mixing plant and transported by cableway to the point of placement in transit mixers and hopper buckets. A total of 17,000 cubic yards of concrete was placed in both houses. The downstream tunnel plug outlet works are located in the inner diversion tunnels, several hundred feet upstream from the outlet portals. They regulate the flow of all stored water released through the upstream intake towers which is not used for the generation of power.

The 25-foot steel outlet pipes leading to the needle valves. branch into three pipes, 13 feet in diameter, which in turn branch into six pipes 86 inches in diameter. A solid block of concrete 100 feet in length, with operating rooms and galleries, encases the outlet pipes for anchorage. The six 72-inch needle valves with 86-inch emergency gates are located in an operating room at the downstream end of the anchor. This room, 35 feet wide, 100 feet long and 66 feet

high, contains equipment similar to that installed in the canyon wall valve houses for the operation and maintenance of the valves and gates.

The emergency gates are housed in blocks of reinforced concrete, the tops of which are used as the operating floor. Needle valves are placed at approximately elevation 653, 10 feet downstream from the emergency gates, on reinforced concrete pedestals built integral with the floor of the operating chamber. The distance between the front wall and the needle valves is spanned by discharge guides which confine the water issuing from the valves. An observation platform is provided on the downstream side of the chamber to view the action of the water in the tunnels as it issues from the needle valves.

Directly downstream from the concrete plug a transition, 97.5 feet in length, is provided from the plug section to the

50-foot diameter tunnel. The transition has a uniform variation from a width of 82 feet to the 50-foot tunnel section.

The 72-inch needle valves will discharge a maximum of 3,670 cubic feet per second for one valve, or approximately 44,000 second-feet for all valves on both sides of the canyon. This capacity, combined with the capacity of the canyon wall outlet works, will produce a total discharge of approximately 90,000 cubic feet per second.

POWER PLANT

The dam will make available initially a firm power output of 4,330,000,000 kilowatt-hours annually. In addition to the firm energy, large amounts of secondary energy will be available in some years.

The power plant is designed for an ultimate installation. of 15 main generating units of 115,000-horsepower capacity each and two main generating units of 55,000-horsepower capacity each, making a total generating capacity of 1,835,000 horsepower. The initial installation in the power-house is composed of four 115,000-horsepower units and one 55,000-horsepower unit. Additional generating units are being installed to conform to the requirements of power

contractors.

Each main turbine is equipped with a butterfly-type hydraulic-rotor-operated, shut-off valve at the inlet to the turbine casing, 14 feet in diameter for the large turbines and 10 feet in diameter for the smaller units. The turbines are of the vertical-shaft, single-runner Francis type, with caststeel spiral casings and single-piece, cast-steel runners. They are designed to operate under a net effective head varying from a minimum of 360 feet to a maximum of 590 feet. The large turbines have a rated output of 115,000 horsepower each at a head of 492.5 feet and will operate at a speed of 180 revolutions per minute. The smaller turbines have a rated output of 55,000 horsepower at a head of 467 feet and will operate at a speed of 257 revolutions per minute. Each large turbine weighs approximately

1,425,000 pounds. The generating equipment consists of 82,500-kilovoltampere, 180-revolutions per minute, 60-cycle, 16,500-volt generators, and 40,000-kilovolt-ampere, 257-revolutions per minute, 60-cycle, 13,800-volt generators. The generators are of the conventional, 2-guide-bearing type with thrust bearing on top of the frame; and each generator has a direct connected main exciter and pilot exciter. Each of the larger units weighs more than 2,000,000 pounds, is 40 feet in diameter, and 32 feet in height. The rotors weigh approximately 1,100,000 pounds each and the shafts are 38 inches in diameter and weigh approximately 100,000 pounds each. Transformers are of the inert-gas-filled outdoor type with shielded windings. The larger units are rated 55,000 kilo

volt-amperes at 287,500 volts, and the smaller units 13,333 kilovolt-amperes at 138,000 volts. The larger transformers are water-cooled and weigh more than 400,000 pounds each. The generator voltage, oil circuit breakers are of the metalenclosed, triple-pole, single-throw, non-oil-throwing indoor type, with vertical lift. All units are operated electrically and are fully automatic.

The power plant is located immediately downstream from the dam and consists of two wings, one on either side of the river, with offices, shops, operating and storage rooms, located in a connecting structure built across the downstream face of the dam, to form a U-shaped structure 1,650 feet in length. Each wing housing the power plant equipment is 650 feet long, 150 feet high above normal tailrace water surface, and 229 feet above the lowest foundation elevation. Construction of the power-house involved 455,000 cubic yards of excavation, 240,000 cubic yards of concrete, 24,000,000 pounds of steel reinforcement, and 12,000,000 pounds of structural steel.

CONSTRUCTION

Plans and specifications for the dam and appurtenant works were prepared in the fall of 1930 and the advertisement issued in December. Bids were opened March 4, 1931, at Denver, Colo. Three regular bids were received, as follows:

Six Companies, Inc., of San Francisco, Calif., $48,890,995.50.
Arundel Corporation, of Baltimore, Md., $53,893,878.70.
Woods Brothers Construction Co., of Lincoln, Nebr., and A. Guthrie
& Co., of Portland, Oreg., $58,653,107.50.

