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be served from the All-American Canal is about 1,000,000 acres, approximately one-half of which is at present irrigated.


INVESTIGATIONS on the Colorado River which finally led to the construction of Boulder Dam were first started by the Reclamation Service in 1904. After a prolonged examination of available storage sites in the upper part of the main valley a reconnaissance was conducted on the river below the mouth of the Virgin River. The preliminary studies resulted in the concentration of work on the better damsites in Boulder and Black canyons in 1918. From 1918 until contracts for construction were awarded in 1931, investigations, surveys, foundation explorations, geological examinations, and engineering studies on feasibility and cost were carried forward for the Boulder Canyon and alternative sites. The Weymouth report, submitted by the Chief Engineer of the Bureau of Reclamation to the Secretary of the Interior in 1924, contains much of the data derived from these investigations.

Congress, by joint resolution (45 Sta. 1011) of May 1928, directed the Secretary of the Interior to appoint a board of five engineers and geologists to review the plans and estimates for the Boulder Canyon Project and report on the safety, the economic and engineering feasibility, and the adequacy of the proposed structure and incidental work. Following a favorable report by this board in November 1928, the Boulder Canyon Project Act was passed by Congress and signed by the President on December 21, 1928.

The purpose of Boulder Canyon Project is defined in the act as follows: “controlling the floods, improving navigation and regulating the flow of the Colorado River, providing for storage and for the delivery of the stored waters thereof for reclamation of public lands and other beneficial uses exclusively within the United States, and for the generation of electrical energy as a means of making the project herein authorized a self-supporting and financially solvent undertaking."

According to preliminary estimates, about 1,900,000 acres of irrigable lands are located below the dam in the United States. The Boulder Canyon Project Act authorized the construction of a main canal from the Colorado River to

blorado River to the Imperial Valley. Work is now in progress on the construction of Imperial Diversion Dam, located about 15 miles northeast of Yuma, Ariz., and on the All-American Canal, to extend from Imperial Dam to a point about 12 miles west of Calexico, Calif. The total irrigable area which can

The Colorado River, including the Gila River, drains an area of about 245,000 square miles, including parts of seven States-Arizona, California, Colorado, New Mexico, Nevada, Utah, and Wyoming. The drainage area above the dam amounts to 167,000 square miles. The average annual runoff with present conditions of upstream irrigation development is estimated at 15,000,000 acre-feet, varying from a maximum of 25,200,000 acre-feet in 1909 to a minimum of 4,300,000 acre-feet in 1934.

Lake Mead, created by Boulder Dam, has a maximum capacity of 30,500,000 acre-feet of water, at which stage the water surface area will be 146,500 acres. The lake will extend 115 miles up the Colorado River, through Boulder Canyon, Virgin Canyon, Iceberg and Travertine Canyons, and into the lower end of Grand Canyon. The width will vary from several hundred feet in the canyons to a maximum of 8 miles.

Initially it is contemplated that 9,500,000 acre-feet of storage capacity will be reserved primarily for flood-control purposes, with incidental use for the production of secondary electrical energy. The remaining capacity of 21,000,000 acre-feet will be used for regulation of water for irrigation, for firm power production, and dead storage for creating power head and a silt pocket.

Future irrigation and storage development upstream from the dam will decrease the inflow and permit a reduction in the storage capacity reserved for flood-control purposes, with a corresponding increase in the live storage capacity. Reductions in the total reservoir capacity due to silting will merely involve a reduction in dead storage. It is estimated that in the first 50-year period of reservoir operation, about 3,000,000 acre-feet of reservoir capacity will be lost by silting.


The site for Boulder Dam is located in Black Canyon, about 30 miles southeast of Las Vegas, Nev. Black Canyon is a youthful topographic feature, deeply entrenched in an

The fluted wall of block 40 in Grand Coulee Dam.

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rial such as would cause trouble in connection with the dam. There is no indication that any of the faults will become paths of leakage when the reservoir is full.


upper mature surface of small relief. The most characteristic features are reddish-black cliffs and exceedingly precipitous slopes. The relatively open arroyos of the uplands approaching the main canyon become narrow ravines or gorges through which the streams from occasional rains plunge to the river over successive falls. The canyon walls at the dam site rise several hundred feet above the river, while the middle of the rock gorge is 110 to 127 feet below low-water elevation.

The rocks forming the foundation and abutments for the dam are of volcanic origin. These rocks compose a group generally latitic in character, higher in alkalies than andesites and basalts.

The rocks throughout are of excellent character, hard, strong, and practically impermeable. Such faults as occur in the region are not active or likely to become so. Neither are they associated with any soft, crushed, or broken mate

Preparations for the construction of Boulder Dam involved many diversified construction operations of considerable magnitude. It was necessary to provide access to the site for transportation of supplies and equipment. Housing facilities had to be constructed to accommodate a population of 5,000 in the middle of a desert. Power had to be provided for construction and domestic purposes.

