SHOSHONE DAM

Shoshone Project, Wyoming

By P. I. Taylor, Assistant Engineer, Washington Office, Bureau of Reclamation

THE Shoshone Dam is located 8 miles from Cody,

Wyo., on Shoshone River, a short distance below the confluence of its north and south forks and near the upper end of Shoshone Canyon. The dam is a monolithic rubble concrete structure of the arch type, the radius of the center line of the top of the dam being 150 feet. The maximum height from the rock foundation to the top of the parapet is 328 feet.

After extensive wash-drill and diamond-drill borings, through a deposit of drift 60 to 90 feet deep, a location for the dam was selected at the narrowest point in the gorge. The abutments and the base of the dam are on the same foundation, which is an extremely hard granite. In design the Shoshone Dam is similar to the Pathfinder in Wyoming, both being high structures in narrow granite gorges. The Shoshone Canyon at the dam site is 70 feet wide at the normal stream bed and 200 feet wide at an elevation of 300 feet above the stream bed. For a distance of 1,400 feet the granite walls continue with little divergence from perpendicular, and are surmounted by a limestone capping extending to over 4,000 feet above the stream bed. The dam has a top thickness of 10 feet with a batter to stream bed level of 15 per cent on the reservoir face and 25 per cent on the downstream face, both faces being vertical below the stream bed.

As no natural deposits of sand or gravel were available near the dam site, it was required in the specifications that both be manufactured of good sound granite. The requirements for concrete were a mix in proportions of 1:22:5 in which sound, clean pieces of granite were hand placed; these stones weighed from 25 to 200 pounds and formed about 25 per cent of the total volume of the masonry.

The volume of concrete in the main structure was 75,000 cubic yards. There were four classes of foundation excavation amounting to a total of 44,000 cubic yards, and 32,000 cubic yards of solid rock excavation in the spillway.

CONSTRUCTION OF THE DAM

Plans for the construction of Shoshone Dam, spillway, and outlet, spillway and road tunnels, were prepared in 1905. Proposals were opened September 5, 1905, and contract was executed September 23, 1905. Work on the outlet tunnel was begun November 21,

and the construction of temporary diversion works was begun December 12, 1905. In May, 1906, work on the outlet tunnel was discontinued on account of high water in the river, and in June of that year the temporary dam was partly destroyed.

In August, 1906, it became necessary to suspend the contract on account of the failure of the contractor to prosecute the work satisfactorily. On September 10 a new contract was executed by the bondsmen of the first contractor, and reconstruction of the temporary dam was begun in November of that year. In April, 1907, the excavation of the outlet tunnel was resumed, and it was completed May 10.

Excavation for the foundation of the Shoshone Dam was begun December 2, 1907, and completed April 1, 1908. The placing of concrete in the dam was begun March 30, 1908, and completed January 16, 1910. The excavation of the road tunnel was begun May 12, 1906, and completed January 17, 1907. The open-cut excavation for the spillway was begun in April, 1906, and completed in April, 1909; and the driving of the spillway tunnel was begun September 3, 1908, and completed in April, 1909. Construction of the upper outlet tunnel for the reservoir was begun December 20, 1909, by Government forces. At the end of May, 1910, the driving of the tunnel was completed, except for the excavation of a part of the bench in the lower end.

OUTLET TUNNELS

The main outlet of the reservoir formed by the dam is a concrete-lined tunnel 488 feet in length, having a section 10 feet wide by 10 feet high at the sides with an arched roof of 16-inch rise. The tunnel was driven through the granite cliff on the south side of the canyon. The elevation of the floor of the tunnel at its upper end is 5,140, and at its lower end 5,137.

The question of providing additional outlets from the reservoir was considered by a board of engineers in 1914. Following its recommendations a bulkhead was installed on the lower end of the tunnel, and between this bulkhead and the sliding gates two Ensign balanced valves discharging into the canyon. The valves were placed in small chambers excavated at the side of the tunnel, and discharge into separate tunnels leading through the canyon wall.

