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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.


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.


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.

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.


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


a basis for the original design, the maximum stress being about 35 tons per square foot by the trial-load method.

In order to prevent leakage in the foundation, two lines of grout holes, at 10-foot centers, were drilled across the entire length at distances of 5 and 13 feet from the upstream face. The holes penetrated 26 feet into the bedrock and were grouted under pressure. To relieve any upward pressure that might occur beyond this grouted curtain, drainage holes were drilled on 10-foot centers at a distance of 27.5 feet from the upstream face. These holes penetrated 26 feet into bedrock and were continued upward into the masonry, emerging into an inspection gallery.

In order to drain any reservoir seepage which might penetrate the concrete, 8-inch vertical drainage conduits were formed in the concrete at about 10-foot centers and at a distance of 12 feet from the upstream face. These conduits extend to within 10 feet of the top of the dam and convey the seepage to the inspection gallery. The inspection gallery is located 25 feet from the upstream face of the dam, and follows the profile of the natural ground surface. It is 14 feet wide by 14 feet high in the river-channel portion of the dam, and 3 feet 6 inches wide, 6 feet 6 inches high at the abutments. The gallery has entrances at each end of the dam and an additional entrance at the lava bench on the left side, directly above the diversion tunnel. Two 24- by 24-inch drains lead from the inspection gallery to the downstream face of the dam to remove any drainage water which may collect in the gallery.

Connecting with the inspection gallery and under the regulating outlets are two operating galleries from which the balanced valves are controlled. These galleries, located 16 feet from the upstream face of the dam, are 87 feet apart in vertical distance, and are connected by a spiral stairway built of reinforced concrete. The lower gallery runs under the lower set of regulating outlets only, while the upper gallery extends along the entire length of the dam, connecting with the inspection gallery at each abutment. The upper operating gallery provides access for inspection of the upper portion of the dam.

Contraction joints, extending to the top of the dam, are provided on radial lines, at 150-foot intervals, from a point 215 feet below the top; at 50-foot intervals from a point 130 feet below; and at 25-foot intervals from a point 70 feet below. The joints were made by building alternate sections of the dam ahead of the others, and allowing them to set before filling intermediate sections. Vertical openings were formed along the joints, three near the bottom of the dam, decreasing to one near the top. The upstream well was 5 feet square, while the other two were 10 feet square in the lower portion of the dam, decreasing to 5 feet square near the top. These wells were filled with concrete during cold weather, at a time when the concrete in the body of the dam was in a contracted state. Five feet from the upstream face of the dam an annealed-copper, Z-strip water stop is

provided in each joint, and downstream from this strip a triangular drain was formed. This drain conveys any water which percolates past the water stop to the inspection or operating gallery.

The principal quantities involved in the construction of the dam before it was raised were: 322,390 cubic yards of excavation, 585,165 cubic yards of concrete, 603,020 pounds of reinforcement steel, 10,490 linear feet of grout holes, in bedrock, 24,540 linear feet of drainage conduits in concrete, 1,067 linear feet of drains, 2,182 linear feet of operating galleries, and 4,168 linear feet of contraction joints.


The spillway is of the side-channel type and is located in a granite cut at the north end of the dam. The weir is adjacent to the dam, along the north bank of the reservoir, extending upstream a distance of 402 feet. From the weir the channel turns the water at right angles, carries it past the dam, and discharges it into Deer Creek, which flows into the Boise River about 800 feet below the dam. The spillway is designed for a capacity of 40,000 second-feet with a head of 10 feet over the crest of the spillway; that is, with the water surface at the top of the parapet.

The crest of the spillway is controlled by six, structuralsteel, drum gates, each 62 feet long and 6 feet high, separated by 6-foot piers. Normal high water can be maintained at an elevation 6 feet above the spillway weir, by means of the gates which raise and lower automatically, or by hand manipulation of valves, as desired.

