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trenches, and a geophysical survey. The total length of the diamond drill holes exceeds 7 miles and the core samples provided early information that the bedrock possesses adequate bearing capacity.
The foundation and abutment material is composed of granite, ranging from a massive or coarse-grained type on the right side to a fine-grained porphyritic type approaching the left abutment. The rock has proved to be hard and sound and well suited to resist the pressures transmitted by the dam. A heavy overburden of silt, with occasional strata of sand or gravel and an upper layer of river drift containing boulders, gravel, and sand in various proportions, covered the bedrock to depths ranging from 20 to 150 feet.
Grand Coulee begins a short distance upstream from the left abutment of the dam and extends more than 50 miles along an irregular course in a southwesterly direction, varying in width from 2 to 5 miles. The rock formation consists of an underlying granite floor which was subsequently covered by successive lava flows. The gorge was eroded when an advancing glacier diverted the Columbia River from its natural course to cut a new outlet at a much higher elevation through the layers of lava which are more than 600 feet in height above the coulee floor. When the glacier
receded, the Columbia returned to its original course, leaving the dry channel 600 feet above river level.
The ancient granite is exposed for a short distance entirely across the coulee floor in addition to a considerable length of margin near the upper end of the basin. The major portions of the floor are covered with variable depths of fine grained silt which has been most heavily deposited by wind action against the east wall. The west side of the floor is strewn with numerous eroded mounds resulting from glacial deposits or broken ledges which reach massive proportions. The base of the west wall is covered to a considerable depth by talus slopes formed by an accumulation of broken ledge fragments. The upper part of the Grand Coulee gorge will be used for a regulatory reservoir in connection with pumping operations by constructing earth and rockfill dams across the north and south ends, approximately 23 miles apart.
PRESENT CONSTRUCTION SCHEDULE
The present construction program is a modification of the one initiated in 1933 because it is better suited to the extensive development of the Columbia River which is now accepted as the ultimate project. Although the initial development included only the lower section of the dam, Congress has authorized and appropriated funds for its completion. The contract for completion of the dam, the west side powerhouse, and the foundation of the pumping plant was awarded in January 1938 to the Consolidated Builders, Inc., on their bid of 834,442,240. The M. W. A. K. contract was then being completed. The contract will require that the dam be completed early in 1942.
Originally the dam was designed as a complete power structure, with provisions for subsequent enlargement to a high dam. This low dam would have been approximately 3,500 feet long, 350 feet above the lowest foundation, and would have contained 3,500,000 cubic yards of concrete. The structure would have impounded 250,000 acre-feet by raising the water surface 150 feet above the low water level in the river, and the reservoir would have extended 70 miles upstream. Because the low dam would have been eventually consolidated within the high dam, except that the upstream face remained common to both structures, regulating gates and bridge facilities were omitted from the 1,800-foot spillway section.
It was planned to complete the west end section of the future power house, some distance downstream from the left abutment, to house three main generating units and two service units capable of producing 520,000 kilowatts. Plans for future development of the power plant called for embedding the penstocks in the concrete of the dam and temporarily capping the projecting ends on the downstream side. A permanent concrete cofferdam would have been placed downstream in the correct location later to form part of the toe for the great dam. Foundation excavation for the two structures in addition to the removal of overburden from the intervening area was estimated at 11,000,000 cubic yards of common excavation and 800,000 cubic yards of rock.
A preliminary contract based on the removal of 2,040,000 cubic yards of earth and rock overburden was awarded to David H. Ryan in November 1933. Occurrence of extensive slides during the progress of the work increased the quantity to 3,004,744 cubic yards. Of this total, 3,000,356 cubic yards were earth excavation and 4,388 cubic yards were rock.
On July 16, 1934, a contract, for construction of the dam as originally planned and power plant, was awarded the Mason-Walsh-Atkinson-Kier companies on their bid of S29,339,300; and on September 25, 1934, they were notified to proceed. Various preliminary activities which had already begun were immediately expanded into a comprehensive construction schedule. Foundation excavation was carried on simultaneously from the west side area, behind steel piling cofferdams and from the area protected by earth dikes on the cast side of the river, in addition to preparations for diverting the river.
The problem of spoil disposal was ingeniously solved by putting into operation an elaborate belt-conveyor system. The contractor's plant, railway and highway bridge, 110,000-volt transmission line, and camp were constructed, in addition to removing approximately half the material to be excavated, before the contract had been in force one year.
In the meantime other important projects were under construction in the Northwest which would temporarily remove the demand for power from Grand Coulee. Severe drought conditions had become widespread, causing large numbers of people to vacate their farms. The dam under construction could afford but limited relief to agriculturists, due to the inadequate amount of storage and the cost of pumping 500 feet vertically above the reservoir to a comparatively small acreage.
