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were specially constructed timber cribs, built to fit the river bed topography. The base section of the cribs, 90 by 64 feet in plan, was made up on shore in sections 15 feet high, and floated into position for sinking by adding gravel to ballast compartments. As the base settled the cribs were built up until the top timbers were at least 10 feet above the river surface.

The principal difference between the upstream and downstream cofferdams was the temporary sluiceways left in the downstream arm which permitted passage of the river flow until all cribs were in place. The openings, 32 feet wide in each 64-foot crib face, were closed by stoplogs to raise the river sufficiently to enter the west abutment area and flood the low blocks in the concrete base. Additional cribs, subsequently placed on top in two lifts, were 68 feet and 36.5 feet, respectively, in the direction of stream flow. The lifts were both 20 feet for the downstream cofferdam, making the top elevation 990; while for the upstream cofferdam the lifts were 20 and 30 feet, carrying the top to elevation 1,000. Heavy timber stringers spanning the cribs tied them together and provided support for trucks engaged in hauling fill material.

Revolving cranes were moved out on the first cribs completed from the shore end to handle timbers for subsequent cribs built in place, excavate trenches for the piling, drive the piling, and fill the cribs with gravel. Steel sheet-piling was driven in 32-foot arcs on the river face of both cofferdams, and connected at the ends of the arcs to timbers projecting from the cribs. The fill was hauled from shore by trucks and dumped inside the piling ahead of placing the material for the 3:1 protective slopes along both sides of the cribs. Additional protection for the ends of the fill was afforded by floating large cribs into position for filling and sinking near the junctions of the newly completed cofferdams and the cell clusters. In constructing the cross-river coffers, about 8,000,000 board feet of timber were used, principally 12- by 12-inch sizes for crib construction, and about 2,200 tons of steel sheet-piling.

EXCAVATION

Excavation and disposal of foundation overburden has been one of the major items of the construction program. Under the original contract it was estimated that 11,000,000 cubic yards of overburden and 800,000 cubic yards of rock would be removed from the foundation area of the dam and power-house. Occurrence of slides in the vicinity of the west abutment, together with the 2,000,000 yards added by excavation of the west power-plant tailrace, have increased the overburden excavation to approximately 15,000,000 cubic yards. Present indications are that the original estimate of 800,000 cubic yards of rock will not be materially changed.

The absence of suitable areas close to the dam site, suffi

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West concrete mixing plant, November 15, 1936.

cicntly large to serve as waste dumps for the excavated material, resulted in putting into operation an elaborate belt conveyor system. The most feasible dump location available for use within several miles was Rattlesnake Canyon, located on the west bank of the river about 1 mile south of the west abutment. A 500-foot difference in elevation between the canyon rim and construction site, together with the bordering cliffs, combined to make the proposed dump area inaccessible for ordinary spoil transportation facilities.

Approximately 12,000,000 yards of the total foundation excavation were in the west abutment area which became the location for the tributary feeders. The excavating equipment consisted of power shovels, varying in capacity from 2 to 5 yards, and fleets of dump trucks and various types of tractor-drawn trailers varying in capacities from 12 to 20 cubic yards. All material was excavated and hauled to four feeder units and dumped on a grillage of steel beams at ground surface.

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Spoil passing through the bars dropped to an apron feeder below, which distributed the irregular supply from the pit at a more uniform rate for the tributary feeders. The feeder belt converged at the hub, which was the control point for the tributary system. A surge hopper and feeder at the hub protected the main conveyor from overloading by smoothing out the surges resulting from several feeder belts. When incoming material began piling up at the surge feeder, all tributary belts were stopped until the overload disappeared.

The main conveyor consisted of approximately 20 conveyor units, varying in length between 156 and 415 feet, depending on the slope. Transfer of material was made directly from the end of one belt to another, the change necessitating a loss in elevation of 6 to 8 feet at each transfer point. The length of the main line was approximately 5,000 feet and the aggregate length of the four tributary lines was 1,500 feet.

The stacker was responsible for the successful operation of the entire system, as it permitted delivery of spoil at the dump without loss of time for changing positions of the discharge end. It consisted of an extensible unit which could be lengthened by adding 50-foot sections; a telescopic unit having a longitudinal travel of 50 feet; and a 175-foot cantilever boom. The three units handled the material in the order named. The boom was pivoted at the loading end and supported at an intermediate point on tractor treads which traveled through an arc of 180°. Both the forward speed of the telescopic unit and the radial movement of the boom were controlled from the operator's station above the tractor.

The conveyor was designed to transport 2,500 cubic yards per hour. The best 7-hour shift record was 17,478 yards and the maximum performance for a three-shift period was 50,700 yards, both amounts representing bank measurement.

