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desire to return the flood water to the river as far from the dam as feasible.
The spillway was designed to discharge 175,000 secondfeet with the water surface at the normal storage level, elevation 1,798. With the water surface at the estimated maximum probable flood stage, elevation 1,807, the spillway will discharge 225,000 second-feet. For this condition 25,000 second-feet will spill over the dam, making a total flood discharge of 250,000 second-feet.
A model of the spillway structure and its discharge channel, including a sufficient portion of the dam to give the actual entrance conditions, was constructed and tested for various reservoir water surface elevations. All tests confirmed the design of the spillway including entrance conditions and flow in the discharge channel under various gate openings. An interesting feature of the discharge channel is that it is curved in plan and profile and superelevated on the horizontal curve.
Two types of outlets are used for passing water through the dam to meet irrigation demands or for draining the reservoir; namely, high-pressure slide gates and needle valves.
The high-pressure slide gates are hydraulically operated and are to be used only under heads less than 100 feet;
but are designed to withstand full reservoir water pressure. There are three of these gates, each 6 feet by 7 feet 6 inches in size. They are placed in the base of an arch barrel spanning a bench on the left bank of the river. They will discharge water into a channel formed by the two buttresses that support this arch barrel. The bottom of the channel is paved to provide better discharge and to prevent possible erosion.
Two 66-inch needle valves, one directly above the other, are placed in the downstream side of the buttress on the left of the high-pressure slide gates. These needle valves are connected to the reservoir by two horizontal 72-inch diameter, %-inch plate-steel outlet pipes. The center line of the lower needle valve is 23 feet higher than the sills of the high-pressure slide gates, and the upper valve is 15 feet above the lower valve. For emergency and repair purposes an 8- by 16-foot steel bulkhead gate operated by gravity and a hand hoist is to be used at the upstream end of the outlet pipes. It will slide along the upstream face of the buttress and between the abutments of adjacent arch barrels.
The discharge from both sets of outlets will be carried through a channel formed by the canyon wall and by a concrete cantilever wall connecting the buttress and a rock promontory about 150 feet downstream, where future plans call for the construction of a canal headworks structure.
The entire outlet works system is designed to provide a discharge of 4,000 second-feet with the water surface in the reservoir at elevation 1,700, or 90 feet above the sills of the slide gates.
The following major items of equipment were on the job shortly after construction started: One 2%-cubic-yard shovel. One 1 ^-cubic-yard shovel. One 1 %-cubic-yard dragline. One Diesel tractor bulldozer.
One rigging tractor.
Six 8-cubic-yard dump trucks.
Three 10-ton trucks.
Four lj'2-ton trucks.
One 20-ton trailer.
Four 2-stage motor-driven compressors.
Additional equipment is to be added as the work progresses. An aggregate processing plant having a capacity of 150 tons per hour and a concrete mixing plant using a 2-cubic-yard tilting mixer are proposed for early erection. It is also planned to erect a 2,000-barrel bulk cement storage silo, with a 60-foot vertical elevator and a 16-foot screw conveyor, having a capacity of 30 tons per hour.
BOISE PROJECT, IDAHO
BY J. L. NEWELL, ASSISTANT ENGINEER, BUREAU OF RECLAMATION
DEADWOOD DAM is constructed across Deadwood River about 25 miles southeast of Cascade, Idaho. The site is 25 miles above the confluence of Deadwood River and the South Fork of Payette River, and 90 miles, measured along the stream, above Black Canyon Diversion Dam. The dam was built to create storage for supplementing the low flow of the Payette River for irrigation purposes and for power use at Black Canyon power plant.
The drainage area above the dam is 108 square miles. At normal high water, elevation 5,334, the dam forms a lake extending 3% miles upstream, covering an area of 3,180 acres and storing 164,000 acre-feet of water.
The dam is located in a narrow canyon at a point where Deadwood River turns about 90° to the right. The walls of the canyon are composed largely of exposed, mediumgrained granite. The right abutment was a projecting shoulder of fissured rock which was hard and sound but separated by cleavage planes into large, rectangular blocks with little or no cohesion.