Six Companies, Inc., of San Francisco, Calif., a company composed of six western contracting firms-Utah Construction Co., Pacific Bridge Co., Kaiser Paving Co., Ltd., MacDonald & Kahn Co., Morrison-Knudsen Co., and J. F. Shea Co., was awarded the contract on April 20, 1931. The construction of Boulder Dam involved approximately 6,000,000 cubic yards of all classes of excavation and placement of 4,400,000 cubic yards of concrete. Additional quantities of other principal work or materials included. drilling grout and drainage holes, 410,000 linear feet; pressure grouting, 422,000 cubic feet; earth and rock fill, more than 1,000,000 cubic yards; gates and valves, 21,670,000 pounds; plate-steel outlet pipes, 88,000,000 pounds; pipe and fittings, 6,700,000 pounds; structural steel, 12,000,000 pounds; miscellaneous metal work, 5,300,000 pounds; and cement, 5,000,000 barrels.

Under the terms of the contract 2,565 days, from March 11, 1931, to April 11, 1938, were allowed the contractor for the completion of the project. On March 1, 1936, the dam and power plant were accepted by the Secretary of the Interior, terminating the contract and marking the end of actual construction on the project 11 days less than 5 years or 2 years 1 month and 28 days ahead of schedule.

Perhaps the most interesting feature of the construction operations was the preparation of concrete. The unprecedented quantity of concrete involved and the exacting conditions governing the manufacture of the same necessitated the installation of unique and interesting equipment and methods of preparation.

The raw material for aggregate was obtained from a natural river deposit, located about 8 miles upstream from the dam site on the Arizona side of the river. This was a typical water-borne deposit of well-rounded particles of excellent quality. The sand was largely quartz and the gravel was made up principally of limestone, intermixed with granite, basalt, and quartzite.

The gravel plant was located about 4 miles upstream from the dam and 2 miles west of the river. It consisted essentially of four structural steel, screening towers, with connecting trusses supporting conveyor belts, and a control tower at the head end carrying a scalping screen. Railway cars delivered the raw material to bunkers, from which it was conveyed by belts to the first tower. Here it was put through the scalping screen to remove the oversize, which was conveyed by belt to the crusher and then returned to the screening system. The sand and cobbles were removed in the first tower and each succeeding tower received the remaining aggregate on vibrating screens, located near the top of the structure, and removed the sizes as required

Imme

4 to 4 inches; 4 to 11⁄2 inches; and 11⁄2 to 3 inches. diately below the floor on which the screens were located, the selected size of aggregate was fed on to a conveyor belt, which was carried by steel trusses to a stone ladder and deposited into live storage over tunnels. The tunnels, equipped with electric-vibrating feeders, contained the lower ends of return belt conveyors to take the material back to the lowest level of the towers, where it was rescreened and washed before being loaded by a shuttle conveyor belt into hopper cars for delivery to the concrete plant.

Specifications for the sand provided for a fineness modulus of not less than 2.75 or more than 3.25. Analysis of the sand contained in the pit run gravel indicated an excess of material of the 28 to 48 mesh sizes. To secure the required modulus it was necessary to remove a part of this material by passing the sand through two rake-type classifiers which segregated and split the 28 and 48 mesh sizes. The necessary amount of material was wasted and the remainder recombined with the rest of the sand. The material was then passed to a bowl-type classifier for washing, to remove the silt, and the final product dewatered by a third raking classifier before transfer to the storage piles by belt conveyor through a tunnel under the loading tracks. The sand was loaded from the stock piles into railroad cars by a railroad crane, equipped with a clamshell bucket.

The entire gravel plant was electrified and controlled from a push-button station on top of the scalping screen tower. The plant was designed to handle 500 tons of materials per hour, and produced 700 tons per hour for extended runs.

Cement was obtained under contract from a number of plants in different localities. To eliminate any variation. in color and chemical characteristics of the various cements, a blending plant was erected on the rim of the canyon, the operation of which was designed to provide a cement which would maintain, in general, the same proportion of cements that was supplied by the mills. Three blends of cement were prescribed, a blend of all low-heat cements, a blend of all standard cements, and a blend of low-heat cement and standard cement.

Cement from each plant was stored in a separate silo and removed by a screw-conveyor system in a predetermined proportion. The action of the screw-conveyor provided the mixing operation and the blended cement was pumped to storage silos or to the mixing plants. The pumping distance to the high-level plant was only 100 feet; but a 9-inch line, 5,600 feet in length, with a total vertical fall of 530 feet, was required to reach the lowlevel plant. A pump station was provided at the beginning of the line.

Two concrete mixing plants were provided to furnish the concrete for the various operations. The main or lowlevel mixing plant was located in the canyon about 4,000

feet upstream from the damsite, on the Nevada side of the river. This plant was used to provide all concrete for lining the diversion tunnels, more than half the concrete for the dam, and concrete for other miscellaneous purposes. The remainder of the concrete was produced at the highlevel mixing plant located on the rim of the Nevada cliff near the blending plant.

A structural steel building was provided at the lowlevel mixing plant with storage bins of laminated timber sides for each of the five classes of aggregates—sand, fine gravel, intermediate gravel, coarse gravel, and cobbles, and for cement. A tipple for receiving and

handling the aggregates was located above the bins.

Cars of aggregate from the gravel plant were dumped into a five compartment track hopper, each compartment of which provided storage for 10 cars of one of the five sizes of aggregate. Material from the track hopper was transported to the tipple by a system of inclined belt conveyors from which the material was distributed to the various bins. Each bin was capable of holding a 12-hour supply of aggregate.

The plant contained two batching units each feeding two 4-cubic-yard tilting mixers. All materials entering the mixers were weighed in automatic remote-control discharge