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tions of loading, including reservoir empty, normal full reservoir operation, and the occurrence of earthquake vibrations during both empty and full conditions. Uplift pressures were assumed to vary as a straight line from full reservoir head at the upstream face to zero or tailwater head at the downstream face, and to be applied over twothirds the horizontal areas of the sections analyzed. In addition to the usual gravity analysis, trial-load twist analyses of the dam on the basis of grouted and ungrouted vertical joints were made for full load operation. Model investigations were made on a cantilever section and on a small reproduction of the entire dam, using plaster-celite materials in constructing both models.

Results obtained from the model investigations and the trial-load twist analysis indicate that in the central portion of the length, the horizontal loads are carried by gravity action through the vertical elements to the foundation; and that the stresses and stability factors are therefore

equal to those computed from the gravity analysis. In the vicinity of the abutments, part of the horizontal load is distributed to the foundation and the remainder is carried horizontally by the twisted structure. The effect of this distribution is to increase the upstream stresses computed from the gravity analysis in the vicinity of the abutments, and to decrease the downstream stresses at the same locations.

Existence of twist in the locality of abrupt change between the planes of the rock floor and abutments has been recognized, and provisions have been made in the design to safeguard the stability of the high structure. Transverse slots, three on the cast side, and two on the west side, each 6 feet wide, extending the full height of the dam, will be constructed at intervals in the vicinity of each abutment where the effect of twist is most pronounced. Heavily reinforced concrete walls will be cantilevered longitudinally across the ends of the openings with their exterior faces forming the upstream and downstream faces of the dam. Three continuous metal strips installed across the 1-inch joint between the ends of the cantilcvered walls and the adjacent section of the dam will permit independent deflection between the two sections thus connected. The slots will be filled with sand prior to grouting all contraction joints. When the deflections occur, as a result of filling the reservoir, the sand will be removed by jotting, and the slots filled with concrete.

SPILLWAY

The spillway is designed to discharge 1,000,000 secondfeet with an effective head on the crest of 30.5 feet. The overflow section, having a gross length of 1,650 feet, will be separated from the abutment sections by training walls, which parallel the downstream face above elevation 980, the top elevation of the massive walls extending downstream beyond the toe. The spillway will consist of eleven 135foot openings, separated by ten 15-foot concrete piers. A structural steel drum gate, 28 feet high and 135 feet long, will be mounted on the crest of each opening to regulate the reservoir level between spillway crest elevation 1,260 and the raised position of the gates at elevation 1,288.0.

Gate regulation is effected hydraulically by filling the chamber with water to raise the gate, or by emptying the chamber to lower the gate, both operations being performed automatically by a control mechanism which will raise or lower the gates through a predetermined range to correspond with an increase or decrease in the reservoir water surface elevation. Complete closure of the gates from the extreme upward position to obtain the maximum discharge is manually controlled. The gate operating chambers and controls will be located in the piers near the upstream face and interconnected by a longitudinal operating gallery.

The movable gate will weigh 7,400 pounds per lineal foot, and for the full length of 135 feet, will total 1,000,000 pounds. The movable gate is hinged from the upstream cantilever, and the massive castings with anchorages and pier plates weigh an additional 287,000 pounds per gate. The control mechanism, complete with all piping and miscellaneous metal work, adds 180,000 pounds, resulting in a total weight of 1,467,000 pounds for each gate and 16,137,000 pounds for the 11 gates required to regulate the spillway discharge.

T he importance of obtaining a crest with a high coefficient of discharge, and methods of dissipating the energy of the descending sheet of water to avoid disastrous effects from scouring, led to numerous studies and experiments. Since model tests had already satisfactorily solved problems in connection with several other dams, a series of studies simulating conditions at Grand Coulee were inaugurated. Four models varying in scale from 1 :184 to 1 :15 were tested to overcome the important effect of scouring at the toe.

The tests were responsible for eliminating the dentated sill which has been successfully used on smaller dams, in favor of a continuous sill which forms the lip of the spillway bucket. Various slopes on the upstream face of the lip had a marked effect on the behavior of the jet and a 1 :1 slope was finally adopted. Likewise the tests showed the bucket having a 50-foot radius was more effective in governing the dissipation of energy than was the 30-foot bucket. All of the tests gave evidence that a curved bucket, placed at a low elevation, will provide the most effective cushion for the high velocity jet. Lowering the bucket and increasing the effective tailwater depth solved the problem of a transverse wave which was observed on all models when the dentated lip was not used.

The spillway crest design was adopted as a result of model studies which began with a 1 : 30 model of the upstream slope of the crest approaching a sharp-crested weir. Measurements of the lower nappe of the jet checked very closely with a theoretical trajectory previously calculated. A crest embodying the elements of the trajectory was used to obtain pressure measurements which indicated the presence of negative pressures during various discharges. The crest was revised to eliminate this feature which subsequent measurements proved has been accomplished.

