TYPE AND SIZE OF DAM

The Grand Coulee Dam is of the straight-gravity type, depending entirely upon the weight of the structure to resist the pressure of water behind it, tending to overturn it or to cause it to slide on its base. The river canyon is too wide for a dam of the arch type. At each side of the 1,650-foot centrally located spillway section, which is surmounted by control gates spanned by concrete arch bridges, will be a power-house and abutment section, each more than a thousand feet long.

The finished dam will be 4,300 feet long at the crest and about 3,000 feet long at the base. The base is 500 feet wide, and covers about 30 acres. The dam will be 30 feet thick at the crust, and will be surmounted by a 30-foot highway. From lowest bedrock, the height will be 550 feet to the crown of the roadway; and the water surface above the dam will be raised about 355 feet above low water-level. Galleries in the dam for inspection, gate control, cooling, grouting, drainage, and other purposes will have a combined length of about 8 miles.

(Top, left): Activity at the Grand Coulee Dam was at its peak in the summer of 1937 with more than 7,000 men at work. The base of the dam was nearing completion when the above view, looking west into the Grand Coulee, was taken. (Top, right): The finished dam as it will appear from the same viewpoint.

ELEVATIONS ON THE PROJECT

The river bed at the side of the Grand Coulee Dam is approximately 910 feet above sea level. The low-water elevation in the river is about 933 feet, and the average high-water level about 978. Bedrock, under a deposit of clay and boulders farming the river bed, was found generally at about elevation 875, but three deep gorges, one extending to elevation 761.5, were found.

The general elevation of the floor of the Grand Coulee is about 1,500, and the walls of the coulee about 2,300 feet above sea level. The lands to be irrigated vary in elevation from about 1,300 near Ephrata to 400 feet near Pasco.

The top of the dam will be at elevation 1,311.08, and the parapet at elevation 1,315. The crest of the concrete in the spillway will be at elevation 1,260, and the tops of the control gates at 1,288.

Aerial view of Grand Coulee Dam, looking northeast over the construction area, the Government town. Coulee Dam (left), and the contractor's town, Mason City, in April 1937. (15th Photo. Sec., U. S. Army)

COMPARATIVE SIZE

The Grand Coulee Dam will be the largest in volume not the second highest of the masonry dams of the world, second in height to only the 726.4-foot Boulder Dam. Even the base of the dam, covered by the Mason-Walsh-Atkinson-Kier contract, completed in the winter of 1937-30, was the biggest man-made structure on earth, far surpassing the great pyramid of Cheops, which for thirty centuries was man's biggest structure, until the Boulder Dam was built. The finished dam will occupy more space than the entire population of the United States—men, women, and children—and will weigh more than twice as much.

Only three families lived in the vicinity of Seaton's Ferry when excavating began at the site of the Grand Coulee Dam late in 1933

VOLUME OF CONCRETE

Nearly 12 million cubic yards of concrete, more than three times that required for the Boulder Dam, will be used in constructing the dams, power plant, and appurtenant works—sufficient to build a monument as high as the Washington Monument and covering six average city blocks.

With the some quantity of concrete a 20-foot pavement could be built about a quarter of the way around the earth, or two times from coast to coast. It would make a pyramid two blocks square and eight blocks high, more than three times the volume of the great pyramid of Cheops.

World's largest cofferdam, 300 feet long, enclosing 60 acres, completed, and the Government camp under construction, 1935

QUANTITY OF CEMENT

About 12 million barrels of cement will be used at the dam, a total of nearly 4 billion pounds or about 48,000 carloads. The daily use has exceeded 60 carloads, and a total of about 960 trainloads will be required. The making of the cement will require the quarrying, crushing, and grinding to a fineness exceeding that of flour, of about 3 million tons of limestone, and the consumption of enormous quantities of power, supplies, refractories, and fuel. The cement, ground into particles less than one four-hundredth of an inch in size, will expose a surface of over 150,000 square miles, an area more than three times that of the State of Pennsylvania.