The principal engineering objections to completing the low dam and later enlarging it to the high structure arose from the difficulties involved in bonding new concrete to concrete which already had become permanently set. Design calculations are based on the assumptions that the entire concrete mass in a vertical section resists the reservoir pressures as a unit. Failure of the concrete to bond perfectly at the joint, with the consequent entrance of water to create hydrostatic pressure, would constitute a serious problem affecting the safety of the dam.
Installation of hydraulic machinery in the power-house which would operate under both the low and high heads resulting from the two-stage construction, must necessarily be uneconomical for one or both conditions. Extensive slides occurring within the foundation area as a result of the contractor's excavation operations had established the likelihood of recurring slides and the desirability of placing the entire concrete base for the high dam to avoid duplication of the excavation work.
The disadvantages of continuing with the original plans were so pronounced that an order for changes was officially issued to the general contractor on June 5, 1935. The principal changes are roughly as follows:
(1) Excavate tailrace channel for east power plant;
(2) Place concrete base for the high dam in lieu of completing the low dam;
(3) Provide longitudinal contraction joints at 50-foot intervals in the mass concrete base and install metal tubing for artificial cooling;
(4) Revise the permanent downstream cofferdams to form the spillway bucket or downstream face;
(5) Eliminate the power house above the level of the turbine floor and install no machinery;
(6) Eliminate all penstock trashrack structures and provide openings in the concrete for future installation of the steel penstocks.
Although an additional 2,000,000 cubic yards of foundation excavation were involved in the order for changes, no time extension was permitted beyond the original 1,650 calendar days allowed for completion.
The concrete dam being built on the foundation structure will be of the massive straight-gravity type having a total length of 4,500 feet. The central portion, consisting of a 1,650-foot overflow section, is designed to pass a flood flow of 1,000,000 second-feet. The abutment sections at either end of the spillway are 30 feet wide at top elevation 1,311.0, with the upstream face extending vertically downward to elevation 1,023.1, below which it has a slope of 0.15. The downstream face is vertical between the top and elevation 1,273.0, below which the slope is 0.80, resulting in a base thickness of 482 feet for a section extending 553 feet above the foundation excavation at elevation 758.
A reinforced concrete arch bridge, consisting of eleven 135-foot spans supported on 15-foot concrete piers, will carry the roadway across the overflow section at a height of 17 feet 3 inches at the crown above the high-water level. With the exception of a metal hand railing, the bridge will conform to the roadway across the abutment sections which provides a sidewalk in addition to a 24-foot width for vehicular traffic.
The concrete base for the dam, under the contract just completed, was limited in height to elevation 1,010, a distance of approximately 250 feet above the lowest foundation excavation. Blocks in the abutment sections varied in height between elevations 1,000 and 1,010, while the blocks in the spillway section had a wider range. The lowest blocks stopped at elevation 935 and the highest were carried up to elevation 1,005. Tiers of blocks at elevation 935 and 945 were used as floodways while diverting the river across the foundation structure.
Approximately 4,500,000 cubic yards of concrete were placed under the first contract. To complete the dam will require an additional 5,700,000 cubic yards, resulting in a structure having a total volume of approximately 10,200,000 cubic yards. The dam and appurtenant works will require approximately 11,250,000 cubic yards.
In a massive structure such as Grand Coulee Dam, where the dissipation of setting heat is a serious problem, the time required to reduce the interior temperature to the average air temperature would amount to many years. Maximum heat loss is accompanied by complete contraction of the concrete, resulting in shrinkage stresses sufficient to develop cracks, unless contraction joints are provided at appropriate intervals. The sole purpose of artificial cooling is to accomplish complete contraction in a fraction of the time that would otherwise be required.
River water will be circulated through a system of 1-inch metal tubing which will remain permanently imbedded in the concrete. The mean temperature of the river water during 8 months of the year is less than 50° F. and averages less than 60° F. during the remaining 4 summer months, thereby eliminating the necessity of mechanically refrigerating the circulating supply.
A system of cooling galleries, located at elevations differing by 50-foot intervals throughout the height of the dam, will provide space for connecting the coolingpipe headers to the imbedded tubing. The principal gallery at each elevation, 6 by 8 feet in size, will parallel the axis of the dam at a distance 31 feet downstream.
Transverse galleries, 5 feet wide and 7 feet high, lead from the longitudinal gallery at distances varying between 600 and 800 feet, this distance being the longitudinal dimension of half the volume served by two adjoining transverse galleries. Vertical shafts, 3.5 feet in diameter, connect the transverse galleries of different elevations and provide means by which the cooling water is distributed at 5-foot vertical spaces between galleries.