OUTLET WORKS

Thirty pairs of outlet conduits, arranged in groups of 10 pairs at each of three elevations which differ by 100 feet, will be installed in the spillway section to regulate outflow during periods of normal operation. The openings will be 8.5 feet in diameter, and the discharge will be controlled by a motor-operated, high-pressure paradox gate and a hydraulically operated, ring-follower emergency gate, installed in tandem near the upstream end of each conduit. The operating mechanism for each pair of conduits, comprising four gates, will be housed in a gate chamber immediately above the gates.

The slope of the conduits approximates the path of a jet having been designed as a parabolic curve. In plan the conduits paired in the same block separate from each other on horizontal curves as they approach the downstream face. This arrangement results in a more uniform distribution of the discharging jets over the downstream area, and also permits the use of a single trashrack structure and gate operating chamber to serve two outlets.

One trashrack structure, semicircular in plan, will form an enclosure for the three pairs of outlets located in the same tier of blocks. The lower pair of conduits are located with their center lines at elevation 934.08 and the other two pairs will eventually be installed directly above at 100-foot intervals.

The upstream sections of the 8.5-foot diameter outlets consist of a circular opening with a heavy metal conduit lining for a distance of 51 feet from the upstream face of the dam. The gates are included within this length. Beyond this point the outlets are heavily reinforced circular openings formed in the mass concrete.

The gates are designed to operate under a maximum head of 250 feet, which will result in a maximum discharge of 275,000 second-feet with all gates open and the reservoir water surface at elevation 1,184. Above this elevation the gates of the lower tier will be closed and the discharge correspondingly reduced. The lower outlets will be completed and the gates and lining installed under the present contract. The two gates for one outlet will weigh 384,000 pounds, and the conduit lining will weigh an additional 143,500 pounds, making a total weight of 527,500 pounds for one complete gate assembly and lining. The 20 sets of gates and liners installed under the first contract will weigh 10,550,000 pounds. The intermediate tier of gates, at elevation 1,034, will operate under a maximum head of 250 feet and correspond physically with the lower gates. The upper tier of gates at elevation 1,134 will operate under a head of 156 feet with the reservoir water surface at elevation 1,290.

POWER PLANT

The foundations for both east and west powerhouses were constructed under the M. W. A. K. companies' contract for the foundation section of the dam. Only the left powerhouse on the west side of the river will be constructed under the contract for completion of the dam, but all the penstocks will be installed. Provision for the 18-foot diameter penstocks was made by leaving 24-foot octagonal openings through the dam. The initial power development will consist of the installation of three 105,000 kilowatt main generating units and two 12,500 kilovolt-ampere stationservice generating units.

The ultimate power plant will consist of two structures, one on either side of the spillway adjacent to the downstream face of the dam. Each structure will house nine main generating units, spaced on 65-foot centers with an erection bay, 86 feet 8 inches long, at the abutment end of each plant. The control building for the plant at the east abutment will be 65 feet long, and for the west plant will be 86 feet 8 inches long. Three station service generating units will be placed in the west powerhouse, together with offices and general station-operating facilities. The total length of the two powerhouses at the east and west abutments will be 736 feet 8 inches, and 758 feet 4 inches, respectively.

The generating rooms will be 80 feet wide and will be spanned by two 375-ton traveling cranes for handling equipment. The transformers will occupy a deck extending from the face of the dam to the powerhouse wall. Beneath the deck, space is allotted for galleries to house

electrical equipment; while downstream from the generating room and below the draft tube operating bridge, the gallery space will be used for housing mechanical equipments In addition, space will be provided for station service transformer vaults, oil storage and purifying equipment, and shop facilities for minor repairs. The vertical distance from the floor of the draft tube to the top of the roof will be 185 feet. The powerhouses at opposite ends of the spillway section will be connected by a 12-foot diameter control tunnel extending longitudinally through the dam.

The main generating units will each have a capacity of 105,000 kilowatts at 120 revolutions per minute. The generators will be 40 feet in diameter and 36 feet high above floor level. The total net weight of each generator will be 2,100,000 pounds and the heaviest part will weigh 1,000,000 pounds. Generators and turbines will be connected by a vertical shaft, 42 inches in diameter.

The hydraulic turbines will operate under a maximum head of 360 feet. They are designed to produce 150,000 horsepower at 120 revolutions per minute with a 335-foot operating head. The draft tubes will have a throat diameter of 15.5 feet and will discharge 4,500 second-feet. The scroll case will be 52 feet in width, measured longitudinally along the axis of the unit, and the weight of the complete unit will approximate 1,500,000 pounds. The turbines will be supplied by 18-foot diameter penstocks which will be controlled by gates at the upstream face of the dam. The penstock intakes will be enclosed by individual trashrack structures.

The three station service generating units installed in the west plant will supply all power for the power plant and pumping plant requirements. The primary distribution at 6,600 volts will be reduced to 440 volts for station service. In addition to the electricity necessary for lighting and motor operation, 2,500,000 watts will be required for space heaters in the plants.

