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15 feet was used between the foot of cut slopes and the top rim of the basin, and a berm width of 20 feet was used beyond the top rim of the basin where there was fill. Compaction of the fill was required between the inside slope of the basin and for the full width of the berm, then down to original ground surface on a 1-1/2 to 1 slope.

The concrete lining for the surge basin was specified to be 4-1/2 inches in thickness and reinforced in both directions with bands of 3/8-inch-diameter round bars. The lining was grooved into panels for additional control of cracking, and placement was required to be on a sand and gravel cushion of 6-inch minimum thickness. An underdrainage system consisting of perforated draintile surrounded by a graded gravel filter was provided under the entire floor area of the basin because of the presence of springs in the vicinity.

257. Penstock Valve Structure. The penstock valve structure, shown on figure 212, is located at the outlet of Tunnel 2 just below the division from tunnel section into the penstocks. The structure provides housing for the butterfly valves, air valves, tunnel drain valves, and control equipment for the butterfly valves. The structure is accessible by road for any equipment that will be required in the maintenance of the valve equipment.

The structural design loadings for the penstock valve structure are as follows:

Roof slab of control house--Dead load plus a live load of 30 pounds per square foot.
Roof slab of valve house around hatch opening--Dead load plus 200 pounds per square
foot, plus concentrated load around opening due to weight of hatch.
Floor of control house--Dead load plus a live load of 100 pounds per square foot.
Sidewalls of penstock valve structure and retaining wall--Equivalent fluid pressure
due to material with a unit weight of 30 pounds per cubic foot. (Surface drain open-
ings should be kept clear so that surface water will be drained off.)

Valve support pedestals--Dead load plus live load exerted by valve during operation.

2. Structural Behavior Instrumentation in Tunnel

258. Instrumentation. Structural behavior instruments were installed at seven locations in the two Spring Creek tunnels. At stations 93+00, 93+50, 126+25, 127+50, 167+00, and 169+00, three strain meters and two pore-pressure meters were attached to the plate liner and embedded in the concrete (table 5). At the surge tank, 12 joint meters were installed.

The strain meters are used to indicate the stress in the steelplate liner. Each end flange of a strain meter is rigidly secured to a mounting bracket welded to the plate liner (fig. 213). Any strain that occurs in the plate liner is also experienced by the meter. Stress is then calculated from the strain indicated by the meter. The pore-pressure meters, located near the strain meters just outside the plate liner, indicate the hydrostatic pressure at that location.

The 12 joint meters in Spring Creek surge tank measure the separation that occurs between the rock excavation and the concrete lining. This separation may be produced by concrete shrinkage, temperature, or other causes. Four meters are located around the periphery of the tank at each of three elevations, 1090, 1130, and 1170.

All meters installed in the Spring Creek installations are of the Carlson unbonded resistance wire type. Signals from all the meters are carried through insulated cables to a reading station on the ground surface. Changes in electrical resistance of wire coils mounted within the meters are then measured and converted to indications of deformation, pore pressure, and temperature.

3. Tunnel Liner

259. Requirement. Tunnel 1 of Spring Creek Power Conduit is steel lined from station 92+30.02 to the downstream portal at station 94+57.00. Tunnel 2 of the conduit is lined from the inlet portal at station 125+50.00 to station 127+64.99 and from station 158+40.00 to the downstream portal at station 170+00.00. An additional 5-foot length of steel pipe, extending beyond the one upstream and each of the two downstream portals, is classed as steel liner. The steel liner was furnished and installed under specifications No. DC-5294.

260. General Description. The steel liners have an inside diameter of 17 feet. The steel liner for the outlet portal of Tunnel 1 is 232 feet long and has a plate thickness of three-fourths inch. The steel liner for the inlet portal of Tunnel 2 is 220 feet long and has a plate thickness of three-fourths inch. The steel liner for the outlet portal of Tunnel 2 is 1, 165 feet 6 inches long and has a plate thickness of 1-1/16 inches at the upstream end which increases to a thickness of 1-7/16 inches at the downstream end. Outside circular stiffener rings 8 by 1-1/4 inches are spaced at 8-foot intervals along each of the tunnel steel liners. Three 1-1/2-inch grout pipe connections are provided at prescribed locations in the top of each of the tunnel steel liners. Two 6-inch pump connections and a hemispherical test head at the outlet portal of Tunnel 1 and a hemispherical test head at the inlet portal of Tunnel 2 are provided for field testing of the monolithic-concrete pipe for watertightness. Four piezometer connections and manifold pipe and fittings are provided at the downstream end of the steel liner for the outlet portal of Tunnel 1 for performance testing.

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Figure 212. --Spring Creek Powerplant penstock valve structure--Plan and sections.

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Table 5.--Summary of instrumentation in Spring Creek Power Conduit Tunnels 1 and 2

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261. Design. The steel liners for the outlet portal of Tunnel 1 and the inlet portal of Tunnel 2 were designed for a maximum total head of 235 and 245 feet, respectively, measured at the downstream ends. The maximum total head of the steel liner for the outlet portal of Tunnel 2 was 472 feet at the downstream end and 340 feet at the upstream end. These heads include the calculated upsurge due to water hammer. The water hammer was computed using a turbine wicket gate closure of 10 seconds from the full-open position. External pressure on the steel liners, based on grout pressure, was 30 pounds per square inch.

262. Materials and Working Stress. Steelplates used in the fabrication of the steel liners for Tunnels 1 and 2 conform to ASTM Designation A 201, grade B, firebox quality. The entire length of each longitudinal and girth seam was fully radiographed in accordance with the ASME Pressure Vessel Code. As prescribed by that code, a maximum allowable joint efficiency of 100 percent and a maximum allowable working stress of 15, 000 pounds per square inch were used in the design computations.

263. Cleaning and Painting. The interior surface of the tunnel steel liners was coated with coal-tar primer and coal-tar enamel. The tunnel steel liners were cleaned and the materials applied in accordance with the applicable sections of the American Water Works Association Standard Specifications AWWA C203-57. Painting was not required for the exterior surface of the tunnel steel liners.

4. Steel Penstocks

264. General Description. The general alinement and profile of the penstock system is shown on figure 214. A plan and profile of the penstocks are shown on figures 215, 216, and 217.

A wye branch at the outlet portal of Tunnel 2 branches from the 17-foot-inside-diameter steel tunnel liner into two 11-foot 6-inch inside-diameter penstocks. These penstocks extend downstream approximately 1, 275 feet to the Spring Creek Powerplant. The wye branch and all penstock bends are secured by embedment in concrete anchors.

The penstock valve structure just downstream from the wye branch houses two 156-inch butterfly valves with bypass piping and eight 10-inch combination air-inlet and air-release valves. The butterfly valves are fully open during normal operation and serve as an emergency or normal shutoff. They are closed during filling operations.

Each of the air valves is protected by a 10-inch gate valve which can be closed to allow maintenance work on the air valves. The air valves release air during filling and admit air during unwatering of the penstocks.

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Figure 214. --Spring Creek Powerplant steel penstocks--General alinement and profile.

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