The Union Pacific Railroad Co. constructed a branch line, 22.7 miles in length, from a point near Las Vegas. Nev., to the site for Boulder City. Under a contract with the Lewis Construction Co., of Los Angeles, Calif., the heated and the administration building is air-conditioned. Family houses were provided with special fireplaces designed for air circulation and were equipped with electric ranges, water heaters, and refrigeration. Six Companies, Inc., constructed more than 660 buildings, practically all of which will be torn down on completion of the project.

Water for Boulder City is obtained by pumping from the Colorado River. After presedimentation near the river, the water is pumped 6%4 miles through two lifts to a 100,000gallon tank at Boulder City for delivery by gravity. to the treatment plant for filtration, softening, and chlorination. After treatment, the water is pumped to a 2,000,000-gallon storage tank on a high knoll and delivered to the city mains by gravity.

The installation of the sewerage system involved the construction of vitrified-clay sewer lines ranging in size from 6 inches to a maximum of 12 inches. A 12-inch outfall line extends about one-fourth of a mile to the disposal plant of the separate sludge digestion type, with gas collection.


Government extended this line 10% miles from Boulder City to the rim of Black Canyon, directly above the dam site, at a cost of $653,551.03. Taking off from the end of Government line, Six Companies, Inc., general contractors for construction of the dam, built 26 miles of railroad; one branch extending to the Arizona gravel deposits at the bottom of Black Canyon, and another to the Nevada side of the gorge at the dam site.

The State of Nevada constructed a new highway between Las Vegas and Boulder City, a distance of 23 miles. This highway was extended by the Government to the rim of Black Canyon under a contract with the General Construction Co. of Seattle, Wash., providing for a gravelbase, oil-surfaced highway, 22 feet wide and 8.3 miles in length, including side roads. From the Government highway, Six Companies, Inc., constructed a road which descended to the lower portals of the Nevada diversion tunnels and a steel suspension bridge across the river to provide access to the Arizona tunnels. Several miles of additional roads also were constructed by the contractor to reach various parts of the work.

The contract for furnishing power was awarded to the Southern Sierras Power Co. (also operating as the NevadaCalifornia Power Co.). A 222-mile transmission line, to deliver power from generating stations near San Bernardino, Calif., was constructed to the dam site. The line, together with a substation near the Nevada rim of the canyon, was constructed by the power company at a cost of approximately $1,500,000. It will serve as a permanent installation to transmit power from the dam. Auxiliary power lines were erected from the substation to various parts of the work and a 7-mile line from the substation to Boulder City was constructed by the Government.

High temperatures at the dam site during the summer months necessitated the selection of a town site where more comfortable living quarters could be provided. A site was selected on the summit of the divide about 7 miles from the dam site, at an elevation 2,500 feet above sea level and 1,855 feet above the river.

The town site covers about 300 acres and was laid out roughly in the shape of a fan with the point to the north. The northern portion of the fan is allotted to Government buildings with groups of residences, from 3 to 7 rooms each, arranged along the lower slopes of the north ridge. Directly in front of the administration building is a 5-acre park from which the two principal streets diverge to outline the business district and provide radial thoroughfares.

The Government construction included an administration building, municipal building and post office, dormitories for unmarried workers and visitors, garage and fire station, schoolhouse, community garages, and 100 residences. Durable materials such as brick, tile, and stucco were used in the buildings, all of which follow the Spanish style of architecture. The larger buildings are furnace

To unwater the dam site, four huge tunnels, two on each side of the river, were driven through the canyon walls to carry the flow of the river during construction. These tunnels, circular in cross section, were excavated 56 feet in diameter and lined with 3 feet of concrete to make a finished section 50 feet in diameter. The combined length of the four tunnels was 15,946 feet and required the removal of approximately 1,500,000 cubic yards of rock. The rock proved to be of excellent quality and no support was required for any portion of the length. The general plan of tunnel advancement was to drive a 12- by 12-foot pioneer heading at the top of the cross section of the 56-foot area. The enlargement followed the pioneer bore to the full width of the tunnel, but only 41 feet high, leaving 15 feet in the invert to be excavated at a later period.

A truck-mounted drill carriage with platforms at four levels, carrying a total of 30 drills and wide enough to drill one-half of the bench in one operation, was devised to carry on the drilling operations. The wing sections on either side of the top heading were drilled from two vertical bars. Forty percent gelatine dynamite was used for the blasting and primers were set from “no delay” up to “15”, making 16 delays in the firing. A 440-volt circuit, with locked safety switches outside the tunnels, was used for detonating.

Mucking was carried on in the tunnel enlarging operations by 100-ton electric shovels, equipped with 3-cubic-yard buckets. The muck was loaded on 10-ton trucks and hauled up roads along the canyon walls to the disposal areas in side canyons.