The necessity for rapid filling of the cells for the west cofferdam to avoid high water was responsible for abandoning the use of trucks and developing a shuttle conveyor. The excavated material from the cofferdam area was transported by belt conveyor to a hopper located midway of the row of cells and about 200 feet back from the river face. The hopper straddled a track 1,600 feet long, parallel with the cells, which served as a runway for an 870-foot traveling conveyor. The track length permitted an 800-foot travel in either direction with some portion of the belt always under the hopper. Two 200-foot boom arms, supported near the discharge ends on traveling frames, were mounted at each end of the traveling conveyor and discharged the material directly into the cells. Placement of approximately 275,000 cubic yards of cell fill and 125,000 cubic yards of protective embankment at the toe and behind the cells was completed in advance of the 1935 flood season.

AGGREGATE

An extensive deposit of aggregate, known as Brett pit, was located on the high mesa east of the river, about \% miles northeast of the dam site. Explorations showed the deposit to be of glacial origin, 300 feet deep, and covering approximately 70 acres. More than ample supply for the dam and appurtenant works was contained in the pit, in all sizes required for manufacturing concrete. Four sizes of coarse aggregate are used in the mass concrete, ranging from a maximum of 6 inches in diameter to one-quarter inch in diameter, comprising about 73 percent as compared to 27 percent sand. An excessive amount of fine material results in disposing of approximately half of the volume excavated. Stripping the pit to a depth of 1 to 3 feet has resulted in the removal of over 500,000 cubic yards of overburden.

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The location of the pit is 900 feet above the river, a factor which influenced the design and construction of a plant and conveyor system, incorporating the advantages of gravity in the preparation and transportation of the aggregate supply. Two 4-yard electric shovels load directly into hoppers over the ends of the 200-foot boom conveyors which were used to fill the coffer cells. Crawler treads, supporting the boom, move the hopper through an arc of 180°, having a radius of 200 feet. The additional 30-foot swing of the shovel results in a cut 460 feet wide, varying in depth between 40 and 100 feet. Radial flexibility results from mounting the discharge end of the boom conveyor on a track frame, supported by rails along both sides of the end conveyor section. This arrangement for telescopic action, together with the provision for extending the belt line in 50-foot sections, keeps the hopper within loading distance as the shovel advances.

Boulders larger than 16 inches in diameter are rejected by a grillage over the boom hopper, while the balance of the material is discharged on a lateral conveyor line feeding a main 60-inch collecting conveyor belt having a capacity of 2,500 tons per hour. The main line terminates in a trestle from which the raw material is stock piled, to move successively through feeder galleries and vibrating feeders to a 60-inch belt running to a crusher house. Oversize rock is reduced to a 6-inch maximum diameter and combined with the smaller aggregate which bypasses the crusher

and is conveyed to a balancing pile. Two feeder galleries under the toe of the pile serve a belt leading to the screening and washing plant, which is a five-story, steel frame building. Each half of the plant, with identical installations of equipment, is capable of handling the incoming load of 1,250 tons per hour.

The screening and washing operations consist of spraying water from nozzles located over the various screens as the different sizes of aggregate are separated. Sizes less than ^-inch diameter remain as a result of the screening processes by which the coarser aggregates are removed first and dumped into chutes leading to the conveyor room for delivery to the plant stock pile. The fine material and wash water is carried by four steel flumes to the dewatering tanks. The coarse sand settles into compartments and is mechanically raked out and deposited on conveyor belts leading to the sand classifiers, while the balance of the material is sent to the waste dump. The classifiers separate three grades of coarse sand which go to drainage bins for blending. The remaining mixture is returned to the clarifiers where the sludge is removed and the reclaimed water goes to a storage tank. Use of clarifiers results in reclaiming about 85 percent of the 20,000 gallons per minute used in the plant operations. The plant capacity is 1,250 tons of finished aggregate per hour.

A 48-inch conveyor belt transports the aggregates from the plant storage a distance of 4,000 feet to the live storage yard near the east end of the dam. Selection of various aggregates from plant storage is electrically operated by remote control from the live storage yard. Incoming aggregate is unloaded from boom arms extending laterally from an airplane-type tripper which travels on rails between two stock piles. Hinged booms permit elevating or lowering the discharge ends to reduce breakage of aggregates. Storage yard capacity of 77,000 cubic yards, plus aggregate plant storage of 13,000 cubic yards, is sufficient quantity to provide both mixing plants with a 3-day run. Aggregate from one row of stockpiles is delivered by conveyor belt to storage bins of the east mixing plant, while the west plant was supplied from the other row by a 36-inch belt which crossed the river on a three-tower steel suspension bridge, which also supported a steel pipe conveying cast-bound cement under pneumatic pressure to the east mixing plant.