PUMPING PLANT

A requisite for development of the 1,200,000 acres of irrigable lands comprising the Columbia Basin Project, is a pumping plant to lift water from the reservoir formed by' Grand Coulee Dam, to the regulatory reservoir, formed by constructing earth and rockfill dams across the upper Coulee.

Near the west abutment the axis of the dam turns upstream at an angle. The pumping plant will be constructed on the downstream face of this section of the dam and will be about 585 feet long, 117.5 feet wide, and 130 feet high. Provision is made for 12 pumping units, spaced on 45-foot centers, and a machine shop constructed above the first 5 pumping units. The machinery will be handled by a 300ton gantry crane.

The motor-operated centrifugal type pumps will each have a capacity of 1,600 second-feet when pumping against a head of 295 feet. Lowering the reservoir water surface to elevation 1,208, will result in pumping against a maximum head of 367 feet. The pumps will be located at elevation 1,205, and will be supplied by intakes protected by trashracks and slide gates. The throat diameter of the pumps will be 12 feet, and the discharge will be conveyed through 12-foot diameter discharge pipes, approximately 680 feet long, to the high level feeder canal which will supply Grand Coulee Reservoir.

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The pumps will be driven by twelve 62,500-horsepower motors, through vertical shafts. Electric current for the 12 motors will be furnished by the six generators adjacent to the left abutment, and connection between the powerhouse and pumping plant will be through a bus gallery which will penetrate the left abutment. The unusual size of the pumps

has resulted in substituting a novel method for starting, in lieu of expensive starting equipment which would otherwise be required. Generators and pump motors will be connected through the main busses, and each will be separately excited by motor-driven exciters operated from the station service units. As the fields of the generators and motors are excited independently, the turbines are started, which in turn will start the generators and motors from rest and bring them up to speed simultaneously.

CONSTRUCTION

The diversion scheme provided for construction of parallel cofferdams through the foundation area, forming a waterway 720 feet wide. Beginning late in 1934, the first cofferdam was completed prior to the flood season of 1935.

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The west cofferdam extended 1,800 feet parallel to the river with the upstream and downstream ends curved back to higher ground. The wall included a permanent 50-foot tier of concrete blocks across the dam location, steel sheetpiling cells filled with overburden, and end sections made up of single lines of piling with timber backing. The total length of 3,000 feet required the use of 13,000 tons of steel sheet-piling having an aggregate length of 667.000 lineal feet.

The greatest obstacle to rapid driving was the coarse river drift overlying a layer of very compact glacial silt. Gravel and boulders were dredged to an average depth of 20 feet along the location for the wall, which necessitated considerable underwater blasting of the largest boulders. The interlocking piling cells were closed before driving was started. The material proved so resistant to driving that

batteries of four steam hammers were mounted on tower gantry frames which were supported on trestles along either side of the cells. The hammers were adjustable to various positions which permitted driving any of the piles in the area beneath the gantry. The average penetration was about 40 feet, but failed to reach bedrock along the river wall. Driving was discontinued when fifty 13,100 footpound strokes failed to secure one inch penetration. Thirty steam hammers in operation on the cofferdam, resulted in completing the driving in 100 days.

Three sizes of cells were used in the first cofferdam. The intermediate section of the wall traversing the dam foundation and the adjacent curve of the upstream shore arm were made up of cells, 40 by 50 feet in plan, with the faces curved on a 40-foot radius. At each end of the intermediate wall a cluster of cells was built to serve as connections between adjoining sectors of the left wall and later as a connection for the cross-river cofferdams. In plan the cell clusters were formed by a series of arcs having radii of 40 feet making an enclosure which was divided by steel piling diaphragms to elevation 965, above which they were replaced by tie rods. The third size of cell was used downstream from the cell cluster near the toe of the dam. Excavation of the tailrace inside the wall excluded the use of a bcrm for stability. As a consequence the cells were made 90 by 36 feet with an arc of 36-foot radius at each face. The west cofferdam was built to elevation 990.

The second cofferdam, forming an enclosure 1,700 feet long along the cast abutment area, was constructed of piling and a timber backwall and the intervening space filled with earth. It was built after the flood season of 1935, to protect the excavation work until the river surface reached elevation 965. Although the wall was flooded during the 1936 season, it enabled the contractor to occupy the area, except during the period when the flood was above the top of the cofferdam and while the east pit was being unwatered.

The second major diversion involved turning the river across the concrete foundation at the left abutment by extending cofferdams diagonally across the stream from the opposite shore. The downstream cofferdam was constructed from a point on the eastern shore to the cell cluster near the toe of the concrete base; and the upstream arm was constructed to the cluster at the upper end of the west cofferdam, from a point upstream on the opposite river bank. Preliminary work consisted of removing the shore arms of the west cofferdam after the diversion channel was excavated to elevation 920. Removal was accomplished by excavating the outside berms and sluicing the fill through openings burned in the cell walls to expose the timber and bracing for dismantling.

Both cross-river cofferdams were started from the eastern shore with a shore arm section consisting of steel sheet-piling corewalls with timber backing. The intermediate section was made up of cribs, built in place, and the western ends

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