The automatic operation of the gates as originally built was accomplished by an arrangement of counterweights, which were attached and detached at intervals as the gate moved up and down. This arrangement has never worked successfully, and is being changed in conjunction with the present raising of the spillway crest. The counterweights are being removed, and the operating mechanism changed to the differential needle-valve type, similar to that installed in the spillways at Boulder Dam and other drum-gate spillways.

The spillway-channel cross section varies from a 7.2-foot bottom width and a 14-foot depth, below the altered spillway crest level at the upstream end of the weir, to a 30foot bottom width and a 50-foot depth at the downstream end of the weir, the side slope varying between % to 1 and 0.8 to 1. The bottom profile of the channel at the upstream end is on a 12-percent grade and decreases gradually to a 1-percent grade at the lower end of the weir. The channel is lined with reinforced concrete, anchored to the rock, and contraction joints are provided about 30 feet apart in the lining. The floor lining is thoroughly drained with vitrified tile drains, and the side lining is provided with 1-inch weep holes.

The highway leading across the dam is carried along the river side of the spillway channel to the lower end, where it crosses over a 96-foot span steel highway bridge 16 feet wide.

The principal quantities contained in the spillway as originally built were: 359,000 cubic yards of excavation, 1,300 cubic yards of backfill, 25,564 cubic yards of concrete, 708,690 pounds of reinforcement steel, 641,770 pounds of gates, machinery, and structural steel, 5,030 linear feet of drains, and 50,200 pounds of structural steel in the highway bridge.


There are 25 outlets through the dam, located in three rows, or tiers, in such a position that when discharging, the water falls into the old river bed. The lower set, consisting of five sluicing outlets, is located at the elevation of the river bed, 248 feet below the top of the parapet. These outlets are 60 inches in diameter and 164 feet long. They are protected by a trash rack and are controlled by 60- by 60-inch slide gates set 28.5 feet from the upstream face of the dam. The gates are operated by oil pressure from the low-level inspection gallery.

A set of 10 outlets, each 129 feet long, is located 197 feet below the top of the dam. Three of these outlets are 72 inches in diameter and are reinforced as penstocks for use in connection with a possible future power development. The remaining seven outlets are 52 inches in diameter and are used to carry irrigation flow through the dam. The outlets at this elevation are protected by trashracks, and are controlled by 58-inch balanced valves operated from the lower operating gallery.

One hundred and ten feet below the parapets is a second set of 10 regulating outlets, each 70 feet long and similar in all respects to the 7 just described, with the exception of the trashrack structure, which is omitted. This set of outlets is controlled from the upper operating gallery.

It is intended that the sluice gates shall be operated at heads of 75 feet or less; but the gates are so designed that they can be opened under a head of 125 feet in an emergency. The discharge at the 75-foot head is about 5,000 second-feet, and at the 125-foot head about 6,700 secondfeet. The regulating outlets are designed to operate under maximum heads of 100 feet, at which head they will discharge 1,000 second-feet each. The total length of all outlet conduits is 2,821 linear feet, which involved the placing of 2,672,300 pounds of gates, machinery, and structural steel.


Before beginning actual construction on the dam, it was necessary to perform considerable work of a preparatory nature, including the construction of a telephone system, railroad, wagon road, power plant, transmission line, saw

mill, construction camp, and diversion works for carrying the river past the dam site during early construction. All work was performed by Government forces with the exception of the grading of the railroad.

A two-circuit telephone line was constructed between the dam site and the main project office at Boise, with branch connections to the sawmill, powerhouse, railroad, and wagonroad construction camps, gaging stations, and the main construction camp at Arrowrock. In all there were constructed 54 miles of telephone line which gave service to a maximum of 54 instruments. Work was started on the line in February 1911, and the first circuit completed April 18, 1911.

The railroad was a standard-gage steam road with 17 miles of main line and 2 miles of siding and spur line. It was built along the Boise River from Barberton, Idaho, the end of the Oregon Short Line branch, to Arrowrock. The maximum grade on the main line was \% percent and the maximum curvature was 12°. The line crossed the river at three points on timber truss bridges.

