Extensive core drilling and auger hole exploration was performed in the area of the basin. These explorations indicated that the rock was weathered quite deeply. The side slopes in the basin were made 1-1/2 to 1 to facilitate placing of the concrete lining and to provide for stability of slopes. The slopes for the excavated area above elevation 1930 were made 2 to 1 to provide stability in the weathered rock. Material forming the embankment on the downhill side was acquired from adjacent excavation, and compacted to 95 percent of the laboratory standard maximum soil density (dry).

The decision to pave the surge basin with reinforced concrete was made for several reasons: (1) The weathered condition of the rock in the basin area would make the slopes unstable when saturated if the basin was not paved, (2) the possibility of saturating the hillside around the basin would contribute to instability of the natural slopes if the basin was not paved, (3) extremely turbulent flows could occur in the basin during upsurge, and (4) backwashing of rocks and debris into the tunnel through an unlined basin would not be tolerable.

A compacted cushion of selected material was provided under the concrete lining to provide a uniform foundation. Drains were provided under the floor and side slopes of the lining to intercept any water which might leak through the lining or which might emanate from natural drainage from the hillside above the basin. Contraction joints in the concrete lining were placed at approximately 22-foot centers, and were provided with rubber water stops to prevent leakage through the joints. Reinforcement bars used in lining of the basin were arranged in groups composed of two 3/8-inch-diameter bars per group. Groups of bars were spaced at 12-inch centers both ways. The basis for this arrangement of reinforcement is given in Technical Memorandum No. 601.3/

A 10-foot-high chain link fence was provided around the entire basin for safety reasons. It will also prevent windblown debris from entering the basin, and offer protection to wildlife.

(c) Structural Design--Tank and Riser. --The bottom of the 44-foot-diameter tank is at elevation 1712.03 and the top is set at elevation 1905.00 (fig. 143). The top of the 16-foot-diameter riser is at elevation 1920.00.

The tank is reinforced for its full height. To eliminate the fixity between the tank wall and floor, a sliding joint with rubber water stop was placed at this intersection. A minimum thickness of 18 inches of concrete was required in the tank wall. In view of the weathered condition of the rock at the surge tank location, it was assumed that steel rib supports would be required to support the excavation during construction. The specifications provided that steel ribs would be permitted inside the 18-inch minimum line, but could not be closer than 12 inches to the inside face of the concrete. The reinforcement is placed within this 12-inch thickness.

The top of the tank projects 2.5 feet above the surge basin floor. This was done to keep gravel wash or rocks from entering the tank and subsequently entering the turbines.

Drains were placed at the invert of the basin on the outside periphery of the tank. These drains will empty the last 2.5 feet of water from the basin into the tank.

Struts were utilized between the riser and the tank walls to strengthen the riser against earthquake loads, which were assumed to act at one-tenth of the gravitational acceleration.

173. Penstock Valve Structure. The penstock valve structure, shown on figure 156, is located at the outlet of the Clear Creek Tunnel 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 valves.

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 of valve house around hatch opening--Dead load plus a live load of 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.

Valve support pedestals --Dead load plus a live load due to operation of valve.

174. Time for Construction. The time allowed in the specifications for construction of the tunnel considered that two adits or points of access to the tunnel would be available to the contractor in addition to the inlet and outlet portals (fig. 125). One of these intermediate access points would involve a construction shaft at draw 1906; the other intermediate access point would involve a construction adit at Crystal Creek. The longest and controlling reach as far as excavation was concerned was the reach between the inlet portal and draw 1906. An assumption was made that the concrete lining operations would be carried out from one heading. By combining the amount of time estimated for mobilization, excavation, cleanup, and concrete lining, it was estimated that 1, 720 days would be needed for tunnel construction. This number of days permitted completion of the tunnel in sufficient time to utilize the programed water releases from Trinity Powerplant, and was therefore the number of days allowed in the specifications for construction of the Clear Creek Tunnel.

3/Zangar, C. N., and Brahtz, J. H. A., "Temperature Reinforcement of Continuous Concrete Slabs, " Technical Memorandum No. 601, Bureau of Reclamation, May 11, 1940 (unpublished).

OMITTED

4. Design Considerations During Construction

175. General. Design problems which arose during construction are discussed in the following paragraphs. See chapter X for a discussion of the actions resulting from these design considerations.

