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in the upper installations, supported by heavy annular rings and cross ribs. A liner plate of %-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 cither 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 % 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 1 !•, 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 21foot tunnel on the Arizona side of the river branches into two tunnels 12 feet 6 inches in diameter about 45 feet from the face of the canyon wall, to serve two smaller turbines.

Downstream from the 21-foot penstock tunnels, the header tunnel was excavated 35 feet in diameter, from which six 11-foot horseshoe-shaped tunnels were provided for the pipes leading to the canyon-wall outlet works. The horseshoe tunnels were not lined until the 102-inch pipes were placed, when concrete was backfilled between the piping and the tunnel walls to form a penstock anchor.

From the upstream intake towers, 41-foot inclined tunnels were excavated to connect with the inner diversion tunnels. The inclines were lined with a 2-foot thickness of concrete to give a finished diameter of 37 feet. Four penstock tunnels, almost horizontal, similar to the tunnels from the 41-foot header tunnels to the location of the power-plant turbines, were provided from the inner diversion tunnels to the power plant site to convey water to the turbines.

Excavation of all tunnels was performed by methods similar to those used in the excavation of the diversion tunnels. The 41-foot tunnels were driven both ways from the construction adits with a rail-mounted jumbo, rebuilt from pieces of equipment used in lining the diversion tunnels. Mucking was done with a 2-yard electric shovel loading into trucks which moved out to the portal of the adit and dumped the material into the canyon for removal. The inclines to the upper headings were excavated by driving an 8- by 8-foot center heading and then enlarging the opening by two drillings. The muck was removed from the lower end of the incline by power shovel and trucks.

All the penstock tunnels on the Arizona side and those from the upper header on the Nevada side were driven inward from the canyon walls. The penstock tunnels from the diversion tunnel on the Nevada side were advanced from the diversion tunnel before diversion. The drilling of the horizontal tunnels from the diversion tunnels was done with a truck-mounted jumbo, drilling a 14-foot round for a fullcircle section. Mucking in the horizontal tunnels was done with a regulation mucking machine, equipped with a lengthened conveyor belt discharging into a truck.

The 2-foot lining of the 41-foot diameter tunnels was placed behind full-circle forms remodeled from one of the diversion tunnel forms. The form provided for placing a 20-foot section of tunnel at one time and was mounted on steel needle beams for moving ahead after each pour. Concrete for the lining was produced at the high-level mixing plant and delivered by cableway to the portal of each construction adit. The concrete was hauled into the tunnel by train or truck and dumped into the hopper of a conveyorbelt system which discharged through a series of chutes into the sides of the form. The closing arch section was placed by pneumatic gun.

Practically all of the concrete in the penstock tunnel lining

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was placed by a concrete pump. The concrete was transported by cableway from the high-level mixing plant in transit mixers which discharged directly into the charging hopper of the concrete pump. All of the horizontal tunnels were lined using the concrete pump, and the lower half of the inclined tunnels was lined by the same means. In most cases the upper half of the inclined tunnels was lined from the header tunnels by means of a chute system.

TUNNEL PLUGS

One of the major considerations in the design and location of the diversion tunnels was their adaptability to other uses after the diversion period. The tunnels were constructed to permit the outer tunnels to be used as spillway tunnels and the inner tunnels as penstock tunnels.

To permit construction of features in the outer tunnels, 50- by 50-foot steel bulkhead gates were installed at the upstream portals of the outer diversion tunnels to shut off the flow of water through the tunnels. These gates were constructed of heavy structural steel, designed to withstand a static head of 295 feet and to close under a head of 60 feet of water. The gates were erected in raised position, to be lowered upon completion of the diversion program. Each gate with steel frames weighed about 3,000,000 pounds.

Where the inclined spillway tunnels join the outer diversion tunnels, concrete tunnel plugs, 393 feet in length, were constructed to seal the tunnels. The plugs are approximately 65 feet in diameter at the widest point and are provided with three tapered shoulders for anchorage. Regulated diversion of the river until the discharge could be handled by the intake towers was provided by four conduits, 6 feet by 7 feet 6 inches in cross section, extending through the plug in the outer Nevada tunnel.

Downstream from the plugs at the spillway inclines, an additional tunnel plug was provided on each side of the river to house the needle valves for the tunnel plug outlet works. The Nevada plug is located about 743 feet from the downstream portal of the tunnel and the Arizona plug about 456 feet from the downstream portal. These plugs were constructed by removing the old lining at the site of the plug and excavating the tunnel to a maximum width of 100 feet and a height of 80 feet to receive the proposed structure.

Where the inner diversion tunnels are joined by the inclined tunnels from the upstream intake towers, concrete plugs 306 feet in length were provided to seal the diversion tunnels. Stoney gates, 50 feet wide and 35 feet high, were provided at the downstream portals of the tunnels to shut out the backwater from the river below the power house and permit maintenance work on the penstocks. The gates were designed for raising and lowering operations under a balanced water head of 35 feet. The operating chains for the gates run from the gate around the hoist sprocket to counterweights. The suspended weight of each gate and chain is about 260,000 pounds and of counterweights and chains 215,500 pounds. The normal hoisting speed is 0.89 feet per minute with a total lift of 58 feet.

