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.

SPILLWAY

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 1⁄2 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.

OUTLET WORKS

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.

PRELIMINARY CONSTRUCTION WORK

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 11⁄2 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 131⁄2 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.

CONSTRUCTION

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

the vicinity of the work, all excavated material which was suitable for concrete was stored for such use.

On the south abutment the lava cap was entirely removed in order that the dam might be founded upon the underlying granite. This material, along with that not suitable for concrete, was deposited upstream and downstream from the dam site. The excavation of the abutments was carried on just ahead of concreting and was completed in November 1915.

Concrete work on the dam started in November 1912, and was completed in November 1915. The dam was virtually built in three sections. The first section, as indicated previously, was built to protect the subsequent work. It consisted of a portion of the dam along the upstream face to a height of about 40 feet above high water in the river, and of ample section to withstand water pressure to that height. The second section brought the full width of the dam to the top of the first section, and the third section completed the dam.

The best progress was made during the months of April, May, June, and July 1914, when more than 200,000 cubic yards of concrete were placed, an average of more than 50,000 cubic yards per month. In June 1914, 56,500 cubic yards were placed in 26 working days, an average of 2,170 cubic yards per day of two 8-hour shifts.

The aggregate for the first and second sections of the dam, 186,000 cubic yards, was obtained from the river-bed excavation. The remainder was hauled over the railroad from a gravel pit located near the Boise Diversion Dam 14 miles below Arrowrock.

The concrete used in the dam was composed of about 1 part sand-cement, 21⁄2 parts sand, 51⁄2 parts gravel, and 3 parts cobbles passing a 5-inch grizzly. A somewhat richer mixture was used in the faces and upper portion of the dam and in the toe of the dam where the maximum stresses occur. A richer sand-cement concrete mix was also used in the spillway weir and lining. A straight portland cement concrete mix was used in the parapet walls, spillway piers, and other thin and heavily reinforced portions of the dam and spillway.

The sand-cement was composed of standard portland cement, with a little less than an equal amount of pulverized granite, reground to a fineness such that 90 percent would pass a No. 200 sieve. Besides the rigid fineness test, the sandcement was required to pass all the standard physical tests for portland cement. The concrete made of this sand-cement was slower in setting and hardening than that made with straight portland cement.

Two 12-ton cableways, each 1,500 feet long, with 60-foot stationary head towers, were installed in such a postion as to command the entire area of the dam. These cableways were operated by electric motors and were employed in excavating and in transporting equipment and materials for construction. Four 10-ton stationary derricks, two 6-ton

traveling derricks and several small stiff legs were also employed in excavating and handling materials on various parts of the dam and spillway.

A steam-operated, dragline excavator, with a 22-yard bucket, was used for excavating part of the foundation of the dam. A 70-ton steam shovel, with a 22-yard dipper, was used for excavating part of the dam foundation, for excavating the spillway, and for loading gravel at the Diversion Dam gravel pit.

The aggregate screening and crushing plant and the concrete mixing plant for constructing the first and second sections of the dam were located on the lava cliff on the south abutment, directly beneath the main cableways. The mixing plant was a 2-unit installation, consisting of 1-yard mixers electrically operated, each served with bins, measuring boxes, scales, and other pertinent equipment.

Material from the river bed suitable for concrete was taken by cableways directly to the screening and crushing plant, where it was processed and stored in stock piles. The aggregate was then hauled to the mixer bins by dump cars and scrapers. Concrete was discharged from the mixers into 2-yard stationary hoppers, from which it was delivered into specially made 2-yard buckets and then into movable hoppers, each suspended from the main and auxiliary cableways. The concrete was distributed from the hoppers into the dam by attached chutes, the free ends of which would be moved to any point desired.

Upon completion of the first and second sections of the dam, the mixing plant was dismantled, moved downstream, and located on the lava bench just below the dam. This plant was in all respects similar to the first one, except that it was a 3-unit installation, with three 1-yard mixers. Aggregates for the second plant were hauled by railroad from the gravel plant at the pit near the Boise Diversion Dam, and deposited into storage bins above the mixers. The concrete was discharged from the mixers into 2-yard trolley cars and carried to a central distributing tower 150 feet high. From this tower the concrete was distributed by buckets and hoppers in the same manner as that employed in the first sections of the dam.

When construction neared the upper portion of the dam, the cableways could no longer conveniently cover the work on the long sweeping area, and the distributing system was changed. A track and dump-car system was employed, the track being supported on collapsible forms running the length of the dam. The concrete was conveyed from the mixers by the main cableways to hoppers placed at both ends of the dam, where it was carried to any point by electric dump cars running on the track. The track and hoppers were raised as the work progressed.

Concrete for the spillway was obtained from the second mixing plant, and was transported from the plant to a dumping hopper at the spillway site by one of the main cableways. The concrete was distributed from the hopper