and coil systems, and returned to the top of the cooling tower where the process was repeated.

Refrigerated water for the second stage of cooling was provided by an ammonia compression system, similar in most details to the systems used in making ice. The refrigerated water was cooled by the action of ammonia, pumped through supply lines into headers supplying the coil system, then returned to the plant for recooling.

An 8-foot cooling slot in the center of the dam, extending through the dam to the upstream face, was provided for the cooling system header pipes. The 6-inch, cork-insulated header pipes extended through the slot to the upstream face of the dam. They connected directly with the loops of 1-inch cooling pipes, which were spaced at 5-foot centers vertically and 5-foot 9-inch centers horizontally. All pipes buried in the concrete were arranged in coils running in a circumferential direction from the cooling slot to the canyon wall and back to the cooling slot. The coils varied in length from 220 to 1,340 feet and averaged 513 feet for 5,880 coils. More than 570 miles of tubing were embedded in the dam.

The cooling pipes were laid directly on top of each 5-foot lift of concrete after the concrete had hardened. They were anchored by wire loops buried in the concrete while the concrete was still plastic. All sections of the pipe were

provided with plain ends and were connected with special couplings which permitted movement.

The temperature of the concrete was obtained by resistance thermometers buried in the concrete or inserted in the end of the cooling pipe at the slot. For cooling tower operations, the average temperature difference for water entering and leaving the concrete was 7.3° F., and for refrigeration plant operations, 11.1° F. The system was designed for a flow of not less than 3 gallons per minute through each coil, but an average actual flow of more than 4 gallons per minute was obtained.

The refrigeration plant was rated at 825 tons of refrigeration, 1 ton being equal to the amount of heat required to melt 1 ton of ice in 24 hours, or 220 B. t. u. per minute. The plant, together with the cooling tower, produced more than 1,700 tons of refrigeration.

CONTRACTION JOINT GROUTING

To insure monolithic action of the dam and to secure the desired stress distribution in the structure, arrangements were made to grout the contraction joints in 50-foot lifts as rapidly as the concrete was cooled and the cooling slot concreted.

Metal grout stops were placed across the radial contraction joints at both the upstream and downstream faces of the dam and across the circumferential joints at each junction with a radial joint. Horizontal grout stops were placed in every joint at 50-foot vertical intervals.

Galleries were provided in the dam at the top of each 50foot lift, for convenience in grouting and observing the behavior of the joints under applications of grout pressure. The lowest gallery was located at elevation 575 and the highest at elevation 975. Above elevation 975, catwalks were erected on the downstream face of the dam at the top of each 50-foot lift, to provide the necessary accommodations for grouting.

For the injection of grout into the joints, a system of pipes with outlets at intervals of 30 to 50 square feet was embedded in the concrete adjacent to the contraction joints. The outlet pipes were connected to headers extending through the grouting galleries in the dam and to both faces of the dam. This lay-out formed a complete circulating system for the distribution of grout and for drawing off water and thin grout not suitable to leave in the joints.

The radial contraction joints in each lift were grouted first. After a 7- to 10-day interval the circumferential joints in the same lift were grouted. Filling of the joints was accomplished by gravity flow, pumping being used only to drive off surplus water and thin grout and to consolidate the film of grout in the joint when the joint was practically filled. Pressures up to 300 pounds per square inch were used in some joints to secure the desired results, but a pressure of 100 pounds per square inch was ordinarily sufficient to drive

the water into the concrete on either side of the joint and produce a grout film of satisfactory density.

Cement for grout was screened through a 200-mesh screen to increase the penetration of the grout into fine seams. An average of one bag of cement was required for 90.79 square feet of radial contraction joint area and 129.09 square feet of circumferential contraction joint area, a total of 33,425 bags of cement being used. The approximate average thickness of the radial contraction joint openings was 0.126 inch and of the circumferential joints 0.087 inch.

SPILLWAYS

A capacity of 400,000 second-feet with the reservoir water surface at elevation 1,232 was selected for the preparation of spillway designs. The designs included in the specifications

were based on a 700-foot free crest, a long concrete-lined channel with an inclined tunnel leading to the diversion tunnels, and a 50- by 50-foot Stoney gate, installed at the upper end of the channel. Subsequent studies and model tests indicated that this type of structure was not suited for the conditions encountered at the dam site.

