THE STORAGE SYSTEM of the Yakima project consists of six reservoirs, with a combined storage capacity of 1,039,000 acre-feet, to conserve the surplus winter and flood run-off for subsequent irrigation use on 277,000 acres of land within completed or proposed divisions of the Yakima project, and to supplement the water supply for 120,000 acres within the Wapato Indian Reservation and about 50,000 acres of privately developed lands under Warren Act contracts. Tieton Reservoir supplies about 20 percent of the storage demand.

Tieton Dam, on the Tieton River 30 miles west of Yakima, Wash., is 42 miles from Yakima by road and 26 miles west of Naches, the nearest railroad point. Construction was started in January 1917, but was discontinued in July 1918, on account of the World War. Work was resumed in April 1921, and the dam was completed in April 1925.

RESERVOIR

The reservoir has a volume of 202,500 acre-feet with a 2-foot depth of water passing over the raised spillway drum gates; and 197,000 acre-feet capacity at elevation 2,926.0, the top of the drum gates when raised. At this elevation the reservoir is 9 miles long and has an area of 2,500 acres. The drainage area upstream from the dam is 187 square miles. The average annual run-off over a period of 18 years was 350,000 acre-feet. The reservoir site was heavily timbered and required the clearing of 2,700 acres.

The damsite is at a narrows in Tieton Canyon with a solid rock (andesite) cliff at the west abutment and a more gently sloping mountain side, with an earth cover 30 to 60 feet deep over the bedrock, at the east abutment. The cover in the river section was 92 feet to shale. The spillway and diversion tunnel are located in the rock cliff at the west end of the dam.

THE DAM

Tieton Dam is an earth, gravel, and rock fill embankment, with a concrete corewall extending from bedrock to the crest and anchored in solid rock on both abutments. The height from the deepest corewall foundation to the

crest is 321 feet. The total yardage in the embankment is 1,995,000 cubic yards, of which 1,570,000 are earth and gravel and the remainder rock. The length of the dam along the crest is 905 feet and the thickness from toe to toe is 1,110 feet through the river section. The total quantity of concrete in the various parts of the structure is 43,600 cubic yards.

Six test pits were sunk to bedrock along the axis of the dam in 1917-18, to test the foundation. These disclosed a cover of 20 to 60 feet of earth on the east abutment, overlying andesite bedrock; and 90 feet of compact gravel, clay, and sand for 150 feet across the river bed, overlying compact shale. The andesite was drilled to a depth of 20 to 65 feet; and the river pit was sunk 22 feet into shale and the shale investigated with a diamond drill to a depth of 67 feet. These investigations gave proof of a satisfactory foundation.

On resuming work in 1921 it was decided to excavate the cut-off trench for the corewall through the overburden by mining methods rather than by open cut. The test pits were rigged as working shafts with frames, cages, and hoists. At the bottom of the shafts, drifts were started in either or both directions and the spoil hoisted to the surface. Mine timbering and lagging were installed, and when the bottom drift had progressed a sufficient distance, the top lagging was taken out at convenient points and enough material picked or barred down into waiting cars to permit the placing of timbers. Then this drift was extended and successive drifts opened in a similar manner. As soon as possible the lower drifts were filled with concrete and the corewall built up from the bottom. All drifts were dug into bedrock until solid rock was found, requiring a minimum penetration of 10 feet. The thickness of the wall below ground was 5 feet, with the base widened so that the pressure on the foundation would not exceed 8 tons per square foot. The wall was carried 20 to 30 feet into the shale across the river bed. Five holes were drilled into the rock, and, when grouted, were found so tight that no further grouting of the corewall foundation was necessary.

Plain concrete of 1:2%: 5 proportions, by volume, was used below ground. The concrete was completed to the ground surface in the river section March 2, 1922. The

corewall above ground varied from 5 feet thick at the ground surface to 1 foot thick at the dam crest, both faces having a batter of 1:100. The wall above ground was heavily reinforced and of the same mix as below ground. Wooden forms were used, and the wall was kept 10 to 40 feet above the pool elevation in the sluice fill. Excavation and concreting in the underground portion of the abutments was kept slightly ahead of the exposed portion. Two 4-foot diameter observation wells were built on the downstream face of the corewall to facilitate future inspection of the corewall and puddle core. The corewall was carried above the top of the embankment to form a parapet 3 feet high. Tieton Dam was designed and built as an earth fill type, using the hydraulic method for separating the fines from the fill material and concentrating them at the upstream side of the corewall and for washing the fines from the material on the downstream side of the corewall. Both

the upstream and downstream slopes are reinforced by rock fills. Watertightness was obtained through the comparatively thin puddle core and the concrete corewall.

The embankment above the corewall was built of earth, gravel, and boulders, excavated from borrow pits by steam and electric shovels, loaded on cars, hauled to the outer edge of the upper slope of the dam and dumped from trestles. Streams of water from hydraulic giants, developing about 85 pounds nozzle pressure, were then directed on the dumped material. This sluicing tended to separate the smaller rocks, sand, and clay and carry them from the dump toward the corewall, leaving the coarsest rocks on the outer slope, well compacted, and the voids filled with smaller rocks and gravel. The suspended material in the water was gradually deposited on a gently sloping beach, extending from the dump to the core pool. The width of the pool was maintained slightly greater than one-third the

distance down from the top of the dam. By the time the sluicing water reached the pool it held in suspension only the finest sand and clay. The fine sand was deposited at the outer edge of the pool, and the clay towards the corewall. Thus was built up a tapering puddle core of impermeable material in front of the corewall.

