LAKE MEAD Construction of Boulder Dam

PLANT OF SIX COMPANIES, INC.

In order to build the dam, power plant and accessory features in an efficient and economical manner and within the time specified by the government contract, Six Companies, Inc., built highways and railroads, erected machine shops, air compressor plants, garages and warehouses, spanned the canyon first by bridges and later with cableways, constructed a gravel screening plant and two mixing plants, acquired power draglines and shovels, trucks, cars, derricks, cranes and similar machines, and designed many original appliances for particular needs. Many trucks of the contractor would transport 16 cubic yards of rock in one load and two were of 50 ton capacity. Undoubtedly this was the greatest massing of specialized equipment ever witnessed on any construction project.

Ten miles of paved highways and 20 miles of standard gauge railroad were built connecting all plants with the canyon activities. The railroad equipment included 150 dump cars and eleven 100-ton steam locomotives, as well as four gasoline and eight electric locomotives.

Means of transportation for men and materials across the river and from canyon rim to river channel were provided first by boats and barges, later by steel suspension bridges, and small cableways, and finally by heavy duty cableways of unique design.

Arizona Sand and Gravel Deposit

The latter were five in number, were operated with loads up to 21 tons, and were so located and devised that supplies could be deposited at any point in the spillway open sections, intake towers, the dam and powerhouse. All of the towers, at terminals of the 3-inch diameter track cables, were movable except the one head tower farthest down-stream on the Nevada side. The movable end structures were built of steel and the largest were 32 feet by 46 feet in base plan and 90 feet high.

The towers traveled on parallel tracks and the Nevada and Arizona structures of each of the four upstream cableways moved in unison. The tendency of a tower to overturn was counteracted primarily by concrete blocks, weighing over a million pounds, placed above the rear track, and by raising the front rail of the front track, or by a central truck bearing against an anchored central rail laid with web horizontal. All five cableways were operated from the head towers located on the Nevada side and were controlled by telephonic orders from men in view of the loading or unloading activity. In their few years of use they were required to convey more than eight million tons of supplies and materials.

Low Level Concrete Mixing Plant

Screening and Washing Plant

SCREENING PLANT

Six Companies, Inc., was required to furnish all concrete aggregates of sand, gravel and cobbles, and to secure these materials from a deposit on the Arizona side of the river approximately 10 miles upstream from the dam. For this purpose, the contractor built a sand and gravel screening and washing plant at a three-way junction on its railway system where one line led to the Arizona deposit, another to the bottom of Black Canyon and the third to the U. S. Construction Railroad, which connects Boulder City with the top of the damsite.

Principal features of the plant were a crusher for oversize rock, a series of structural steel towers connected by belt conveyors and equipped with large vibrating screens, a battery of sand washers and classifiers, and a system of stockpiling, reclassifying aggregates and loading into railroad cars. Materials from the deposit were loaded by a 5-cubic yard electric dragline into trains of ten 50-ton dump cars and hauled across seven miles of standard gauge line, including an 800-foot pile trestle bridge over the river, to the screening plant. Here the screens separated the pit run into sand, three sizes of gravel and 3-inch to 9-inch cobbles, and conveyed these classified aggregates to stock-piles. Later they were hauled by train direct to the mixing plants or to storage piles. Some of these latter were near the plant and others are above the high water surface of the reservoir, along the railroad line to the top of the damsite.

The plant was the largest of its type ever built and classified materials at an average rate of 700 tons an hour and a maximum of 1,000 tons an hour. It was shut down in November, 1934 after classifying 7,857,000 tons of aggregates.

Blending Plant and High Level Mixing Plant

MIXING PLANTS

Concrete mixing plants of the contractor were two in number, one of which was located near the bottom of the canyon, 4,000 feet upstream from the dam and the other is situated on the Nevada rim of the canyon 600 feet downstream from the damsite. Both were equipped in the most modern manner with immense storage bins for aggregates, automatic weighing batchers, automatic recorders and several 4-cubic yard mixers. The low level plant was shut down in December, 1934.

The aggregates of sand, gravel and cobbles were transported from the track hoppers to the bins in the low level plant by long belt conveyors, but are dumped directly from the railroad cars into the aggregate bins at the high level plant.

Water was secured from the Colorado River, desilted in the contractor's treatment plant and pumped to storage tanks near each mixing plant.

Cement is purchased by the government from lowest bidders in 350,000 barrel to 1,500,000 barrel contracts and shipped in bulk to a cement blending plant situated on the south side of the aggregate bins at the high level mixing plant. The cement is unloaded and conveyed to the silos of the blending plant by Fuller-Kinyon compressed air pumps, passes through vane feeders beneath the silos to helical conveyors which mixes the cements from each silo and delivers the product to a Fluxo pneumatic pump. The latter machine then transported the cement through a 6-inch pipeline to the silos of the high level plant or through a 9-inch line a mile in length, including a drop of 500 feet, to the low level plant. The flow through this latter line was at the rate of 450 barrels of cement an hour.

The purpose of the blending plant is to combine cements of different chemical characteristics or to mix the same type from various factories to secure a uniform product.

When a mixing plant was placed in operation the aggregates, water and cement were fed first into butchers which automatically filled to the designated weight, then passed to a mixer hopper and into the mixer. After a 2-1/2—minute mixing period the resultant concrete was dumped into buckets and transported to the pouring site. The weights of cement, water and aggregates, and the relative amount of water (consistency) of each concrete batch were automatically recorded.

The low level plant contained three 4-cubic yard mixers and the high level plant five of the same size, making possible the production of 32 cubic yards (64 tons) of concrete in 3 minutes, or a theoretical daily capacity of 13,150 cubic yards. If mixed for the dam this volume of concrete would require 44 cars of cement, 418 cars of aggregates, and 350,000 gallons of water. The actual maximum output in one day has been in excess of 10,000 cubic yards.