A second outlet tunnel, also on the south side of the canyon, has its inlet floor at elevation 5,250. It is

THE

ARROWROCK DAM

Boise Project, Idaho

By P. I. Taylor, Assistant Engineer, Washington Office, Bureau of Reclamation

HE Arrowrock reservoir stores water for the irrigation of lands of the Arrowrock division of the Boise project in southwestern Idaho. The dam is located on the Boise River, upstream about 22 miles, in an easterly direction from the city of Boise, and 17 miles by railroad from Barberton, the end of the Oregon Short Line Railroad branch out of Boise. The dam is built at a point about 4 miles. below the junction of the two forks of the river. The reservoir is practically 17 miles long, covers an area at maximum storage of 2,888 acres, and in shape resembles the letter Y due to the water backing up both forks. The capacity of the reservoir with movable crest weir gates raised is approximately 250,000 acre-feet.

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 cost were made of storage works for the several sites, and in 1910 a party was sent out with a diamond-drill outfit to explore the foundations at the most promising dam sites. Through a process of elimination the Arrowrock site was chosen as the most favorable, and testing of the foundation was continued more in detail at this site during the latter part of 1910 and early part of 1911. Fifty-nine diamond-drill holes were sunk over the proposed foundation area and along the spillway, besides several test pits and tunnels to determine depth to bedrock and its character. The conclusions drawn from the results of testing were favorable and were confirmed by the report of Prof. W. O. Crosby, geologist, who inspected the site in 1910.

DESCRIPTION OF DAM

The dam is a concrete structure, built with a gravity section upon a curved plan, the radius of the upstream face being 672.5 feet. It rests upon a granite foundation of excellent quality, especially good in the lower portions, where it is subjected to the greatest loads. The general bedrock level in the old river bed is from 60 to 80 feet below the present river bed, and the dam section has an area of about 1 acre at that depth. The dam is 348.5 feet high, 223 feet thick at its base, 15.5 feet thick at the thinnest point near the top, and carries a 16-foot roadway across its top, which is 1,100 feet long.

Two lines of grout holes at 10-foot centers were drilled across the entire length of the dam at distances of 5 feet and 13 feet, respectively, from the upstream face for the purpose of cutting off any passage of water under the dam. These holes penetrated 26 feet into

the bedrock and were grouted under pressure. In case water by chance finds a way past this line of defense, provision for relieving any upward pressure that may be exerted is made by a line of drainage holes drilled 26 feet into bedrock at 10-foot centers at a distance of 27.5 feet from the upstream face.

In order to drain any reservoir seepage water which may penetrate the concrete of the dam itself, 8-inch vertical drainage conduits were formed in the concrete at 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 any water which enters them to the inspection gallery, to which the rock drains also lead. Water is conveyed from the inspection gallery to the downstream face of the dam. through 24-inch drains located at convenient points.

Connecting with the inspection gallery and occupying positions just under the regulating outlets are the 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 one extends along the entire length of the dam, connecting with the inspection gallery at each abutment. The upper operating gallery gives access for the inspection of the upper portion of the dam.

Contraction joints are built at 150-foot centers, extending to the top of the dam from a point 215 feet below; at 50-foot centers from a point 130 feet below; and at 25-foot centers from a point 70 feet below. These joints were made by building alternate sections of the dam ahead of the others, allowing these to "set up" before filling in against them. Along these joints openings, called wells, were formed out; three near the bottom, decreasing to one near the top. The upstream well is 5 feet square, while the other two are 10 feet square. Five feet from the upstream face of the dam an annealed copper Z strip was placed in the joint to cut off the flow of water through it. Back of each upstream or second well a drain was formed in the joint which leads either to the inspection or operating gallery. The contraction joint 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.

There are 25 outlets through the dam. At the elevation of the river bed, 248 feet below the top of the parapet, are 5 sluice outlets 60 inches in diameter,