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SALT RIVER PROJECT, ARIZONA
BY E. R. DEXTER, ENGINEER, BUREAU OF RECLAMATION
BARTLETT DAM, now being constructed in central Arizona about 54 miles northeast of Phoenix, is located on the Verde River, the chief tributary of the Salt River. The dam is being constructed for the Salt River Valley Water Users Association by the Bureau of Reclamation to serve as a regulative means for conserving seasonal run-off. It will provide supplementary storage for irrigation of Indian lands along theVerde River and the Salt River Project now supplied by the natural flow of the Salt and Verde Rivers, augmented by storage impounded at Roosevelt, Horse Mesa, Mormon Flat, and Stewart Mountain Dams, all located on Salt River.
The contract for construction was awarded to the Barrett and Hilp and Macco Corporation of Clearwater, Calif., and notice to proceed with construction was given August 11, 1936. The formal date for completion of the project is May 9, 1939.
The annual run-off of Verde River near its mouth averages 573,000 acre-feet with a maximum of 1,822,000 acrefeet occurring in 1905, and a minimum of 117,000 acre-feet occurring in 1900. The drainage area above the gaging station is 6,100 square miles; and above the dam site, 5,600 square miles. From records on Salt River below the mouth of Verde River, the maximum discharge on the Verde River was that of March 1893, which was estimated at 144,000 second-feet. The second highest discharge was that of February 1891, which was similarly estimated at 135,000 second-feet.
The normal water surface for reservoir storage is at elevation 1,798. This provides a storage of 200,000 acre-feet, a surface area of approximately 3,170 acres, a maximum reservoir length of approximately 14 miles, and a maximum width of about three-quarters of a mile.
abutments. There is a relatively large area a short distance above the normal water surface elevation of the reservoir, but to utilize this area would require the construction of several small dams between promontories and also would necessitate a considerable increase in quantities of materials for the main dam. Economic considerations did not warrant the construction of a larger reservoir.
At the dam site the difference in elevation between the promontories and the stream bed is approximately 300 feet. The topographic differences are due to the effect of erosion and the weather upon rock of fairly uniform hardness.
The reservoir and the dam site lie wholly in a granite formation. Both fine- and coarse-grained granite make up the rock formations at the dam site. However, the dam is located entirely on the fine-grained granite, which is of excellent quality. On the left abutment the granite appears fresh and durable fairly close to the surface, while on the right abutment it is considerably weathered. The spillway channel is located for the greater part of its length in the coarse-grained granite. It is to be protected against erosion by concrete lining.
According to drilling records, nearly 70 feet of fill in the river channel covers the fine-grained granite in the dam foundation. It is probable that this fill is a coarse sand and gravel containing no large boulders. All drilling was done in the river bed, with foundation exploratory work above river level being done by test pits, trenches, and tunnels.
Drilling records confirmed the existence of a fault in the river bed formations, which had been indicated by rock conditions at the sides. The log of an inclined drill hole showed that the fault, approximately two feet wide and filled with gravel, traverses the river at the foundation of the dam. No concern was expressed by geologists over any danger of future movement, as all surface evidence indicates that a considerable period of time has elapsed since the fault was formed. Adequate concrete plugs are to be placed in the fault to provide support wherever a buttress or an arch barrel crosses it.
The Verde River flows in a southerly direction to its junction with Salt River; but it flows almost directly west at the dam site. The bottom of the valley at the dam site is approximately 275 feet wide, nearly half of which is occupied by the stream. The confining slopes are fairly steep, joining less abrupt surfaces on top of the ridges forming the two
Bartlett Dam is of the concrete multiple-arch type. This type was selected after investigations of several types; namely, earth and rock fills, straight concrete gravity, tance between these surfaces decreasing uniformly from the bottom of the buttress to the top and from the upstream edge to the downstream edge.
The upstream edge of the buttress, or springing line of the arches, has a slope from the vertical of 0.9. The downstream edge has a slope of 0.36 from the vertical. Thicknesses of the main walls vary from 24 inches near the top, elevation 1,795.5, to 7 feet near the base of the maximum buttress, elevation 1,540, these thicknesses applying only along the upstream edges. The thicknesses of the main walls along the downstream edges vary from 24 inches at elevation 1,795.5 to approximately 4.5 feet at elevation 1,540. The upstream face slab has a normal thickness, excluding fillets on the under side, varying from 24 inches at elevation 1,795.5 to 7 feet at elevation 1,540, the same thickness as the upstream edge of the main walls. The stiffener walls and the downstream face slab have a normal and constant thickness of 18 inches for their entire height. Footings for the buttresses will have a minimum depth and a base thickness equal to and varying as the thickness of the main walls.