In the stream bed, sound rock was encountered practically at the surface; the slight amount of river fill consisting of clay, sand, and gravel. A fault zone on the right side of the stream was excavated to a depth of 10 feet below the general foundation level. At this depth the fault seams narrowed down to a width of one-half to 2 inches and were filled with an impermeable, dark-colored gouge. It was concluded, from the geology of the formation, that fault movements had ceased and were not likely to recur.
The left abutment was an earth-covered slope, just upstream from a precipitous shoulder of exposed granite. Under the earth blanket the rock was massive, without open seams, and fairly hard.
Deadwood Dam is a constant radius concrete arch with an upstream radius of 290 feet, a gravity section tangent on the right abutment, and concentric arches with long
radius fillets at the ends of the lower intrados curves. A thrust block is constructed at the junction between the arch and gravity sections to take the radial shear transmitted by the upper elements of the arch section. The dam has a top length of 750 feet and a maximum height of 165 feet, measured from the main foundation bedrock to the roadway at elevation 5,340. The top width of the dam is 9 feet. The upper portion of the arch has a slope on the downstream face of 1 horizontal to 4.55 vertical. This slope is flattened out in the lower portions as increased thickness is required.
The thrust block has a top width of 9 feet, a slope on the downstream face of 1 horizontal to 1 vertical, and a length of 35 feet. The upstream face of the thrust block is vertical and coincides with the upstream face of the arch. The concrete gravity section has a 9-foot top, a vertical upstream face, and a slope on the downstream face of 1 horizontal to 2% vertical. The gravity section has a maximum height of 30 feet from the main foundation bedrock to the roadway. Its length, including the thrust block, is about 235 feet.
Exclusive of the spillway, all sections are provided with a 3%-foot parapet on the upstream side and a pipe handrailing on the downstream side.
The dam was built in 4-foot horizontal layers and was divided by vertical, radial contraction joints every 25 feet, except at the thrust block. The vertical joints were provided with a system of grout pipes for grouting the structure after shrinkage had taken place. Both vertical and horizontal joints were provided with keys.
The following types of dams were considered as suitable for this site:
(a) Concrete arch overflow dam with outlet conduits through the dam. (b) A rock-fill dam with the upstream slope faced with concrete. This dam was to have a separate unlined spillway and a tunnel for outlet control.
Comparative estimates were made and in January 1928, the concrete-arch type was selected as best for the site.
The arch dam was designed by the trial-load method and the data used in the design were as given below:
1. Weight of water 62.5 pounds per cubic foot.
2. Weight of concrete 150 pounds per cubic foot.
3. Modulus of elasticity of concrete in compression and tension 2,000,000 pounds per square inch.
4. Modulus of elasticity of concrete in shear 833,000 pounds per square inch, reduced to 666,700 pounds per square inch to allow for nonlinear distribution of shear, the analytical studies being made on the basis of a straight-line distribution.
5. Ratio of modulus of elasticity of foundation and abutment rock to modulus of elasticity of concrete, unity.
6. Maximum allowable compressive stress in concrete, 600 pounds per square inch.
7. No loads assumed to be transferred by tension forces.
8. Maximum elevation of reservoir water surface, 5,343.5.
9. No tail water assumed.
10. Elevation of spillway crest, 5,334.0.
11. Elevation of crest at roadway, 5,340.0.
12. Elevation of top of parapet, 5,343.5.
13. Uplift pressures were assumed to vary from full reservoir pressure at the upstream face of the dam to zero at the downstream face, these pressures to be applied to one-half the horizontal area of the base and to one-half the horizontal areas of the concrete sections at elevations investigated above the base.
14. Maximum range in average concrete temperature for arches of different thickness as caused by seasonal variations in air and water temperatures at the faces of the dam.
15. Grouting of radial contraction joints to be deferred until setting heat is completely dissipated and the average concrete temperature lowered to or below the mean annual concrete temperature corresponding to the mean annual air and water temperatures at the faces of the dam.
16. Reservoir water level to be drawn down to an elevation well below the crest of the spillway in the fall of the year so that ice loads will not subject the dam to severe strains.
