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of operation. Other preliminary construction included the camp buildings, refrigeration plant, telephone lines, and a water supply

THE DAM

The dam is located in a narrow canyon, about 200 feet wide at the river level, with nearly vertical cliffs rising on either sides. The foundation was explored by drilling three lines of holes across the bottom. The rock encountered was a hard, tough, fine-grained sandstone with a few strata of quartzite. It dips upstream at an angle of about 29° from the horizontal, approximately at right angles to the dam. At the upstream face of the dam a minor fault was uncovered with a displacement of about 8 inches. The rock is of a good quality and required very little excavation for the foundation. A few seams were encountered in the foundation, most of which were filled with fine silt. Several warm prings found in the base were piped together and carried

to the powerhouse to furnish warm water. Seams carrying river water were grouted.

The river was diverted through a tunnel driven beneath the south abutment at river level. The tunnel was designed for use as an outlet after the dam was completed and was 12 feet wide, 10 feet high, and about 480 feet long. The intake is located 125 feet upstream from the dam and is covered with a reinforced concrete structure about 69 feet high, in which provision was made for installing trashracks. The diversion capacity was about 1,300 second-feet with water at the top of the cofferdams. To care for additional flow of the river, it was originally planned to build a sluiceway over the cofferdams. Owing to the unusual size of floods occurring at the beginning of construction, the sluiceway idea was abandoned and work on the lower part of the dam was carried on only at such times as the tunnel could handle the river diversion. A high flood with an estimated peak discharge of 130,000 second-feet in Novem

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ber 1905 completely destroyed all work completed in the canyon and delayed construction for several months.

The dam is of the arch-gravity type with a radius of 410 feet. It was designed as a gravity structure, the arch plan being added to give greater stability. The dimensions of the dam are: Length at the top, including spillways, 1,125 feet; length at the bottom, 235 feet; thickness at the top, 16 feet; thickness at the bottom, 184 feet; maximum height, 284 feet. It is constructed of cyclopean rubble masonry, the large stones being set in portland cement mortar, and the vertical joints being filled with concrete and spalls. The face stone was rough cut and laid in courses with mortar joints of about 2 inches. The masonry in the dam totals 343,000 cubic yards. The spillway excavation furnished practically all the stone required. Where the fault was encountered within masonry lines, it was cleaned out and a masonry cut-off built across the canyon. The curve of the arch misses the fault in the abutment areas.

Materials were transported from the top of the canyon to the dam by two cableways of about 1,200-foot span, and were placed in the dam by stiff-leg derricks. Pumping within the cofferdams was done by hydraulic ejectors operating under a head of about 220 feet from the power canal. The concrete mixing plant was located on the south side of the canyon. The faces of the dam were kept moist and were cooled in hot weather by spraying water from a line of pipes having holes drilled at intervals.

Six high-pressure, Stoney gates, arranged in three pairs, are located in the diversion tunnel at the upstream face of the dam. They are 4 feet 6 inches wide by 10 feet high and are operated by hydraulic cylinders located in a circular

well on the upstream face of the dam. Water can also be drawn from the reservoir through an outlet located on the north side of the dam. The inlet is 123 feet below the top of the dam and the outflow is controlled by three 58-inch balanced valves. Three 60-inch, cast-iron pipes led from the valves to a 9-foot diameter, concrete lined tunnel with the outlet end under the edge of the cliff in the north spillway.

There are two spillways, one on either end of the dam, with a total initial length of 402 feet and a total capacity of 150,000 second-feet. As first constructed, the spillways were of the plain weir type with uncontrolled crests 20 feet below the roadway surface. A highway, 16 feet wide, crosses the dam, with the approach across each spillway being over a three-span, reinforced concrete arch bridge.

The last of the masonry was placed in the main structure in February 1911, the dedication being held on March 18, 1911, at which the late ex-President Theodore Roosevelt was present, the dam having been named in his honor.

POWER STRUCTURES

It was decided at the beginning of the work to utilize the fall of the river above the reservoir limits and to install a temporary hydroelectric unit until the permanent power plant could be placed in operation. A power canal 195 miles long, with a capacity of 225 second-feet, was built along the south side of the river. A concrete diversion dam 450 feet long and about 7 feet high, was built at the intake. Distributed along the length of the canal are 21 tunnels with a total length of about 2 miles. The lower end of the

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canal was connected to a 1,200-horsepower hydroelectric unit, operating under a head of 220 feet, water being supplied through a 7-foot diameter penstock which passes under the left abutment of the dam. The unit was installed in a cave behind the permanent plant and was placed in operation in April 1906.