The buttresses were designed in accordance with a modification of the theory presented in a paper entitled “Economical Design of Buttresses for High Dams and of Cellular Gravity Dams”, by Fredrik Vogt, Det. Kgl. Norske Videnskabers Selskab, Trondhjem, Norway, Forhandlinger, Bd. II, No. 40, p. 141, 1929; and of the theory presented by Herman Schorer in his paper entitled “The Buttressed Dam of Uniform Strength” published in the Transactions of the American Society of Civil Engineers, vol. 96, 1932. A limiting compressive stress of 600 pounds per square inch and a limiting value of 0.7 for sliding factors for normal conditions, using a unit weight of concrete of 150 pounds per cubic foot, were used in the design. Check computations were made using the conventional methods of gravity analysis on horizontal sections.
Buttresses are to be constructed in vertical column units
Bartlett Dam model, view from above.
concrete arch, diamond-head buttress, Ambursen slab and buttress, massive multiple arch, and lightweight reinforced multiple arch. The dam will have a maximum height of 273 feet, measured from the lowest point in the foundation to the top of the parapet, elevation 1,803. When completed it will be the highest multiple-arch dam in the world. The dam is slightly curved in plan so as to most economically fit the topography, the radius to the axis being 1,379.7 feet. The crest length, including the 10 arch-barrel sections, the gravity blocks at each end, and the spillway gate structure at the right abutment, is approximately 950 feet.
The dam was designed for a normal reservoir water surface at elevation 1,798, thus providing a 5-foot freeboard below the top of the parapet. Stresses and stability factors were checked for a maximum water surface 4 feet above the top of the parapet. Effects of earthquake disturbances on the stability of the dam were also considered. Assumptions for earthquake effects were that the dam would move horizontally upstream and downstream, that the period of vibration would equal one second, and that the acceleration would equal one-tenth gravity.
Buttresses are of the hollow type, consisting of (1) two main walls each directly supporting an abutment of an arch barrel, (2) an upstream face slab, (3) a downstream face slab, and (4) vertical stiffener walls between the main walls for providing greater lateral stability for the buttress. Each main wall has a vertical plane surface on the inside and an inclined plane surface on the outside, with the dis
by leaving 18-inch openings at 41-foot 6-inch centers in the main walls, these to be filled with concrete after the initial shrinkage has taken place. The sides of the openings were designed to transfer computed stresses from one column unit to the next by means of keyways or steps at 5-foot vertical centers, the slope of the faces of the keyways being determined from the computed directions of the resultant forces acting through the buttress. Openings are provided through the top and bottom of the stiffener walls and through the main walls to permit ventilation of the inside of the buttress and to aid in securing a more uniform distribution of temperature in the main walls.
Reinforcement steel consisting of vertical bars and inclined bars parallel to the upstream edge of the buttress, was placed near the outer surfaces of the main walls to provide some means of counteracting the effects of the variation of the outside air temperatures. In placing the steel the specifications call for lapping the inclined bars in the vertical openings. Bond for the laps will be provided when the openings are filled. The upstream face slab is reinforced to carry the loads due to water pressure and temperature changes, also those due to the reactions of the arches at their abutments; and to assure better structural action for the buttress. Stiffener walls and the downstream face slab are reinforced by steel which ties the reinforcement in the outer surfaces of the main walls and aids in making a more rigid buttress.
All arch elements of the barrel are circular, have a total central angle of 180°, and have constant thicknesses in a plane normal to the springing line. The radius of the downstream face is constant and is equal to 24 feet. The
thickness of the arch barrel along the springing line varies uniformly from 24 inches at elevation 1,795.5 to 7 feet at elevation 1,540. At any elevation along the springing line the thicknesses of the arch elements, the upstream face slab, and the main walls of the buttresses are equal.
Surmounting the arch barrels are horizontal ribs having a vertical dimension of 3 feet and a horizontal thickness of 4 feet. These ribs serve three purposes; first, to give support and stiffness to the top of the barrel; second, to support a parapet; and third, to provide a walkway across the dam. A 2-inch standard railing flanks the downstream edge. A trussed walkway through the buttresses, at about the elevation of the upper needle valve outlet, and a stairway in a buttress near the spillway are provided for operating purposes.
The arches were designed using the following assumptions for normal operating conditions:
1. For reservoir empty conditions, with the arches subjected only to loads induced by their own weight and by temperature variations, the maximum allowable compressive stress for concrete was 750 pounds per square inch and the maximum allowable tensile stress for reinforcement steel was 20,000 pounds per square inch. The concrete was assumed to take no tension.
2. For reservoir full conditions, with the arches subjected to water loads in addition to their own weight and the effects of temperature variations, the maximum allowable compressive stress for concrete was 650 pounds per square inch and the maximum allowable tensile stress for reinforcement steel was 10,000 pounds per square inch. The concrete was assumed to take no tension.
double purpose, namely, to function as straight gravity dams and to prevent water pressures from acting against the sides of the end buttresses. The gravity block at the right abutment was also designed to provide a suitable entrance for the spillway.