The maximum inclined cantilever compression occurred at the downstream edge of the base of the crown cantilever and amounted to 472 pounds per square inch. The maximum horizontal stresses occurred at the abutment of the arch at elevation 5,220 and amounted to 305 pounds per square inch compression at the intrados and 184 pounds per square inch tension at the extrados. Cracked arches were not calculated, but the opening of the vertical contraction joints at the upstream face will reduce the tension to zero and increase the maximum compression to about 500 pounds per square inch.
The spillway is of the uncontrolled overflow type, being essentially a notch 100 feet long in the center of the arch section, 6 feet below the roadway, and 9.5 feet below the top of the parapet. The top of the spillway is built on an 8-foot radius, with the downstream portion ending on an
inclined tangent extending a little beyond the dam proper.
The bottom of the spillway section is provided with a bucket on a 50-foot radius that ends in a horizontal tangent at elevation 5,193.0. The bucket is 10 feet wider on each side than the spillway.
The spillway will discharge 11,000 second-feet with the reservoir water surface at the top of the parapet. Maximum discharge indicated by records at this point on the Deadwood River is about 2,000 second-feet.
The outlet works consist of two 66-inch inside diameter conduits through the base of the dam at the downstream end of which are installed 54-inch internal, differential needle valves protected by 4-foot 6-inch by 4-foot 6-inch emergency slide gates. The gates are protected from debris by a semicircular trashrack on the upstream face.
The valve house joins the dam on the downstream face of the dam and is of reinforced concrete construction. The lower floor is built to elevation 5.197.58, about 1.5 feet above the natural river bed. A traveling crane with a 10-ton hoist is provided for use in installing and repairing the gates and valves.
The maximum capacity of the two needle valves is 2,050 second-feet.
The contract for the construction of Deadwood Dam was awarded to the LUah Construction Co. of Ogden, Utah, July 23, 1929. The contractor commenced work in August 1929 and completed the dam in April 1931. The hauling of materials was an important item in construction. The 67-mile road from Cascade is entirely through mountainous country with an average grade of 4% percent and is closed by snow from November to June. Approximately 17,000 tons of material were hauled from Cascade to the dam at an average cost of 4 cents per ton-mile.
For temporary diversion, a wooden flume 24 by 8 feet in size was built at the left side of the channel. This flume was used successfully until the river was diverted through the diversion opening formed in the dam to the left of the outlet works. This opening was formed with radial sides and slightly arched top, 10 by 6 feet in cross section at the upstream face, and was provided with stop-log slots, keyway, copper seals, grout pipes, and two inclined openings from the downstream face to facilitate later filling with concrete. The river was diverted through this opening on July 21, 1930, and continuously thereafter until the emergency gates in the outlet conduits were in place and closed.
Foundation excavation was commenced in September 1929. The main part of the foundation was excavated by blasting and the material loaded on trucks with a power shovel. The excavation was finished by hand and the material removed by wagons and cableway skips. The cut-off trenches under the upstream toe and under the lower edge of the spillway apron were excavated by repeated shallow blastings and by barring and wedging. The total amount of excavation was 27,969 cubic yards.
Grout holes in the foundation rock were drilled at 5-foot centers in the bottom of the cut-off trench. The holes were 1 inch in diameter with a maximum depth of 40 feet. Grouting of the foundation was done after much of the mass concrete was in place. Holes were cleaned with air and water before grouting. Grout, with an average watercement ratio of 1.0, was forced into the holes with a maximum pressure of 100 pounds. In general, the rock did not take an appreciable amount of grout. There was 152 holes aggregating 4,389 lineal feet, and 574 sacks of cement were used in grouting, with an average of 3.75 sacks per hole.
Drain holes, two inches in diameter and 40 feet deep, were drilled 3 feet inside the upstream face. These holes were at 5-foot centers and connected with the pipe drainage system of the dam. The holes were not drilled until all of the adjacent grout holes within a minimum distance of 150 feet had been drilled and grouted.
Concrete aggregates were obtained from gravel pits along the river channel from 1 to 3 miles above the dam site. The plant for the production of aggregates was placed upstream from the dam site, above the left abutment. The plant was arranged to deliver the finished product by conveyor to the supply bins above the mixer.
Concrete was mixed in a 2-cubic-yard mixer, the mixer being set to discharge directly into the cableway bucket. Practically all of the concrete was placed by a cableway with bucket and chute system.