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were constructed, one each between stations 5+00 and 5+34, 5+37 and 5+74, 5+88 and 6+93, and 6+39 and 6+60. Completed by January 1961, the buttresses averaged about 30 feet in thickness.

214. CONCRETE. (a) Mixing and Placing Methods.-Concrete for the power plant area was manufactured in the central batching plant, located on the elevation 3540 bench, excavated out of the right canyon wall upstream of the dam and approximately 700 feet upstream of the powerplant. The concrete was delivered to the construction area by high-line cableway using concrete buckets. The first concrete in the powerplant area was placed on May 5, 1960, for penstock support 2B, using one of the main 50-ton high-line cableway with a 12-cubic-yard concrete bucket. The 50-ton cableways were used in the powerplant area to place concrete until late November 1960 when the third high-line cableway, with 25-ton capacity, was placed in service. The remaining concrete placed in the powerplant area was placed using the third high-line cableway with an 8-cubic-yard bucket. The concrete in the downstream part of the tailrace slab and the river outlet area could not be reached using the highline. For these locations, the structural concrete was transferred to a hopper and was placed using a crane and a smaller bucket. Mass concrete for the river outlet was placed using a pumpcrete machine, located within the reach of the highline used to deliver concrete from the concrete batching plant. Concrete mixes varied from 6-inch maximum size aggregate for the mass concrete to 3/4-inch maximum size aggregate, depending upon the complexity of the particular placement and taking into account cover for embedded items, spacing of reinforcing steel, massiveness of the placement, and amount and spacing of the embedded materials. Generally, the typical sizes of aggregate used were as follows:

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Details of concrete materials, methods, and equipment used in the manufacturing, control, mix designs and compressive strengths are covered in appropriate sections of this publication. Additional information is presented in the following subsections.

(b) Forms.-Wooden forms were used for the mass concrete in the powerplant area until the mass concrete was of sufficient height above rock, then steel panel forms were used. Powerplant area structural forms, both exterior and interior finish surfaces, were made of 3/4-inch exterior plywood sheathing on 2- by 6-inch studs spaced on 16-inch centers and with double wales of 3- by 6-inch timbers on 16-inch to 3-foot centers.

Tongue-and-groove sheathing was required to be used for the exterior surfaces of the superstructure concrete on the south elevation of the machine shop, the west elevation of the powerplant, and the remaining surfaces of the cable and elevator tower, for architectural appearances. Forms were tied together with 1/2-inch steel rods and 3/4-inch she-bolts. Form tie spacing ranged from 16-inch centers to a maximum of about 3 feet, taking into account various factors such as size, shape, and depth of the placement. The contractor was quite generous in his use of form ties, resulting from having several form failures early in the concrete construction phase. All wood forms were constructed at the carpenter shop located on the east rim. The quality of the forms constructed at the carpenter shop was outstanding, resulting in excellent concrete surfaces. Exterior concrete forms were usually stripped within 24 hours after placement, with repairs being made as required and she-bolt holes being plugged shortly thereafter.

(c) Placement Schedule.-The first mass concrete in the powerplant substructure was placed on May 31, 1960, in the upstream section of unit bay 1 between the 5- and 3-lines to elevation 3105. Placement in unit bays 3 and 5 followed. Concrete placement methods and equipment closely followed the procedures used in the concrete construction for the dam, and the mass concrete in the powerplant structure was completed in December 1960 with a placement in unit bay 8.

Placement of the reinforced substructure concrete began in June 1960 and continued in the logical sequence of construction, followed by initiation of intermediate structure concrete construction in September 1960. By this time, the pattern of concrete placement was established in a routine sequence of unit bays 1, 3, 7, and 5, followed by unit bays 2, 4, 6, and

8. The mass concrete for the outlet works encasement downstream of the dam also became a part of the sequence placements between the dam concrete and the powerplant as the work progressed. During the early part of 1961, concrete placing in the powerplant averaged about 13,000 cubic yards per month. This average dropped to about 1,700 cubic yards per month during the last 6 months of the year, due partly to concentration of effort on placement in the dam and partly to the greater forming and reinforcing steel work requirements in the structural concrete of the powerplant. Concrete construction in the machine shop and service bay followed on an appropriate schedule. Intermediate structure concrete was completed in November 1961 with a placement in unit bay 8.

