1.46

1.66

2.18

The dam was divided into columns or blocks by radial and circumferential contraction joints. The blocks ranged in size from 25 by 30 feet at the downstream face at the base of the dam to 50 by 60 feet at the upstream face. Joints were interlocked by keys, formed to provide maximum cross-sectional area for resistance to shear after grouting. The rate of placing concrete was restricted so that not more than one horizontal layer, 5 feet in depth, could be placed in 72 hours, and not more than 35 feet in depth could be placed in 30 days. A maximum vertical difference of 35 feet in the top surface of the blocks was permitted.

The form system consisted of metal-lined wooden panels of the cantilever type, providing for a 5-foot lift, with the vertical timbers extending down another 5 feet against the last pour. The fact that the corners of the columnar blocks of concrete in the dam extended vertically from the lowest point of the block to the top permitted forms to be designed and built for each separate block and used for successive lifts without recutting.

Eight-cubic-yard, bottom-discharge buckets were used in placing, to prevent segregation with a minimum variation in water content. The buckets were all-welded construction of 4-inch steel plate, 6 feet in diameter and 8 feet in height. The bottom consisted of two semicircular doors, hinged at the sides and equipped with safety latches to eliminate the possibility of accidental dumping. The loaded buckets weighed approximately 20 tons and were handled by cableways. In placing concrete the buckets were lowered to rest on the concrete, the safety appliances released, and the doors opened. The concrete moved into final position with little handling, some shoveling being required and the cobbles pushed down. Vibrators were used near corners and around the drain pipes and gallery forms.

Each lift was finished with 26- by 5%1⁄2-inch horizontal keys, spaced 10 feet apart. As soon as the initial set had taken place, the surface was washed with an air and water jet under high pressure, to remove any laitance or porous concrete along the horizontal joints. Before the next lift was placed, the treatment was repeated, together with any

Arizona side, downstream face, January 31, 1934. brushing or chipping required to furnish satisfactory bond with the new concrete. Curing was effected on the sides of the blocks by sprinkling from perforated pipes attached to the forms. The top surface of each block was kept wet by hose sprinkling.

To prevent volume change in the large mass of concrete, an artificial cooling system was provided to remove the heat developed during the hardening process and to concentrate the cooling and shrinking period to a relatively short period of time. The cooling and shrinking of the mass was accomplished by circulating water through 1-inch O. D. 14-gage tubing buried in the concrete. The refrigeration process was carried on in two stages: First, circulation of air-cooled water through the pipe system; and, second, circulation of refrigerated water through the same system.

An atmospheric type of cooling tower was erected on the crest of the downstream cofferdam, to supply the air-cooled water for the first stage of cooling. This structure was 150 feet in length, 16 feet in width, and 43 feet in height. The low humidity in the region and the natural draft in the canyon combined to make this method of cooling very effective. Water falling over the tower was collected in a basin at the foot of the structure, from which it flowed to the pumping plant. It was then pumped through a supply header to the dam, circulated through the header

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 11⁄2 inches thick