The removal of the invert section was performed in a similar operation to that used on the bench heading. The same drilling carriages with the top part removed and with

two folding wings built on either side were used for this operation. The drills were mounted on the wings, curved on a 28-foot radius, and the whole of the invert section drilled in one operation.

Trimming and scaling operations were performed coincidently with, and a short distance ahead of, the invert excavation. A horseshoe-shaped steel framework with an outside diameter of 50 feet, mounted on wheels which traveled on rails laid true to line and grade was used to measure the rocks projecting within the allowable clearances.

For the purpose of applying concrete lining, the circular section was divided into three parts—the lower 74° for the invert, to be placed first; above this section, on either side, the side wall portions of 88° each, to be placed second; and above the side wall sections the roof or arch section of 110°, to be placed last.

Specifications required an average lining thickness of 36 inches with a 24-inch minimum. Concrete for the invert and side wall sections was composed of 1 part cement, 2.1 parts sand, and 4.7 parts gravel. Aggregate was graded as follows: 14 to 24 inch, 32 percent; 34 to 14 inches, 32 percent; 13 to 3 inches, 36 percent. Concrete for the arch section was 1 part cement, 2.5 parts sand, and 4.3 parts gravel. The larger size of aggregate, 142 to 3 inches, was omitted from this mix. The slump of the concrete used in the various sections was: invert, 39 inches; side walls, 4% inches; and arch, 5 inches.

Concreting operations were started at the upstream portals and carried progressively toward the downstream ends of the tunnels. All concrete was mixed at the low level plant, located a short distance upstream from the tunnel inlets on the Nevada side of the river. Truck transportation was used exclusively, consisting of either two 2-cubic yard form buckets, carried on platform trucks, or 4-cubic yard agitator bodies.

The first step in the lining program involved the construction of continuous concrete shelves to serve as rail bases for a 10-ton electrically operated gantry crane, to be used for concrete placing. Invert forms, consisting of steel side plates carrying curved steel screeds, were then set in place and pouring started. Concrete was hauled by truck to the forms where the gantry crane took up the buckets and deposited the contents, one on either side of the screeds. The concrete was then puddled into place and when sufficiently high the screeds were moved outward toward the side walls to form the required concave surface. The surface was finished by workmen on a movable platform suspended just above the concrete, using wooden floats and steel trowels.

Bulkheads for contraction joints in the invert were provided every 40 feet, except in those portions of the two outer tunnels which were later to be used as spillways, where the distance was 26 feet 8 inches. This same spacing of contraction joints also applied to the side walls and arch sections.

The use of the invert for trucking in subsequent lining

work necessitated the placing of about 3 feet of fine rock spoil, to protect the concrete. After the invert section was placed, two concrete rail shelves were constructed on the the invert to support the side wall and arch form equipment. A huge structural steel framework or jumbo, weighing about 270 tons for an 80-foot section, was provided to support the wall forms which were composed of 74-inch steel plate. The jumbo was completely equipped to handle concrete with an electric crane operating on top of the form and a system of chutes, seven high on each side of the form, spaced horizontally 9 feet center to center.

Placing was started through the lowest row of chutes and carried progressively upward. Proper rotation was maintained to place each side of the 40- or 26-foot 8-inch section simultaneously. Men were stationed back of the forms to puddle the concrete against the rock. As the concrete rose in the forms, the chutes were withdrawn and the form openings covered with steel plates.

Temporary timber bulkheads were used to provide construction joints between pours. These bulkheads were framed to form a simple offset keyway 1)2 inches deep and 10 inches wide. A keyway, approximately 10 inches wide and 2 inches deep, also was formed on top of the side wall sections to receive the arch section. With the completion of two 40-foot pours, or three pours of the shorter section, the form was left in position for 12 hours, then moved to the next position.

The forms and concrete placing equipment for the arch section included an 80-foot length of form supported by a structural steel framework or jumbo, a separate gun carriage containing two 2-cubic yard pneumatic concrete guns with the necessary hoisting equipment, and a separate traveler connected to the gun jumbo to support two placement pipes. This equipment was operated on the same rails that were used for the side wall jumbo and was kept not less than 150 feet behind the completed side walls.

Four-cubic-yard agitator bodies were hoisted off trucks and the concrete divided between the two guns. Operation of the concrete guns followed standard procedure, using a compressed air supply. A large air receiver was used with each gun and the nominal pressure was about 100 pounds, although this often dropped nearly 50 percent at the finish of a 2-yard charge of concrete. From the guns, 6-inch discharge pipes carried the concrete to the arch forms.

Curing of the invert concrete was performed by the roadway which kept the surface moist. A 2 week's spray cure was provided by the specifications, but when this procedure was attempted on the side walls the water collected in the roadway, softening the road. Also, during the summer months the temperature in the tunnels was extremely high, and the humidity resulting from the spraying made working conditions unbearable. As a result, an asphalt oil coating, applied with compressed air (the Hunt process), was used for curing.

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