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CEMENT

Cement storage is provided by eight 5,000-barrel steel silos in addition to two identical silos for storing the blended cement which is pumped to the mixing plants through steel pipe lines. The pipe line supplying the west plant is 2,000 feet long, and the one crossing the river to the east plant was 6,000 feet long.

Each mixing plant, constructed of structural steel except the reinforced concrete supporting columns and mixing floor, is octagonal in plan, having a diameter of 42 feet and a height of 102 feet. The top of the plant is devoted to aggregate and cement storage space. The floor below the bins is occupied by eight weighing batchers, one for each size of aggregate, two for cement and one for water, the mechanism of which is operated by air pressure and electrically controlled. Ingredients for one mixer are contained in the batchers which discharge simultaneously into a collecting cone. A battery of four 4-yard tilting mixers on the

lower floor are charged successively by the cone through a retractable chute which charges the mixer through the front opening while in the mixing position. The mixers discharge by tilting downward through an arc of 75° from a mixing position of 15° above horizontal.

CONCRETE

Two construction trestles, parallel to the axis of the dam, were used to transport the concrete from the plants. The high trestle, at elevation 1,024, is 93 feet downstream, while the trestle at elevation 950 is 328 feet downstream. The trestles will be extended as construction progresses until both are continuous between the east and west plants. When concrete is dispatched to the low trestle a concrete chute connects the mixer with a 4-yard skip which discharges into a hopper for the concrete buckets on cars at the lower level. Concrete for the high trestle is discharged directly from the hopper underneath the mixers into the 4-yard buckets mounted on flat cars. Traveling cranes, with rigid truss arms extending laterally from the trestle, and auxiliary revolving cranes, transfer the bottom-dump buckets of concrete from the cars to the placing areas.

Preparations for depositing concrete in the various blocks of the dam are made by setting forms for a 5-foot lift and installing all items to be embedded in the concrete. The buckets are lowered into place from the trestles and tripped over the point where the batch is to be placed. By means of electric vibrators and spading the concrete is compacted and leveled into layers approximately 1-foot thick, until the block is carried up to the top of the forms. After 24 hours, construction of the next form is begun, and placing of concrete is permitted after an elapse of 72 hours.

Government inspectors maintain control over concrete production and placing. Aggregates and cements are tested individually, in addition to testing numerous samples of concrete taken from the mixers. Samples cured for various periods are tested in a 200,000-pound compression machine operated in connection with the testing laboratory near the construction site.

BIBLIOGRAPHY

Report on Columbia Basin Irrigation Project. Engineering News-Record, Sept. 24, 1925.

Planning the Columbia Basin Development. Reclamation Era, July 1927.

Columbia Basin Project Report Shows Feasibility. Reclamation Era, March 1932.

Preliminary Work Starts on Coulee Dam. Pacific Builder and Engineer, Sept. 2, 1933.

Grand Coulee Dam and Power Plant on Columbia River. Western Construction News, issues of April, May, and June 1934.

World's Largest Construction Conveyor Speeds Excavation at Grand Coulee. Western Construction News, March 1935.

Cofferdam, 3,000 Feet Long Built at Grand Coulee. Western Construction News, June 1935.

Grand Coulee Project and Dam: Ten Months' Construction Progress; Constructing the First Cofferdam: I— Planning for Excavation Disposal; II—The Belt Convey

ors in Operation; III—Designing the Belt System; Making Aggregate at Grand Coulee. Engineering NewsRecord. Aug. 1, 1935.

Preparing Millions of Yards of Aggregate for Grand Coulee Dam. Western Construction News, November 1935.

Grand Coulee Dam Concreting Plant is Notable for Design Efficiency. Western Construction News, February 1936.

Aggregate Production for Grand Coulee Dam. Reclamation Era, June 1936.

Details of Concreting Procedure at Grand Coulee Dam. Western Construction News, July 1936.

Program for Grand Coulee's Second Cofferdam. Engineering News-Record, Oct. 1, 1936.

Pour 7,000 yards of Concrete Daily at Grand Coulee. Southwest Builder and Contractor, October 23, 1936.

Diverting the Columbia at Grand Coulee With Timber Cribs and Gravel Fills. Western Construction News, December 1936.

Experiments Aid in Design at Grand Coulee. Civil Engineering, November 1936.

Columbia Basin and Grand Coulee Project. Civil Engineering, September 1934.

Foundation Conditions for Grand Coulee and Bonneville Projects. Civil Engineering, February 1935.

Aggregate Transported Across Columbia by Long Suspension Belt. Engineering News-Record, Nov. 14, 1935.

Concrete Mixing and Placing. Engineering News-Record, Jan. 23, 1936.

Handles 2,500 Tons per Hour: Wastes 1,000 to 1,500 Tons of Sand. Rock Products, March 1936.

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