Approximately 88,000 cubic yards of concrete were placed in all tunnel plugs, all of which was produced at the highlevel mixing plant. In general concrete was placed in 5-foot lifts, by pumps and transit mixers, and cooling pipes were placed on top of the concrete in a manner similar to that used in the dam. Cooling and grouting of the plugs was performed to effect a tight seal in the tunnels.

PLATE-STEEL OUTLET PIPES .

The possibility of using the concrete-lined tunnels as pressure tunnels was considered, but the analysis of the stress distribution in the rock surrounding the tunnels disclosed zones subjected to tensile stresses along which fractures might occur and leakage develop when the tunnels were under pressure. Studies and investigations disclosed the fact that it was not economically feasible to reinforce the lining as a protection against leakage. The possibility of placing welded steel lining in the tunnels to prevent leakage was also investigated; but it was discovered that a steel lining, sufficiently thick to resist breaking by the expansion of the tunnels under full pressure, would contain enough material to construct a pipe 35 feet in diameter.

A study of the requirements for the power plant and for release of water from the reservoir showed that four steel pipes, each 30 feet in diameter, installed in the tunnels as detached conduits, would satisfy the conditions. Further study of flow conditions and water hammer revealed that by using high-strength steel plates and providing longitudinal joints so welded that the joint efficiency would have a

strength equal to that in the plates, the maximum plate thickness would not exceed 2% inches.

Accordingly, designs were prepared on this basis and specifications issued covering the fabrication and installation of the pipes. Award was made to the Babcock & Wilcox Co., of Barberton, Ohio, for the lump sum of 510,908,000.

The most economical system of pipes was composed of four 30-foot headers; four branch penstocks, 13 feet in diameter, leading to the turbines from each 30-foot header; and a 25-foot header beyond the penstock pipes, connecting with smaller branch pipes leading to the outlet valves.

A stress of 18,000 pounds per square inch was used in the design of the pipes, no allowance for corrosion being made. This resulted in plate thicknesses of from l'Ji'j to 2% inches for the 30-foot pipe, 1% to 25/}i inches for the 25-foot pipe, lYu to l/i6 inches for the 13-foot pipe, and % to % inch for the 8!2-foot pipe.

The 30-, 25-, and 13-foot pipes are supported on reinforced concrete piers, one pair of piers being used for each pipe section. The piers were designed to resist the overturning tendencies of the empty pipe caused by temperature movements transferred to the supporting plate when the pipe was being installed. The piers were also proportioned to carry the dead load of the pipe and water with a maximum compression on the concrete of 800 pounds per square inch.

Each conduit system is anchored at both upstream and downstream ends and around the bends and manifolds. Two intermediate anchors are provided in the upper tunnels and three for the headers in the lower tunnels. Two anchors are provided for the 13-foot penstocks, one around the lower bend near the power house and one near the connection to the 30-foot header.

The large diameter of the greater part of the penstocks made it impossible to ship completed sections of the pipe by rail to the dam site. Accordingly, a complete field fabrication plant was erected by the contractor about \)i miles from the rim of the canyon. The shop building was a steel-frame and galvanized-steel-covercd structure, 85 feet wide and 520 feet long, equipped with three 75-ton overhead traveling cranes for handling steel plates and pipe sections.

The total weight of the steel required to complete over 14,000 feet of the various sizes of pipe was approximately 88,000,000 pounds. Carbon-steel plates of high strength were developed for use in the construction of the pipe. Three plates were required for the construction of the 30foot pipe sections, two plates for the 25-foot sections, and one plate for the 13- and 8},-foot sections.

Each plate was laid out and marked to the required size and shape, and the edges planed for welding grooves. From the planer the plates were moved to vertical platebending rolls for shaping to cylindrical form. To form the special conical sections of pipe and to bulldoze the edges of the plates before starting them into the

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How Boulder Dam Works, as illustrated by cutting away the Arizona wall to reveal penstocks and outlets. A similar set of penstocks and

outlets is embedded in the Nevada wall.

bending rolls, two 1,500-ton presses, working as a unit, were provided.

Following the rolling, the plates were welded together, the number of welds and operations depending on the size of the pipe to be fabricated. All welding was performed by automatic arc-welding machines, so arranged that they could be operated either on the top or inside of the pipe sections. Each deposit of weld metal was chipped clean of slag, wire brushed, and cold worked by peening before the next deposit of metal was applied.

After the completion of welding, every inch of weld was photographed by X-ray to render visible for inspection any internal defects. Two 300,000-volt portable X-ray machines were used for this operation. Following the inspection of the welds and the approval of each section, the pipes

were given a stress-relieving treatment by heating to a temperature of about 1,200° F. in a specially built furnace. This temperature was maintained for a period of time depending upon the thickness of the pipe; and the furnace then allowed to cool to a temperature of about 600° F. during a 3-hour period, after which the pipes were removed. After cooling, the ends of each pipe section were machined to provide an accurate field fit in the girth joint.