The height of the water in the pool was regulated through conduits in the west abutment, connected to the diversion tunnel. The coarse material near the trestles washed to a slope of 1+1; the slope to the pool from the edge of the coarse material was 1:10. The top 20 feet was finished with top soil from the river bottom. Large boulders from the borrow pit and rock from the tunnel and spillway were placed as an added layer outside of the coarse sluice fill. The upper 100 feet of the slope was finished off with 4 feet of riprap placed on a 3:1 slope.

The earth embankment material below the corewall was secured from the coarser material in the borrow pit, hauled to the lower side of the corewall at the toe or on the slope

on a 1:1 slope, and dumped from trestles. This material was sluiced in the same way the material upstream of the corewall was treated; but no pool was maintained and the fines were carried off by the water to leave a fill grading from very coarse material at the outer slope through finer rock and gravel to coarse sand at the corewall. This left a heavy, compact, free-draining fill below the corewall. Clean rock from the tunnel and spillway was used to build out the embankment to a 22:1 slope. The top was finished one foot above the proposed height, to allow for settlement, using 18 inches of spillway rock and 6 inches of clean gravel. In constructing the sluiced fills, great care was taken to maintain the corewall in as vertical a position as possible. Readings were taken at least once daily; and the relative heights of the pool above the wall, and the fill below, were maintained to keep the wall nearly vertical. Especial care was taken to prevent permanent movement downstream. The maximum movement in any one day was 0.03 of a foot. and the maximum for any one point during the time of

observation was 0.20 of a foot. The depth in the pool was varied to allow the fines to settle out before the water was discharged into the diversion tunnel, the variation in the pool elevation being from 2 to 8 feet, due to varying fines in the borrow pit material.

Pressure cells, pipes with well points connected to pressure gages, and 3-inch open pipes with gate valves were placed in the fill upstream from the corewall to study the consolidation of the puddle material. The ends of these test pipes are in the observation wells so that it is always possible to obtain data. Brass plugs were set in the bottoms of the observation wells, so that movement of the corewall can be observed at any time.

SPILLWAY

The spillway consists of a side-overflow concrete structure with six 65- by 8-foot drum gates installed on the crest. The axis of the spillway crest is approximately at right angles to the axis of the dam. The flow from the spillway is carried well below the dam in a concrete lined channel. The spillway and the channel are located in solid rock at the west end of the dam. With the gates down the crest elevation is 2,918 and the discharge capacity 30,000 second-feet with 8 feet of water on the crest and an 11-foot freeboard on the dam. The spillway will discharge 50,000 second-feet with 12 feet of water over the spillway and a freeboard of 7 feet on the dam. The maximum discharge recorded was 8,450 second-feet on December 22, 1933.

The drum gates weigh about 45 tons each. Each gate is controlled independently of the others and each may be operated by hand. The two gates nearest the dam are provided with automatic siphonic controls, so that the elevation of the reservoir water surface can be maintained at a certain point with varying amounts of water passing the

crest.

The foundation rock under the crest was thoroughly grouted. A box drain under the channel floor runs the length of the overflow section and partly the length of the waste channel; pipe drains and broken rock drains connect into this from underneath the floor and the concrete lined side slopes of the channel.

OUTLET WORKS

Diversion of the river for the construction of the corewall and embankment was provided by a tunnel 22 by 19%1⁄2 feet through the west abutment of the dam. The tunnel was designed to be the permanent outlet works for releasing water during the irrigation season and to control the reservoir water surface elevation, if required, below the spillway crest. The tunnel is 2,120 feet long, about 700 feet of this being downstream from the axis of the dam. A reach of 160 feet at the upper end is located in earth and is lined

with concrete, the lining being a 17-foot horseshoe section. The lining was extended as a conduit for 60 feet outside the tunnel in order to keep the trashracks and intake well away from the hillside at the tunnel portal.

Emergency gates are located in a gate chamber excavated above the tunnel on the line of the corewall. The tunnel is lined for 500 feet upstream from the gate chamber. The gate chamber was thoroughly grouted. An auxiliary intake is located upstream from the spillway with a sill elevation of 2,820. The elevation of the sill of the intake at the upper end of the tunnel is 2,761; and 175,000 acre-feet can be drawn from the reservoir below the crest of the raised spillway gates.

The outlet works consist of two 5- by 6-foot, hydraulically operated, emergency slide gates, discharging into two 72inch, welded steel pipes provided with two 60-inch and one 24-inch balanced valves for control. The emergency slide gates are provided with two independent sets of controls, one in the gate chamber immediately above the gates and the other in a house on top of the dam. The pumps in the gate chamber are operated by a motor deriving its power from the generator located in the balanced valve control house. The pumps in the house on top of the dam are operated by a gasoline engine.

The two 72-inch pipes are 700 feet long, and are provided with expansion joints and air relief valves. A turbine and 30-kilowatt generator in the balanced valve control house supplies power for operating the emergency gates, lights about the dam, and for the watchman's house and the camp. The balanced valves have a combined discharge of 2,400 second-feet with water at the spillway crest elevation of 2,926. The trashrack area at the tunnel entrance is 850 square feet, or 14 times the area of the emergency gates.

CONSTRUCTION

Tieton Dam was built by Government forces, with small contracts let for hauling, clearing, woodcutting, and similar items. The construction season for the above ground operations was from April to October, inclusive, high water coming in May and the larger floods in November and December. The first work required was the reconstruction of 21 miles of road from the highway into the camp. This included the building of five bridges. The area which could be devoted to camp purposes was so limited that twostory bunkhouses with a central heating plant and steam heat were used. Twelve bunkhouses with individual rooms to accommodate 470 men were built. The camp was largely a family camp, consisting of 112 houses. The largest force employed was 570 men. An excellent water supply system, operating under a head of 125 feet, supplied water for domestic use and fire protection. A 1,000-kilowatt, hydroelectric plant furnished light and power for the job and camp.