Provision was made in the main powerhouse for the ultimate installation of six units. The building, located just below the dam on the south side of the river, was constructed by Government forces. It is about 125 feet long by 36 feet wide, and has a total height of 33 feet from the floor to the top of the crane rail. It was constructed of concrete with stone facing. Two penstocks were originally provided, comprising the 7-foot diameter steel pipe connected to the power canal and a 10-foot diameter steel pipe passing through the dam. The intake for the 10-foot penstock is located about 165 feet below the top of the dam and is covered with a movable trashrack. The 10-foot diameter penstock has a drop of 50 feet between the intake and the powerhouse. A 10-foot diameter cylinder gate is installed at the powerhouse for emergency closure.

The first three power units were placed in operation in 1909. These units were 1,200-horsepower, 3-phase, 25cycle, 2,300-volt machines, operated by vertical type turbines under a head of 220 feet obtained from the power canal.

Two other units of the same type and output were installed and connected to the 10-foot penstock. The runners of these machines were designed for heads varying from 112 to 228 feet, to take care of changing elevation of the reservoir surface. The sixth unit is of the variable head type with a capacity of about 5,000 horsepower and operating in connection with the 10-foot penstock. It was not installed until 1915. Two reservoir outlets, controlled by two 43-inch balanced valves, were installed in the upstream end of the powerhouse and connected to the 10-foot diameter penstock. The transformer house was located farther downstream on the south side, below the spillway channel. It was built of concrete with stone facing. The voltage was stepped up from 2,300 to 45,000 volts. The power generated is largely used for drainage and irrigation pumping needs on the project.

COSTS

The cost of the dam and reservoir up to 1913 totaled approximately $3,650,000. Lumber brought in from Globe would have cost $60 to $65 per thousand board feet. The portable sawmill produced 2,964,000 board feet at a cost of $44 per thousand delivered to the work. The cement mill produced 338,452 barrels of portland cement at a cost

of $3.14 per barrel. Cement purchased on the market would have cost about $4.89 per barrel delivered at the dam. The unit cost of crushing 87,000 cubic yards of sand was $1.58 per cubic yard. Unit contract prices were as follows:

Per cubic

yard ........... $1.75

Earth excavation .................
Rock excavation (below low water) ............ 5.00
Rock excavation (above low water) ............ 1.50
Rubble masonry in dam ............ ... 3.15
Rubble masonry in wing walls . ............. 4.75
Bridges (each). .................... 7,500

Materials were furnished by the Government. The cost of diverting the river, but not including the diversion tunnel, and unwatering foundations was included in the prices bid for excavation and placing masonry.

station power equipment, bringing the total installed capacity up to 24,000 horsepower. An additional story was erected on top of the powerhouse and new transformer equipment installed. The voltage is stepped up on the transmission lines to 110,000 volts.

The north outlet works have been altered by plugging the 9-foot tunnel and moving the outlet over to the left of the spillway. Three 54-inch diameter, steel liners were installed and three 54-inch needle valves placed on the outlet end. These valves were later removed and placed in service at the Mormon Flat and Stewart Mountain Dams. Two 54-inch butterfly valves are located on the outlet ends of the two outside pipes, the center pipe being close by a bulkhead. The three 58-inch balanced valves were removed from the inlets and trash bars installed.

The spillway crests were raised 5 feet, soon after the dam was completed, to provide additional storage. In 1923 the spillways were provided with radial gates and piers to further increase the reservoir storage. The crest was lowered 9 inches to compensate for the effect of the piers on discharge capacity. The radial gates are 15 feet 9 inches high by 20 feet long, 10 being installed on the south and 9 on the north spillway. The spillway was repaired and altered in 1936. The crest was then lowered 6 feet, to increase the discharge capacity, new motor operated hoists were installed and changes were made on the piers and gates. The power canal inlet was repaired and altered during the same year.

SUPPLEMENTARY WORK

LATER POWER DEVELOPMENT

The original power and outlet equipment at Roosevelt Dam was in a pioneer stage of development at the time of its installation. Since that time changes and refinements have been made in the various features. The project was turned over to the Salt River Valley Water Users' Association in 1917. All changes previous to that date were made by the Bureau of Reclamation. Since then the association has made most of the changes, subject to the approval of the Bureau.

The Stoney gates in the south tunnel did not operate satisfactorily under high heads. Two 58-inch-diameter bronze pipes were installed in a concrete plug downstream from the Stoney gates. Each pipe was equipped with a 30- by 38-inch bronze slide gate. Two 48-inch-diameter steel pipes were connected to the bronze pipes and extended to the outlet of the tunnel. The power canal was often washed out in places during heavy rains. To eliminate the interruptions from this source, two riser connections were made from the two 48-inch pipes to the 7-foot penstock. Two 38-inch needle valves were installed on the downstream end of the 48-inch pipes to permit operation of the three 1,200-horsepower machines from the reservoir when necessary. In 1923 the two 43-inch, balanced valves were removed from the powerhouse and a 10,000 horsepower unit installed in the space they had occupied. A 14-footdiameter tunnel was driven from the powerhouse to a chamber in the south tunnel. In this tunnel a 10-foot diameter penstock was installed to operate the new unit. Changes have been made in some of the units and in the

In 1923 the association, with the approval of the Bureau, undertook to develop the power resources of the river into one coordinated system. Horse Mesa, Mormon Flat, and Stewart Mountain Dams were built between Roosevelt Dam and Granite Reef Diversion Dam. These dams provide additional storage of irrigation water as well as creating an effective power system. Almost the entire fall of the river within these limits is now utilized for generation of power.