(a) Water. --The original designs for the tunnel had considered that ground waters would be encountered during construction, but a somewhat wider distribution than that which actually occurred had been anticipated. It had also been expected that the greatest flow concentrations might occur under the overlying drainage courses where the height of ground cover was not great. Instead of the conditions anticipated, most of the ground-water flow was encountered over a distance in the tunnel of about 1-1/2 miles in length, and under generally the greatest height of ground cover. Despite the concentrated volumes and potential pressure of the ground water throughout the wettest portions of the tunnel, it was decided to sustain a grouting program following placement of lining, that would insure contact between concrete and rock. See chapter X for the actual grouting performed. In order to permit development of a grout pressure of 500 pounds per square inch behind the lining, which it was believed would be required for successful grouting, the design strength of the concrete was increased from 3,000 to 5,000 pounds per square inch between tunnel stations 145 and 280. The greatest pressure of ground water which had been measured was 432 pounds per square inch.

As high-pressure grouting would make development of a continuous ground-water relieving drain behind the lining difficult, it was decided, as an additional safety factor and in lieu of continuous drains, to provide weep holes through the lining after high-pressure grouting was completed.

The general criteria advanced for determining number, location, and depth of weep holes were as follows:

Weep holes were to be provided wherever ground water exists and ground cover over the tunnel exceeds 400 feet; weep holes would be 1-1/2 or 2 inches in diameter. Maximum spacing of weep holes was not to exceed about 20 feet along the tunnel with additional holes at specific locations as required. A capacity of 10 gallons per minute was suggested for the 1-1/2-inch-diameter holes and a capacity of 20 gallons per minute for the 2-inch-diameter holes; the number of holes required was to be determined by dividing the maximum total inflow encountered at any location by these respective capacities, except that the maximum spacing of holes would be in any event 20 feet along the tunnel wherever the ground cover exceeded 400 feet and water had been encountered. It should be noted that these criteria were developed before the high-pressure grouting program had been carried out, and therefore could not take into account the changing patterns of ground-water flow into the tunnel which the application of grout would bring about.

As there was little possibility of water loss occurring from the tunnel into the ground-water zones adjacent to the tunnel, the basic and underlying purpose of the weep holes was to provide relief through the lining so that differential pressures across the lining would be low under any condition of tunnel operation.

(b) Concrete Finish on Tunnel Lining. --Specifications had provided for formed surfaces of tunnel lining to have an F2 finish and for unformed surfaces to have a U2 finish. A careful review of the tunnel hydraulics indicated that a finish superior to the U2 would be justified. The specifications requirement for finish of the unformed portion of the tunnel lining was therefore modified to include the application of steel trowels following the U2 wood float finish. The finish to be secured, however, was not specified to equal the U3 or hard steel trowel finish.

(c) Reinforcement of Tunnel Lining. --Conditions which developed after excavation of the tunnel had been made required a review of the reinforcement requirements which had been furnished on the specifications drawings. Specifications generally had shown reinforcement in the vicinity of the portals and at other points where ground cover over the tunnel was low. Also, provisions had been made in the specifications for placing reinforcement at undesignated locations in the tunnel where this might be considered necessary.

Following excavation of the tunnel, field inspections were required to review and determine finally the requirements for reinforcement bars in the concrete lining. Although results of the detailed inspections are contained in the field trip reports, and are otherwise given on the as-built drawings for the tunnel, the following criteria are those which were generally followed:

(1) Circular reinforcement hoops were required wherever the ratio of ground cover over the tunnel to the internal bursting head was less than 1.0.

(2) Reinforcement was required in heavily timbered reaches where the amount of rock exposure was not great enough to permit concrete to rock contact over at least one-half of the area.

(3) Reinforcement bars were required in wet reaches of tunnel wherever the development of adequate rock support for the lining could not be assured by grouting.

(4) Reinforcement bars were required in reaches of the tunnel where the rock quality indicated inadequate support for the tunnel lining.

The types, size, and spacing of reinforcement bars varied depending upon the purpose, location, and sometimes the internal pressure head in the tunnel. Some details of the reinforcement and locations at which it was provided are shown on figure 133.

