (weir controlled)

Length of tunnel (forebay to surge tank), 56,000 feet

Diameter of tunnel (forebay to surge tank), 17 feet 6 inches concrete lined

Plant capacity, 130 megawatts (two units)

Distance from surge tank to penstock valve structure, about 700 feet.

(Note: Tunnel reduces to a diameter of 15 feet 8 inches 385 feet upstream of valve

structure; bifurcation to two 11-foot 6-inch diameter conduits occurs just upstream
of valve structure.)

Length of penstocks, about 2, 100 feet

Penstocks: Two at 10-foot 6-inch diameter beginning downstream of the butterfly valves
in the penstock valve structure

Pressure relief valves: None to be provided

Maximum flow through tunnel, 3,200 second-feet2/

Requirement for "Load On": Pickup of second half of load within 4 seconds and hold undi-
minished throughout the surge. (Will accommodate pickup from zero to full load on both
units in about 5 minutes.)

Requirement for "Load Off": Rejection of full load both units in a period of 4 seconds
Maximum total head loss (losses + hy) assumed between forebay and surge tank for down--
surge for flow of 3, 200 second-feet: 130 feet

Minimum total head loss (losses + hy) assumed between forebay and surge tank for upsurge
for 3, 200 second-feet: 75 feet

Because the tunnel traversed through reaches of low cover, it was advisable to keep the pressure head on the tunnel to a minimum. Preliminary, comparative estimates indicated that construction costs for a differential surge tank with an overflow basin would be approximately the same as for a restricted orifice tank. The lower differential tank with a surge basin would decrease the pressure head on the entire system, which would result in a more secure installation, particularly in reaches of the tunnel where cover was low. Because of the length of tunnel, the same results could not have been obtained by use of a restricted orifice type of tank. The quantity of reinforcement required for the tunnel would also be less, as would the quantity of steel for the penstock lines. Consequently, the design of the surge tank proceeded on the basis of constructing a differential tank with a surge basin.

The invert elevation in the surge basin (1902. 50) was set 0. 5-foot higher than the maximum water surface in Lewiston Reservoir (1902.0). This will permit the basin to be empty at all times except for periods of upsurge. Suitable surface topography in the vicinity where the surge basin could be located was limited. It was desired to limit the maximum fill section for a basin to 30 feet if this were found to be compatible with surge requirements. Preliminary surge studies indicated this could be done reasonably. Preliminary estimates also indicated that an increase of 1 foot of head on the tunnel would result in a requirement of spending an additional $7,000 to $8,000 for reinforcement. The size of tank was made 44 feet in diameter and the size of riser was made 16 feet in diameter. These diameters provide adequately for hydraulic stability.

The optimum throttling for flow from the riser ports into the tank for upsurge was not determined. For the purpose of satisfactorily controlling pressure rise in the tunnel upstream of the surge tank, the 2/Subsequent to completion of designs for the tank, the rated Clear Creek Powerplant capacity was increased to 134 megawatts. This would indicate a maximum flow of about 3, 680 second-feet.