To transport the completed pipe sections from the fabrication plant to the dam site, a specially constructed trailer, weighing 41 tons and designed to carry a maximum load of 200 tons, was provided. The trailer was 22 feet in width and 37 feet in length with a turning radius of 100 feet. Each of the four corners of the trailer was supported by two axles, each carrying two rubber-tired wheels. Hydraulie steering was provided for each end and braking was applied by compressed air.

For handling the penstock pipe and power plant equipment at the dam, a 150-ton cableway was constructed by the Government. This cableway installation spans the canyon for a distance of 1,200 feet at the downstream end of the power house. A 90-foot structural steel tower was provided on the Nevada side of the canyon, but no tower was required on the Arizona side. The track is composed of six 3%-inch, 6 by 37, parallel cables, spaced at 18?2-inch centers. Each of these cables is connected to eye-bolts and cross beams anchored in tunnels filled with concrete on either side of the canyon.

The hoist house containing all of the operating machinery is located on the Nevada side of the canyon between the anchorage and the head tower. Two main hoist drums, 13 feet in diameter and 17 feet in length, provide sufficient area to wind the hoist cable in a single layer. A separate drum is provided for the conveying line. The carriage conveying speed is 240 feet per minute, the hoisting and lowering speeds are 120 and 30 feet per minute, and provisions are made for inching speeds for both conveying and hoisting. Operation of the cableway is by remote control from the main control station, supported on cantilever I-beams over the canyon, or by interlocking change-over switch to any of four stations at the landing platforms.

The contract for furnishing and erecting the cableway equipment was awarded to the Lidgerwood Manufacturing Co., of Elizabeth, N. J., for SI 72,110.

A car of special design was used to transport the pipe sections from the cableway landing through the construction adit to the main tunnels where another special car was used to haul the sections to position. Hoisting winches located inside the intake towers were used to haul the cars up the inclined portions of the tunnels.

The installation of the 30-foot pipe sections was commenced at the upper ends of the tunnels at the base of the intake towers. The installation of the 25-foot headers below the construction adits was commenced at the lower ends of the tunnels, progressing in an upstream direction to the adits. The last pipe section closing the gap left in the 25-foot conduit was located opposite the adits.

OUTLET WORKS

The primary purpose of the outlet works is to provide regulation of the reservoir and to supply water for downstream use in addition to that passing through the power plant. The capacities are so designed that with ideal operation of the outlet works, there will be little occasion for the use of the spillways.

Outlet works capacities were designed for a discharge of 100,000 second-feet at elevation 1,150 with reductions in capacity for lower water surface elevations. In the pre

liminary designs the outlet capacities were provided by forty 72-inch-diameter needle valves, 16 at elevation 820 and 16 at elevation 945, located at the canyon walls, and 8 at elevation 662 in the tunnel plugs. By revising the outlet tunnels, increasing the size of the needle valves at elevation 820, and increasing the number of valves in each tunnel plug from 4 to 6, it was possible to decrease the total number of valves needed to 24, thereby eliminating all of the valves at elevation 945, and reducing the number of valves at elevation 820 to 6 on either side of the river.

The outlet works, similar in design on both sides of the river, may be divided into two systems: first, the canyon wall outlet system consisting of the valves and structures on the canyon walls at elevation 820; and second, the tunnel plug system consisting of the valves and tunnel plugs at elevation 662 in the inner diversion tunnels.

The canyon wall outlet valves are located on each side of the river gorge, approximately 800 feet downstream from the dam, and are supported on a bench blasted out of the walls. The six 84-inch needle valves on each side of the canyon discharge from elevation 820, approimattely 175 feet above the normal elevation of the river. The valves are pointed at an angle of 60° downstream, to give the issuing water a downstream component, and are located so that the jets from the two sides of the river meet approximately in the stream bed.

The valves are housed in reinforced concrete structures containing all the operating and maintenance equipment. The houses are similar in design, except for slight differences in size caused by the angles at which the valves pass through the houses. The Arizona house is 206 feet in length, 36 feet in width, and 64.5 feet in height from the lower floor to the top of the roof. The Nevada house is 190 feet long, 37 feet wide, and the same height as the Arizona house. The steel outlet conduits enter the house through the back wall and connect with 96-inch Paradox emergency gates embedded in concrete blocks which form the operating floor for the gates and valves. Directly in front of the gate blocks are the 84-inch needle valves, supported on reinforced concrete pedestals built integral with the foundation. On the front of the needle valves are attached conical sheet metal discharge guides which carry the water through discharge apertures in the front wall. Each house is equipped with a 30-ton crane, with 10-ton auxiliary for installation and maintenance of heavy equipment.

Each 84-inch needle valve will discharge an average of about 3,800 cubic feet per second, providing an average total discharge of approximately 46,000 cubic feet per second for all 12 valves. The 96-inch Paradox emergency gates are closed when the needle valves are not in operation, to relieve the valves of the pressure head of the reservoir.

Excavation of the benches for the valve houses involved the removal of approximately 56,000 cubic yards of rock. Concrete for the houses was manufactured at the high-level

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