A grout curtain is to be provided in the foundation rock under the arch footings, under the footings of the upstream face slabs of the buttresses, and under the gravity sections. It is planned to drill alternate holes to depths varying from about 100 feet at the maximum section of the dam to about 50 feet at the abutments, and to drill intermediate holes to depths varying from about 75 feet at the maximum section of the dam to about 25 feet at the abutments. The holes are to be spaced at 5-foot centers. A grout curtain is also to be provided beneath the spillway structure.
Drainage is to be provided for the gravity sections of the dam, the floor of the spillway gate structure, and the spillway channel, to prevent the development of uplift.
The estimated major quantities involved in the construction of the dam are as follows:
3. Maximum allowable compressive stress for the reinforcement steel, 12,000 pounds per square inch.
4., Sustained modulus of elasticity for concrete, 3,500,000 pounds per square inch.
5. Modulus of elasticity of reinforcing steel, 30,000,000 pounds per square inch.
6. n=8.57. 7. Unit weight of concrete, 150 pounds per cubic foot. 8. Unit weight of water, 62.5 pounds per cubic foot.
9. Coefficient of expansion and contraction of concrete 0.000005.
Arches were analyzed by the elastic method in which deformations of the arch due to rib-shortening and shear were considered in addition to those caused by bending. There was also included in the analysis the rotational yielding of the arch abutments due to the elastic properties of the supporting buttresses.
Weather bureau records were closely studied to determine the temperature conditions under which the dam will be operated. The records of air temperature variations, the temperature of the water in the reservoir having similar climatic conditions, and the thermal conductivity of concrete were then correlated and a temperature variation was determined for which the arches may be subjected for both reservoir full and empty conditions.
Design of reinforcement steel was based on the analysis of the constant thickness, circular arch elements normal to the springing line. All longitudinal steel was designed to function in the plane of the arch elements and was spaced at 8-inch centers throughout the arch barrel. All lapping of longitudinal bars is to be made near the one-eighth points of the arch adjacent to the crown, and at the oneeighth point adjacent to the abutments. The steel between the abutments and the adjacent one-eighth points is to be placed and extended into the buttress a minimum distance of 3 feet before the buttress concrete is poured. This is done to avoid lapping the steel at the abutments where maximum moments occur. A construction joint with a 2-inch depth keyway is to be formed at the junction of the arches and the buttress. Spacer bars, normal to the longitudinal bars, are to be spaced at 12-inch centers between the abutments and the adjacent one-eighth points and at 24-inch centers through the remainder of the arch.
Arch barrel footings are designed to have a minimum depth of 114 times the normal thickness of the arch barrel at the point of support, and a base width of not less than the normal thickness.
Two relatively small gravity sections, one at each end of the multiple arch portion of the dam, complete the structure, except for the spillway. The section at the left abutment will have a maximum height at the axis of approximately 45 feet, and the one at the right abutment will have a maximum height at the axis of approximately 90 feet. The gravity sections were designed to serve a
1. Excavation, common, for dam.......... 140,000 cubic yards. 2. Excavation, rock, for dam......... 75,000 cubic yards. 3. Excavation, common, for spillway... 10,000 cubic yards. 4. Excavation, rock, for spillway....... 112,000 cubic yards. 5. Grout holes, total depth...............
29,000 lineal feet. 6. Pressure grouting ...........
43,500 cubic feet. 7. Concrete in buttresses and outlet works
structures. ............ ........ 95,500 cubic yards. 8. Concrete in arches.......
35,000 cubic yards. 9. Concrete in gravity sections and training wall....
11,000 cubic yards. 10. Concrete in spillway gate structure and
spillway channel lining .............. 15,900 cubic yards. 11. Reinforcement bars...
5,800,000 pounds. 12. Spillway gates and accessories. ......... 2,150,000 pounds. 13. Trashrack metalwork . ......
274,000 pounds. metalwork................. 14. High-pressure slide gates and metal conduit linings ..............
489,000 pounds. 15. Bulkhead gate and accessories ........... 26,000 pounds. 16. Needle-valve outlet pipes .............. 142,000 pounds. 17, Needle valves and accessories.......... 175,000 pounds. 18. Metal stairways, walkways, and ladders.. 111,000 pounds. 19. Pipe handrails . . . .................... 20,000 pounds.
The spillway structure is a Stoney gate type, except that a system of fixed wheels, instead of rollers, is being used for the gates to bear against. The structure is composed of three 50- by 50-foot gates which will rest on a sill having a crest at elevation 1,748. Water flowing through the gates will be carried by a channel, lined with concrete for approximately three-fourths of its excavated length, and discharged into a natural draw which will carry it into the river about 900 feet downstream from the dam. For the protection of the dam the location of the channel was determined partly by the topography and partly by the
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.
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, 36-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.
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;