(d) Concrete Cores. --Plans were made for the removal of 6-inch-diameter concrete cores from the tunnel lining. Results of taking and testing the cores were published in a laboratory report. 4/

(e) Closure of Construction Adit. --The specifications for the tunnel required that any adits or shafts used by the contractor for construction purposes were to be closed at the direct expense of the contractor, and according to designs approved by the contracting officer. The details of the closure as finally approved and constructed are shown on drawing No. 416-229-3543.*

5. Structural Behavior Instrumentation in Tunnel

176. Instrumentation. Structural behavior instruments were installed at four stations in Clear Creek Tunnel. At each location, indications of stress in the reinforcement steel, hydrostatic pressure outside the concrete lining, and temperature are measured. Table 4 is a summary of instrumentation in the tunnel. Identical installations were made at stations 462+36 and 563+50. The equipment at each station consists of two pore-pressure meters located to indicate the hydrostatic pressure at the rock-concrete contact and three reinforcement meters to indicate stress in the 2-1/4-inch-diameter circumferential bars. Figure 144 shows two reinforcement meters installed in the tunnel. Indications of temperature are also obtained from each of the five meters.

Three reinforcement meters, two pore-pressure meters, and two temperature meters were installed at station 370+30 and at station 378+20. Although the same measured values are obtained, operation of these instruments differs from that of the meters at the two downstream stations. The sensing element of the meters installed at stations 462+36 and 563+50 is the Carlson unbonded resistance wire gage while the transducer for the meters embedded at stations 370+30 and 378+20 is the vibrating wire gage. The Carlson strain meter operates on the principle that the electrical resistance of a coil of wire located within the meter is proportional to the stress applied to the wire. Any strain experienced by the reinforcement bar is transferred to the meter, and a measurable change in the resistance of the coil occurs. For the vibrating wire meter, a single wire within the meter is subjected to the same strain that occurs in the reinforcement bar. The change in stress of the wire produces a measurable change in the natural frequency of vibration of the wire. This installation is the first time the Bureau of Reclamation has used vibrating wire instruments to obtain structural behavior measurements.

Signals from either type of instrument are carried through insulated cables to a reading station on the ground surface. (See fig. 145.)

6. Tunnel Liner

177. Requirement. The Clear Creek Tunnel is lined with steel pipe from station 564+50.52 to its downstream portal at station 568+30.49. An additional 5-foot length of steel pipe, extending beyond the portal, is also classed as tunnel liner. The tunnel liner was furnished and installed under specifications No. DC4802, and fabrication details are shown on figure 134.

178. General Description. The tunnel liner (fig. 146) has an inside diameter of 15 feet 8 inches, and a total length of about 385 feet. The upstream portion of the liner, about 338 feet in length, has a plate thickness of seven-eighths inch; the plate thickness for the remainder of the liner is 1 inch. Outside circular stiffener rings, 8 by 1-1/4 inches, are spaced at 8-foot intervals along the liner. Three 1-1/2-inch grout pipe connections have been provided in the top of the liner at each of eight prescribed locations.

179. Hydraulic Design Data. The tunnel liner was designed for a maximum total head of 334 feet, measured at the downstream end. The maximum total head at the upstream end of the tunnel liner was 290 feet. These heads include the calculated upsurge due to water hammer, which was computed using a turbine wicket gate closure of 10 seconds from the full open position. External pressure on the liner, assuming saturation to ground surface, was estimated as 45 pounds per square inch.

180. Materials. Steelplate selected for the tunnel liner conforms with ASTM Designation A 201, grade B, firebox quality. This is a carbon-silicon plate of intermediate tensile range for fusion-welded boilers and other pressure vessels.

181. Design. Design of the liner is in accordance with applicable requirements of section W of the 1951 edition of the API-ASME Code for the Design, Construction, Inspection, and Repair of Unfired Pressure Vessels for Petroleum Liquids and Gases. All longitudinal and girth joints in the shell were radiographed, but radiographs were not required for fillet welds or butt welds in the stiffener rings. Thermal stress-relieving and hydrostatic testing was not required. The basic design stress was 15, 000 pounds per square inch, and the joint efficiency was 90 percent, giving an allowable stress of 13, 500 pounds per square inch. Stiffener rings were designed to provide support for the pipe resting on the ground and against a collapsing pressure of 45 pounds per square inch.

182. Cleaning and Painting. The interior surface of the tunnel liner was coated with coal-tar primer and coal-tar enamel. The tunnel liner was cleaned and the materials applied in accordance with the

4/Hoagland, G. G., "Evaluation of Concrete Lining for Clear Creek Power Tunnel, " Concrete Laboratory Report No. C-1125, Bureau of Reclamation, October 12, 1964 (unpublished).

*Not included.