Placement of the superstructure concrete began in unit bay 1 in October 1961. In November 1961, the contractor was advised that the powerplant cable and elevator tower above elevation 3233.33 and unit bays 1 and 2 above elevation 3231.69 were being redesigned to provide fallout protection. No work was performed above those elevations until the details of the revisions were furnished, and all superstructure concrete was completed early in July 1962. The high bay windows in the powerplant unit bay outside walls were also deleted for additional fallout protection and security purposes.

(d) Cooling.-The powerplant unit bay mass concrete was placed in 5-foot lifts and was not cooled. The mass concrete for the service bay, machine shop, and river outlet was required to be cooled to 60° F. One-inch outside-diameter aluminum tubing on nominal 5-foot spacing was embedded in the mass concrete, and refrigerated water was circulated through the cooling coils at a rate of approximately 4 gallons per minute for a period of 12 days. Upon completion of placement of the mass concrete, the concrete temperature was lowered to the required 60° F. Thermocouples and insert thermometers placed in embedded tubing were used to control the progress of the cooling. In March of 1961, following completion of the required cooling, the contraction joint, cooling pipe, and thermometer tubing downstream of the m-line were grouted. The machine shop and service bay cooling pipe was grouted during June of 1961.

The service bay mass concrete cooling was accomplished using the chilled water supply system used for cooling the mass concrete in the dam. A small refrigeration plant with chiller and cooling tower was set up downstream of unit 8 near the m-line to cool the machine shop and river outlet mass concrete. This same plant setup was first used to cool the dam mass

concrete prior to placing the main refrigeration plant chilled water system in service.

(e) Curing.-Concrete curing was performed as required by the specifications. The exterior surfaces of the mass concrete and powerplant structure concrete were cured using gray membrane curing compound. Horizontal construction joints, floors, and stair treads were cured with water for at least 14 days. Finished floors were covered with plywood for protection during the construction phase; but even so, gouging and spalling of these surfaces required considerable repairs by the prime contractor. Interior surfaces of walls, columns, ceilings, soffits, vertical construction joints, and construction joints were cured by leaving the forms in place for at least 4 days. Mass concrete that contained pozzolan was required to be cured for a minimum of 21 days or until covered with concrete. Dry-packed she-bolt holes were cured by covering with masking tape.

(f) Tailrace Slab.-The contractor was instructed to construct a reinforced concrete slab (fig. 280) in the tailrace of the powerplant by order for changes No. 7, in lieu of riprap required by the specifications. The tailrace area was laid out in 17 sections, designated A through Q, with 10 slabs placed in sections A through H, and 9 slabs in sections I through Q. The tailrace slabs were about 30 feet square and were 8 inches in depth for a distance of 150 feet from the powerplant and 12 inches deep for the remaining length of the slab. Following dewatering and backfill compaction, the work was started in September 1961 in section A-1, downstream from unit bay 8, and was completed in April 1962. For placing sections farthest downstream which were beyond cableway reach, it was necessary to transfer the concrete bucket to a crane.

Placing of concrete in the powerplant was suspended on December 11, 1961, due to severe winter weather, and was not resumed until the middle of February 1962, when the weather moderated. Placement of the tailrace slab was resumed late in December, and portable wooden shelters shelters were constructed to protect the slab placement. Electrically driven forced-air heaters were used to assist in the protection against low temperatures. The tailrace slab collapsed 3 years later in April 1965, after the river outlets were operated for an extensive period (see section 57).

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Figure 280.-Concrete being placed in the tailrace in front of unit bays 7 and 8 of the powerplant. P557-420-6457, October 12, 1961.

subcontractor Judson Pacific-Murphy Corp., a subsidiary of Yuba Consolidated Industries, Inc.

