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was obtained from the hill lying directly west of the dam site. Material for the left embankment section was obtained from a borrow pit at the east end of the dam. Both embankments were constructed by depositing the earth in 6-inch layers, sprinkling, and rolling with a 10-ton roller. Rock for riprapping the embankments was obtained from the excavation for the base of the left abutment section, the excavated material being stored in piles in front of the dam until needed.

Construction of the permanent bridge over the dam was begun in February 1927, the work being started at both ends and proceeding toward the center. The construction plant consisting of a %-yard concrete mixer, a circular saw for cutting forms, and a hoisting tower, was placed near the center of the left abutment section. Concrete was delivered to the forms by trucks, running along the 8-foot top of the dam. Practically all exposed concrete was poured in metal-lined forms thus obtaining a smooth finish. Concrete work was finished August 30, the asphalt pavement was completed September 14, and the bridge was opened to traffic September 24, 1927.

COST DATA

Table 1 gives the quantities, total costs, and unit costs of the various items included in the construction of the dam. Table 2 gives similar data for the highway bridge. The contractor's profit amounted to about 14.1 percent of the total in the case of the dam, and to about 6.8 percent of the total in the case of the bridge. The contractor cooperated with the engineers in keeping accurate detailed records of cost of all classes of work. Wages paid by the contractor were from S3.60 to $4.80 per day for common labor, from $4.80 to $8 per day for skilled labor, from $125 to $250 per month for miscellaneous employees on a monthly rate of

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Quantities are given in percentages of the total cost as well as in absolute values. It will be noticed that the cost of the dam alone amounted to only about 35 percent of the total cost of the reservoir. The expense incurred because of the fact that the city of American Falls was originally built in the reservoir site amounted to about 23 percent of the total, the expenditures for rural property affected to about 12.4 percent, the expenditures for Indian land adjustments to about 9.8 percent, and the expenditures made to cover the interests of the Idaho Power Co., to about 8.7 percent. The total cost of the reservoir was $7,355,315, which was $541,685 less than the engineer's estimate and an average of only $4.33 per acre-foot.

Table 3.—Cost of American Falls Reservoir

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BIBLIOGRAPHY

Reclamation Bureau New Dam in Idaho, Eng. and Contr., Waterworks Issue, Oct. 8, 1924, p. 804.

Large Concrete Dam to be Built at American Falls, Engineering News-Record, Nov. 6, 1924, pp. 741 and 742.

American Falls Dam on Snake River Will Supply Water to Twenty Irrigation Districts, by R. E. Shepherd, Western Construction News, Apr. 10, 1926, pp. 41-43.

American Falls Project Completed at Cost of $8,500,000, Modern Irrigation, June 1927, pp. 51 and 64.

New Town site of American Falls, Idaho, Reclamation Era, February 1928, pp. 22 and 23.

Moving the City of American Falls, by Ivan E. Houk, Western Construction News, Oct. 25, 1928, pp. 648-653.

American Falls Dam, Minidoka Project, Idaho, by Ivan E. Houk, Reclamation Era, February 1929, pp. 26-29.

American Falls Dam, by Ivan E. Houk, Western Construction News, Sept. 25, 1929, pp. 480-490.

STONY GORGE DAM

ORLAND PROJECT, CALIFORNIA

BY S. E. ROCKWELL, ENGINEER, BUREAU OF RECLAMATION

THE ORLAND PROJECT is located in the north central portion of California and receives its water supply from Stony Creek, a tributary of the Sacramento River. Construction was started in 1908 for a project of 14,000 acres, which was extended in 1914 to 20,500 acres. The principal storage features of the extended project prior to the construction of Stony Gorge Dam included the East Park Reservoir, with a capacity of 51,000 acre-feet, on Little Stony Creek, a tributary of Stony Creek, and a 4-mile feed canal, extending from the East Park Reservoir to a diversion from Stony Creek.

While these storage facilities were adequate during the early stages of development of the project and during years of ample run-off, they were found entirely inadequate in later years.

The heavy shortages and the financial losses suffered by the water users on the project during the years 1918, 1920, and 1924 resulted in an appropriation by the Congress in the spring of 1926 for starting construction of the Stony Gorge Dam to provide a storage capacity of 50,200 acrefeet.

The Stony Gorge Dam is located on Stony Creek about 25 miles north and west of Willows, Calif., the nearest railroad point on the main line of the Southern Pacific Railroad, and about 8 miles west of Fruto, the nearest point on a branch line of the same railroad.

Stony Gorge Dam creates a reservoir 5% miles long, with surface area of 1,280 acres at a normal water surface elevation of 841 feet above sea level. The drainage area above the dam is 275 square miles, from which the largest flood in 9 years of record, 1919 to 1928, inclusive, 12,000 cubic feet per second, occurred during the construction period on March 26, 1928. However, it is believed from comparison with other stream records of longer duration that much greater floods are to be expected, and a spillway capacity of 30,000 cubic feet per second was adopted in the design of the dam.