SPILLWAY GATES, AND OUTLETS

Between the power-house sections at the ends of the dam is a spillway section 1,650 feet long over which water not required for storage or for power generation or irrigation will be allowed to flow, forming a spectacular waterfall twice as high as Niagara. The rate of flow, and to a certain extent the quantity of water held in storage, will be controlled by 11 drum gates at the crest of the spillway each gate 28 feet high and 135 feet long.

Columbia River diverted through base of dam built inside west cofferdam. Excavation under way in old channel, 1937

The spillway will have a capacity of a million cubic feet a second; and, if that capacity should ever be realized, it will be necessary to dissipate at the foot of the dam the energy of the falling water equivalent to 32 million horsepower. This will be accomplished, and erosion of the river below the dam will be prevented, by an upwardly curved bucket at the toe of the dam, where a trough 100 feet wide and 30 feet deep is formed behind a concrete wall across the river bed at elevation 900, 33 feet below low tail-water surface.

Through the dam there will be sixty 8-1/2-foot gate-controlled outlet tunnels, twenty at elevation 934, the approximate level of low water, twenty at elevation 1,034, and twenty at elevation 1,134. The tunnels are arranged in pairs, and their entrances are protected by trashracks. The upstream ends of the outlet tunnels are lined with heavily ribbed semi-steel conduits, set in the concrete as protection against erosion and the effects of cavitation, which will be reduced or eliminated by the scientific shaping of the entrances to the tunnels. A ring-follower sliding-leaf valve, hydraulically operated, and an electrically operated "paradox" valve, with leaf and wedge on rollers, will control the flow of water so each tunnel for the purpose of regulating the flow of the river in seasons of low water, or emptying the storage reservoir. The 60 tunnels have a combined length of about 2-1/2 miles.

The outlet tunnels will have a capacity of 253,000 cubic feet per second and the turbines fully loaded will pass 81,000 second-feet. These, with the spillway will have a total capacity nearly three times the maximum recorded flow of the river, and nearly double the estimated flood of 1894.

Foundation of dam completed, cofferdams removed, and the Columbia River flowing through low gaps in the base of the dam, 1938

FOUNDATION EXPLORATION

Diamond drill holes to the extent of about 33,000 feet were put down into the granite foundation on which the dam rests. Occasional holes were drilled to depths varying from 660 to 880 feet. In all instances there was found light-colored, dense granite, suitable, according to the board of consulting engineers, to bear any load that might be put upon it.

After uncovering bedrock, additional exploratory work was done with Calyx drills, extracting rock cores 36 inches in diameter, which permits a detailed examination to be made of both the core and the hole from which it was taken. Eighteen such holes were drilled to depths varying from 29 to 68 feet.

Bedrock is quite uniform in character, and in general was prepared for foundations by the removal of weathered surface rock to a depth of 6 to 10 feet.

A 10-million-yard hole on the west bank to make room for dam, forebay, and tailbay

FOUNDATION GROUTING

To seal the cracks and crevices in the granite bedrock under the dam, formed millions of years ago when the molten rock solidified and shrunk, grout of cement and water is forced down through holes drilled into the rock. Of such holes, 30 feet deep and 20 feet apart, five rows were drilled under the upstream edge of the dam and entirely across the river canyon. The rows were spaced 20 feet apart, and the holes in each row are staggered with respect to those in the next row. The 30-foot holes were grouted under pressures up to 250 pounds per square inch before any concrete was placed close to them.

After considerable concrete was in place on bedrock, a row of holes 75 feet deep and spaced 20 feet apart was drilled diagonally downward into the rook under the dam through the curved fillet of concrete which connects the upstream face of the dam with bedrock. An effectual seal against leakage under the dam will be created by the grouting of these holes and a single row of holes 150 feet to 200 feet deep, to be spaced 10 feet apart and drilled from the drainage gallery in the dam close to bedrock. Grouting pressures in the deep holes may run as high as 1,000 pounds per square inch.

As an added precaution against the uplifting effect of any leakage under the upstream edge of the dam, there will be one row of uplift pressure relief holes spaced 10 feet apart, permanently open into the drainage gallery at the base of the dam.

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