Preliminary surveys were started in October 1910, and construction started in January 1911. Track laying was started in July 1911, and completed on November 9, 1911. The average construction cost of the railroad to the Government was about $19,400 per mile. During the period of operation, the trains traveled a total of 111,300 trainmiles, and carried 13,968,000 ton-miles of freight and 90,000 passengers. Of the freight carried, concrete aggregate was the largest single item. Materials for 427,650 cubic yards of concrete were handled from the gravel pit to Arrowrock, a distance of \3% miles. A total of 25,052 cars were hauled, the maximum rate being 100 cars per day.

The entire construction plant at Arrowrock, with the exception of the steam shovel, dragline excavator, and dinkey engines, was operated by electricity. To furnish the necessary power, a power plant was constructed at the Boise Diversion Dam, 14 miles below Arrowrock. The plant, a 1,500-kilowatt, 3-unit installation, generating 3-phase, 60-cycle alternating current at 2,300 volts, was designed for a normal effective head of 24 feet, and for a normal capacity at maximum efficiency of 2,200 horsepower. A duplicate transmission line was built from the power plant to Arrowrock, and a single line to Barberton, 3 miles from the plant, where it connected with the line owned by the local power company. These lines were first used on September 16, 1911.

To furnish lumber for camp and construction purposes, a sawmill was built on Cottonwood Creek, 17 miles above Arrowrock, in the Boise National Forest. The mill was operated from April 1911 to September 1913, and produced during that period 6,747,000 feet of lumber.

The construction camp was located a quarter of a mile below the dam site and was built to accommodate about 900 men. The camp was complete with a mess house, a mercantile store, a bathhouse, a laundry, a hospital, a club house, cottages, and dormitories. All buildings were electrically lighted and the main buildings were steam heated. The camp was served with a complete water and sewage system, the water being obtained from Deer Creek, about 1?4 miles above the camp.


The diversion works consisted of an upper and lower cofferdam and a tunnel constructed through the granite below the lava bench at the south abutment of the dam. The tunnel was designed for 20,000 second-feet and the cofferdams were designed to withstand overtopping in case of a larger flood.

The tunnel, which was designed for a velocity of 30 feet per second, was 470 feet long and had a section 30 feet wide and 25 feet high, with an arched top having a 10-foot rise. The bottom and sides of the tunnel were lined with concrete, and the arched top was supported by timbers. The entrance and exit of the tunnel were both made bell-shaped, to avoid loss of head and to increase the discharge. The upper cofferdam was about 200 feet long and 40 feet high, and was built of timber cribs filled with rock, gravel, and fine material sluiced into the crevices. The faces of the cribs were of solid timber construction, with the joints calked with oakum, and a sheet piling cut-off was provided. The lower cofferdam was 150 feet long and 25 feet high with construction similar to that for the upstream cofferdam.

To take care of the seepage, electrically driven centrifugal pumps were mounted on cars which could be raised or lowered on an inclined track as needed.

In December 1914, when there was no longer any need for the diversion tunnel, it was plugged with concrete for a length of 190 feet under the section of the dam.


The excavation in the river bed was removed in two sections. The excavation first made was of a width upstream and downstream only sufficient to permit construction of a portion of the dam along its upstream side. This portion of the dam was built as a gravity section to a height sufficient to protect the work during the following flood season. This work was successfully accomplished between the flood seasons of 1912 and 1913, which would not have been possible had the entire excavation been completed before concreting began. The completed section of the dam then served as an effective cofferdam to prevent flood damage to the subsequent work and to prevent seepage from the upstream side into the pit during the remainder of the excavating and concreting.

The material in the river bed was largely gravel and sand, with perhaps 5 to 10 percent boulders exceeding two-man size. Inasmuch as concrete materials were not plentiful in

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