The final result of the design studies and model tests, indicated that the side-channel type of spillway with drum gate crest was better adapted to the topographic conditions and to the balancing of the reservoir discharges. Two identical spillways, each with a clear crest length of 400 feet, were provided, one on either side of the canyon. The discharge from the spillways will be carried through inclined tunnels, 50 feet in diameter and 600 feet in length, into the outer diversion tunnels on either side of the river which will serve as permanent spillway outlets.

The topography at each site necessitated the construction of a gravity dam of overflow profile for each weir section, the design and construction of which were practically the same as for a dam of similar proportions. The maximum height for the Nevada section was 75 feet, and for the Arizona section, 85 feet. At the quarter points on the crest of these overflow sections, piers were provided to divide the crest into 100-foot sections for the structural steel drum gates. More than 600,000 cubic yards of rock were removed for the construction of the spillways.

The face of the weir that forms part of one side of the spillway channel was designed as a parabola tangent to a 1⁄2:1 slope. The opposite side of the channel was provided with a similar slope. These slopes provide a width of 125 feet at the elevation of the weir crest at the upper end of the structure and 165 feet at the lower end. The spillway channel has a uniform bottom width of 40 feet and the floor slopes on a 12 percent grade from a depth of 75 feet at the upper end to a depth of 128 feet at a point opposite the lower end of the crest.

From the downstream end of the last 100-foot gate section, a 55-foot length of channel was added to improve the hydraulic conditions and to reduce the disturbance at the entrance to the transition leading to the inclined tunnel. At the lower end of this section a step about 36 feet high was introduced to provide increased depth for the reduction of the disturbance caused by crossflow. The crest of the step is the beginning of a parabolic curve forming the bottom of the transition leading to the 50-foot inclined tunnel.

An elaborate drainage system was provided for the spillways, consisting of porous concrete drain tile running vertically under the construction joints in the lining, a 21⁄2- by 5-foot manway extending under the center of the channel floor, and a 12-inch layer of porous concrete under the bottom of the entire channel.

The sides and bottom of the spillway channel and transition were lined with an average thickness of 24 inches of concrete. The channel lining was built in panels which were secured to the rock by hooked, 1%-inch square bars, grouted into holes drilled into the rock a minimum distance of 5 feet. Concrete from the high-level mixing plant was delivered in 4-cubic-yard agitators on trucks, and either moved directly by cableway to the pour behind the forms or transferred to 2-cubic-yard buckets for more convenient handling. More than 127,000 cubic yards of concrete were placed in the spillways.

Each spillway is provided with four structural steel drum gates, each 100 feet in length and 16 feet in height. The gates were built up of structural steel members, the outer surface of 1⁄2- and -inch steel plate being supported directly on girders at 28-inch centers. Each gate weighs approximately 500,000 pounds.

In the lowered position, the top face of the gates will provide a curved surface to complete the outline of the weir

crest. Hinged at the top and upstream side of the concrete weirs, the gates will float in recesses in the weir section and will operate to provide a maximum depth of about 24 feet over the fixed crest.

Automatic control with optional manual operation is provided for raising and lowering the gates. When in raised position a gate may be held continuously in that position by the pressure of water against its bottom, until the water surface of the reservoir rises above a fixed point, when by action of a float the gate is automatically lowered. As the flood peak decreases, the gate will rise automatically. Aside from the automatic control, the gate can be operated manually so as gradually to empty the flood control portion of the reservoir without creation of flood conditions down stream.

INTAKE TOWERS

For the release of water from the reservoir under normal conditions, four intake towers, two on either side of the canyon upstream from the dam, were provided to control the flow to the canyon wall outlet works, the tunnel plug outlet works, and the power plant turbines.

The towers, located symmetrically with respect to the dam, were constructed on rock benches or shelves excavated in the canyon walls. Each tower consists of an inner barrel with a nominal inside diameter of 29 feet 8 inches, surrounded by 12 radial buttresses or fins to accommodate the trashrack sections and afford structural support for the barrel. The width between faces of opposite fins varies from 82 feet at the base of the towers to 63 feet 8% inches at the top of the parapets which are 342 feet above the bases of the structures.