Two pump barges on the reservoir surface are each equipped with a group of electric motor-driven centrifugal pumps and control apparatus. The total rated capacity for the pump installations are 3,500 and 5,500 gallons per minute, respectively, when pumping against a maximum head of 220 feet. The supply will be pumped through intake pipes embedded in the concrete between the longitudinal gallery and the upstream face. Headers, from 14 inches to 8 inches in diameter, convey the supply along the longitudinal galleries until it is diverted through 10- to 4inch headers, running along the transverse galleries and thence to the terminal connections in the vertical shafts for radial distribution. Each header is also provided with cooling pipe connections at 5-foot centers for lateral distribution from the walls of the galleries.
Each embedded coil varies in length between 600 and 800 feet. It is estimated that the total length of coils required for the dam will exceed 2,200 miles. The return flow is collected by return headers which increase in size between the far ends of the transverse galleries and the longitudinal galleries, from where it is conveyed by a 30-inch-diameter
drain outlet to the stilling basin. When the concrete served by a cooling circuit has cooled sufficiently, the header system will be dismantled and reinstalled at a higher elevation. This will limit the total requirement to approximately 500 tons of header piping, valves, and fittings for the entire cooling program.
Provision against foundation percolation will be made by an extensive grouting program which has as its objective the filling of all seams or crevices in the foundation area. The consistency of the grout will be varied by combining cement and water in different ratios, depending on the reception of the mixture by the rock. The pressures used will vary from 100 to 600 pounds per square inch.
Preliminary operations consisted of sealing the upstream portion of the foundation to a depth of 20 feet along a strip approximately 60 feet wide parallel with the axis of the dam. The first drilling and low pressure grouting were completed prior to placing concrete and was done to consolidate the surface rock preparatory to subsequent high-pressure grouting. Other areas in the immediate vicinity which indicated the presence of seams or fractures were likewise grouted with low pressures. After concrete had been placed, grout, under intermediate pressure, was forced through a series of inclined holes drilled through the upstream concrete foundation and 75 feet into the bedrock at 20-foot centers. The operation was completed in advance of diverting the river over the concrete base.
Prior to completing the dam the final grouting operations will be carried out from a longitudinal grouting and drainage gallery located near bedrock, 17.5 feet downstream from the axis. Holes at 20-foot centers will be drilled through the intervening concrete foundation and approximately 150 feet into the bedrock. Intermediate holes will be drilled, as required, to a depth of 200 feet, and both series will be grouted under high pressure. When grouting operations have ceased, a line of drain holes at 10-foot centers will penetrate the foundation to eliminate any possibility of uplift pressure resulting from the accumulation of water. Drainage will be effected by pumping from the foundation into galleries at higher elevations, from where the water will be conveyed by gravity to the stilling basin.
Following preliminary grouting, preparations were made for placing concrete by constructing the initial forms to fit the irregular rock surface. The forms enclose one block, generally 50 feet square, which is the unit of construction. Provisions for future grouting of the joints between adjoining blocks are made prior to commencing the pour. Vertical riser pipes, one-half inch in diameter, are attached to the forms at 6-foot intervals so that the flanges of the grout outlets are coincident with the joints. The outlets are spaced 10 feet apart on the risers and are staggered in relation to adjacent risers. The l%- and 2-inch grout headers and return pipes are installed preparatory to being embedded within the block. Vertical key ways are formed on the joints, transverse to the axis of the dam, while along the longitudinal joints the keyways extend horizontally.
Blocks are poured successively in 5-foot lifts with a minimum time interval of 72 hours between consecutive pours. The difference in top elevations between adjoining blocks is limited to 25 feet. A tier includes the blocks in a single row between adjacent transverse contraction joints. The longitudinal joints are not continuous, being offset 25 feet between adjoining tiers, which results in the blocks at the upstream and downstream faces having dimensions which differ from 50 feet in the transverse direction.
When contraction has occurred as a result of cooling, joints will be grouted. Transverse joints will be grouted from the longitudinal galleries, and the longitudinal joints
will be grouted from the transverse galleries. Grout stops across the joint between adjoining blocks in the tiers will confine the grout to the transverse joints. Additional metal stops will be installed across the transverse joints at the upstream and downstream faces to prevent escape of the grout or the entrance of water. A triangular cut-off groove, formed horizontally along the top of the 50-foot lifts, stops the vertical flow of grout as it is forced upward through the contraction joints from the grouting galleries at the bottom of the 50-foot lifts. The galleries serve for both grouting and cooling operations.
The design for the dam was determined by numerous technical studies and model investigations of the stability of the structure. The studies were made for various condi