BIBLIOGRAPHY

The Roosevelt Masonry Dam (and Specifications). Engi

neering News, Jan. 12, 1905. Progress on Roosevelt Dam. Engineering News, Sept. 10,

1908. Construction of Roosevelt Dam (Including Costs). Engineering Record Dec. 31, 1910.

ARROW ROCK DAM

BOISE RIVER PROJECT, BOISE, IDAHO

BY CARL J. HOFFMAN, ASSOCIATE ENGINEER, BUREAU OF RECLAMATION

Fifty-nine diamond-drill holes were sunk over the proposed foundation area and along the spillway site, yielding cores from 30 to 50 feet. The drill holes were supplemented by exploration of numerous test pits, shafts, and tunnels above water line. The foundation was found to be a good quality of hard granite, nearly 90 feet below river bed at the deepest place and about 65 to 70 feet deep over most of the area. The overlying material was mostly sand, gravel, and boulders suitable for use in concrete.

The canyon at the dam site is fairly narrow, with high, steep, bare granite cliffs on the north side, and a less precipitous slope, covered for some distance by a cap of lava, on the south side. This lava cap rises almost vertically from its contact with the granite, which starts about 20 feet above the water surface, to a height of about 50 feet. The top of the lava bench is nearly level, intersecting the granite cliff farther back; and, like the granite, is covered with a thin layer of overburden.

THE DAM

THE BOISE IRRIGATION PROJECT in southwestern Idaho has an irrigable area of 166,000 acres, practically all of which is developed. The Arrowrock Reservoir, part of the storage system for the project, is used to impound surplus winter and flood waters for use on project lands and to supplement the supply for about 110,000 acres of lands, under Warren Act contracts along the Boise River. The dam is located on the Boise River, about 4 miles below the junction of the north and south forks and about 22 miles upstream from Boise.

RESERVOIR The Boise River, above Arrowrock, drains about 2,200 square miles on the western slope of the Sawtooth Mountain Range, where run-off occurs principally from melting snows. The average annual flow of the river at Boise is 2 million acre-feet, varying from high flood rates in the spring to a few hundred second-feet late in the summer.

Arrowrock Dam was completed in 1916, but the dam was completed to a sufficient height to allow storage of 180,000 acre-feet in the spring of 1915. The original storage capacity of the reservoir was 276,500 acre-feet at elevation 3,211, which was the top of the spillway drum gates when raised. Subsequent silting reduced the capacity to 271,500 acre-feet. Incident to repairs being made on the dam at the present writing, the height is being increased 5 feet, with a corresponding raise of the spillway crest, increasing the storage capacity to 286,500 acre-feet.

The reservoir is about 17 miles long, and covers an area at maximum storage of about 3,000 acres. In shape it resembles the letter“Y”, due to water backing up both forks of the river.

DAM SITE In 1903 and 1904 reconnaissance surveys were made at several reservoir sites on the upper waters of the Boise River, including rough surveys of several dam sites. Estimates of costs of storage works were made, and in 1910 a party was sent out with a diamond-drill outfit to explore the foundations at the most promising site. Through a process of elimination the Arrowrock site was chosen as most favorable, and testing of the foundation was continued more in detail at this site during the latter part of 1910 and the early part of 1911.

In 1916, when Arrowrock Dam was completed, it had the distinction of being the highest dam in the world. The dam is a concrete structure of the curved gravity type, the radius of the upstream face being 669.5 feet, measured at the parapet. The dam as originally built was 348.5 feet high to the top of the parapet, 223 feet thick at the base, and 15.5 feet thick at the thinnest point near the top. It carried a 16-foot roadway along its top length of 1,100 feet, the crown of the road being at elevation 3,211 and 4-foot parapets being provided at the sides. At present the height of the dam is being increased by raising the top 5 feet. New parapets, 3.75 feet high, are being constructed with their tops at elevation 3,219.75.

The dam was originally designed for a maximum allowable stress of 30 tons per square foot, computed as a straight gravity dam. No dependence was placed on arch action, and no allowance was made for uplift, backwater, or ice pressure.

In 1935 the dam was analyzed by the trial-load method including allowances for earthquake shocks, tangential shear and twist, temperature changes, and both cantilever and arch action. The stresses in the heightened dam were found to be increased only slightly over those assumed as

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