The first shipments of steel consisting primarily of welded steel plate arrived at the jobsite in April and May 1959, amounting to some 2 million pounds. Further deliveries were deferred for about 6 months and an additional 2.82 million pounds of steel was shipped over a 3-month period ending in March 1960. Subsequently, a change in the structural steel requirements added 383,000 pounds and increased the structural steel erected into the powerplant to the final total of 5.2 million pounds.

Erection of steel began in unit bay 1 in June 1961 (fig. 281), and was completed in July. This work was then temporarily suspended until September for completion of intermediate concrete construction. A rubber-tired mobile crane was used to handle the steel members, which were field erected with bolted connections. The crane boom was equipped with a

boom extension for erection of the control and office area superstructure over unit bays 1 and 2. Work was resumed in September 1961, and the erection of the superstructure steel was completed in January 1962. Items of special interest on the superstructure include exposure of the outside steel column flanges with concrete exterior walls placed between the columns. Given the option to use precast or cast-in-place walls, the contractor elected to construct the walls in place. Also, cast-in-place roof slabs were used for the machine shop and service bay portions of the structure for protection from falling rock as these bays are adjacent to the canyon wall.

The roof of the unit bays, except unit bay 3, consisted of precast concrete decking, waterproofing membrane, and lightweight cover slabs. These precast concrete decking units were covered with a vapor barrier and a five-ply membrane, and the lightweight cover slabs were 3 inches in thickness. Some difficulties were experienced in protecting the membrane from

damage during installation of the cover slab and related roof work. Unit bay 3, which would also be a visitors' observation deck, was constructed with a 4-inch regular concrete deck slab, placed on the structural steel, and a five-ply membrane and 3-inch-thick cover slab of regular concrete.

In an amendatory agreement dated May 7, 1962, in order to insure readiness of the powerplant for work under the completion contract, the contractor agreed to complete the

walls, floors, structural roof slabs, and all interior installations for unit bays 1, 2, and 3 of the powerplant structure by June 15, 1962; and the same work in unit bays 4 through 8, the service bay and the machine shop bay of the powerplant structure, and the paving of the powerplant parking area by August 15, 1962. A s the completion date for all work in the powerplant was established by the specifications September 23, 1962, meeting the earlier completion date involved additional cost to the contractor for additional materials, labor, equipment and other expenses, and the contractor was reimbursed $128,000 for expediting this

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the powerplant and various other locations. This concrete work also included bonded concrete surfacing, the 4-inch floor surfacing, and 3-1/2-inch terrazzo underbed on the several floor surfaces in the powerplant and other structures. This concrete was batched at a plant in Page, Ariz., transported to the powerplant in transit mixers, and placed by conventional methods. There were several instances where the underbed in the generator floor and on the generator balcony was placed too high, despite

Figure 281.-View looking down on unit bays 1, 2, and 3 of the powerplant. Note steel erection in unit bay 1. P557-420-6105, June 22, 1961.

work. All work pertaining to unit bays 1, 2, and 3 was accepted on July 15, 1962; the remaining work under the amendatory agreement was substantially complete and accepted on August 15, 1962; and all remaining work involved in the powerplant construction was completed and accepted on September 23, 1962.

216. SECOND-STAGE CONCRETE. Concrete placed under the completion contract included blockout concrete, a control cable entry structure and certain lined sections, and miscellaneous concrete in

properly set screeds, and it was necessary for the contractor to bush these surfaces to allow for an adequate thickness in finish terrazzo. Owing to delays in scheduling the terrazzo work, these rough

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caused inconvenience and additional maintenance.

The completion contractor elected to

use prepacked methods for placing second-stage concrete about the embedded parts of turbines (fig. 282), which was allowed under an alternative provision of the specifications. This concrete was placed by a subcontractor, Lee Turzillo and Cleveland Cement Co., Inc., a joint venture.

As this method is not often used, the

aggregate placing and grouting methods utilized for this work are described below in greater detail than usual for second-stage concrete.

Prior to placing aggregate, the first-stage concrete was sandblasted initial cleanup performed. Reinforcing around the draft tube liner was followed by the installation of the grout pipe and grout observation well piping. The grout pipe was installed on 5-foot centers for adequate grout coverage.

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