The great height and relative slenderness of the intake towers made necessary the preparation of designs capable of withstanding earthquake shocks. The fins supporting the barrel were heavily reinforced to meet these conditions and an unusually large quantity of reinforcement steel amounting to approximately 4,000,000 pounds or more than 160 pounds per cubic yard of concrete was provided for each structure. The sections of the barrels between the cylinder gates and above the upper gates vary in thickness from 6 to 2 feet, except for the sections immediately above the lower gate and immediately above and below the upper gate which were thickened to provide for the extra stress concentrations at the gate sections.

Two sets of gate openings are provided in each barrel, one at the base of the tower, at sill elevation 895, and the other 150 feet higher, at elevation 1.045. Each set consists of 12 gate openings, rectangular in cross section, protected by conduit lining castings. Combined closure of each set of gate openings is provided on the inside of the barrel by cylinder gates 32 feet in diameter and 11 feet in height. The gates are composed of a steel cylindrical outer shell, 2 inches thick in the lower installations and 11⁄21⁄2 inches thick

in the upper installations, supported by heavy annular rings and cross ribs. A liner plate of 4-inch thickness is added on the inside of the lower gates to offer an unobstructed passage to water flowing through the tower from the upper gate openings.

Each gate contains a monel metal seal around the base and circular monel metal seals are affixed to the top of the gate and to the openings. The weight of each upper gate is 150,000 pounds and the lower gates 240,000 pounds. The total weight of the material required for the gates, nose, throat, and entrance liners is more than 7,000,000 pounds. Three electrically operated screw-stem hoists are provided to raise and lower the gates individually or simultaneously as required. The weight of the moving parts of a lower gate is 340,000 pounds, and of an upper gate 220,000 pounds. The normal travel of the gates is approximately 10 feet and the hoisting mechanisms are arranged to open or close either or both gates in 52 minutes.

A set of 12 bulkhead gates is provided on the outside of the barrel for each set of gate openings. These gates will be used to unwater the towers for inspection and maintenance of the cylinder gates and seats. The gates are steel castings of cellular construction, 11 feet 6 inches in height and 7 feet 2 inches in width. A crane hoist, mounted on a circular track in each tower, will raise and lower the gates by a

single cable. Each section of upper gate bulkhead weighs 9,500 pounds and each lower gate 13,500 pounds.

Reinforced concrete beams connecting the fins were placed at intervals of approximately 10 feet 7 inches from the base of the dam to elevation 1,200. These beams support steel trashrack sections, 10 feet 7 inches high and 12 feet 8 inches wide, which were lowered to rest on the base of the tower or on the preceding trashrack section through grooves provided in the fins. The trashrack sections were constructed of vertical cross bars, 5 inches by 4 inch in section, and horizontal spacer bars 3 inches by inch in section. The trashrack section on each tower extends from the base to elevation 1,200. The total weight of all sections amounted to 7,024,000 pounds.

The cranes and hoisting equipment are located in hoist houses rising 56 feet above the top of each tower. Two plate-girder type bridges are provided on each side of the river to connect the towers with the dam. One bridge spans the distance between the upstream and downstream intake towers and the other the distance between the downstream intake tower and the dam. The bridges between the towers are 118 feet in length and the bridges between the downstream towers and the dam are 93 feet 8 inches and 109 feet 11⁄2 inches on the Arizona and Nevada sides, respectively.

Excavation for the intake towers involving 361,000 cubic yards of rock was performed by high-scaling methods. The completed cuts were 110 feet in diameter at the base, and 338 feet in depth on the Nevada side of the river. Concrete for these structures was produced at the highlevel mixing plant and handled by derricks located between the towers and near the top of each canyon wall. The concrete was conveyed to a hopper erected immediately above the center of the tower and flowed through short chutes to the barrel, fins, and trashrack beams. A total of 93,674 cubic yards of concrete was placed in all towers.

PENSTOCK AND OUTLET TUNNELS

From each of the two downstream intake towers a header tunnel was driven through a vertical curve to elevation 820, then downstream approximately parallel with and 170 feet above the diversion tunnels, to the location of the canyonwall outlet works. These tunnels were excavated 41 feet in diameter and lined with a 24-inch thickness of concrete. To facilitate the installation of the penstock headers, construction adits, 26 feet wide and 43 feet high, were driven from the canyon wall to the line of the tunnel.

From each of the penstock header tunnels four tunnels were driven on an incline to elevation 637 at the back wall of the power house to reach the location of the power-plant turbines. These were excavated 21 feet in diameter and lined with 18 inches of concrete to form a completed tunnel section 18 feet in diameter. The downstream 21