INVESTIGATIONS

Following the usual practice of the Bureau of Reclamation, careful investigations were made to determine the suitability of the damsite and reservoir basin for water retention, and to decide on the type of dam to be constructed. The investigations included 10 core-drill test

holes in the creek bottom, and a number of open test pits in the abutments where solid rock was not exposed.

The reservoir basin is situated within the general region embracing the Knoxville formation, which consists of a thick and well indurated series of clay shales, hard sandstone, and pebble or boulder conglomerates. The reservoir basin is generally underlain by shale beds. These have a north and south strike, parallel to Stony Creek Valley, and a dip of about 60° to the east; therefore any leakage from the reservoir must travel normal to the bedding of the shale, except at the ends of the reservoir. This condition, when taken with the highly impervious character of the shale, gives ample assurance against appreciable seepage loss from the reservoir basin.

At the north end of the reservoir basin, Stony Creek turns westward and cuts through a low range of hills which forms the damsite, the location of the creek at the damsite being predetermined by the presence of a fault line. This fault constituted the most unfavorable condition at the damsite and it was naturally given special attention in the geological investigations.

Looking northward at the north abutment of the damsite, the following rock formations are exposed from east to west:

(a) Sandstone with a negligible content of shale. Surface exposure about 500 feet east and west.

(b) Pebble and boulder conglomerate, with some sandstone but practically no shale intermixed. Surface exposure of conglomerate about 75 feet.

(c) Interbedded shales, shaly sandstone, and sandstone—the shaly phase predominating. Surface exposure about 200 feet.

The same sequence of formation is exposed in the south abutment, except that due to faulting all of the strata have been moved eastward about 150 feet. The main conglomerate strata appear to be somewhat thinner on the south abutment and in the creek bed (south of the fault line) than on the north abutment. The evidence of faulting is, however, very conclusive from the similarity, sequence, and position of the rock formations.

All of the rock formations at the damsite are comparatively impervious, and as leakage past the dam would have to pass through the strata normal to the bedding, there appears to be no possibility of appreciable leakage except along the fault line which was thoroughly grouted.

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THE DAM

Before adopting the Ambursen type of dam, preliminary designs and estimates were prepared for six different alternative types, including, (1), rock fill with concrete face; (2), earth fill with concrete core wall; (3), earth and rock fill; (4), concrete gravity section; (5), multiple arch; (6), Ambursen.

The rock fill with concrete face, the earth fill with concrete core wall, and the earth and rock fill were all eliminated on account of their high cost, and for the reason that any considerable movement along the fault line would cause the complete and sudden destruction of any of these types, resulting in large property damage and possible loss of life. Likewise, the multiple arch dam, notwithstanding its low estimated cost, was eliminated because any failure due to movement along the fault line would probably result

in the complete and sudden destruction of the greater portion of the dam, due to the condition that every individual arch is dependent on the two adjoining arches for arch thrust support. The concrete gravity section was eliminated on account of the high cost.

In adopting the Ambursen type the engineers gave full consideration to its low cost, and particularly to the limited extent of failure likely to result from movement along the fault line. In the noncontinuous type of Ambursen dam, the face slabs act as simple beams, spanning from buttress to buttress, and are free to adjust themselves to any slight movement of the buttress. This articulated construction is particularly well adapted to the Stony.Gorge site, and is believed to give immunity against failure from any minor movement along the fault line. While a fault movement of several feet might result in the failure of one or two bays of the structure, such a failure would probably not extend beyond the immediate bays affected by the fault movement, as each bay of the structure is self-supporting and independent of the adjoining bays for lateral stability.

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The location of the dam with respect to the geology was based on three important considerations, including (1) utilization of the most suitable rock strata for the dam foundations and cut-off trench; (2) utilization of the most resistant rock strata available in the creek channel to withstand erosive velocities below the spillway apron; and (3) the position of the spillway structure, outlet works, and the buttress of the dam with respect to the main fault and secondary fault lines.

The final position of the dam was largely determined by the third criterion, wherein the spillway, outlet features, and buttresses were located to avoid as far as possible the main fault line. The spillway section was located on the south side, and the outlet works on the north side, of the main fault. This was considered very important in order to protect the spillway gates and the outlet gates and valves, against damage from fault movement. Buttress no. 34 crosses the main fault, unavoidably; and buttresses nos. 28 to 33, inclusive, cross secondary faults. Should any future movement be limited to the main fault, as would be the most likely case, the spillway and outlet works would escape damage, and the failure of the dam would probably be confined to the two bays between buttresses no. 33 and no. 35. If thus confined, the damage to the structure would be relatively slight, and the damage from released waters, while probably extensive, would be very much less than from complete failure of the dam.

The dam is 125 feet high above the stream bed and 142.5 feet high above the lowest concrete in the cut-off trench. It is 868 feet long on top, including 310 feet in the left abutment section, 108 feet in the spillway section, and 450 feet in the right abutment section. The spillway section is provided with an ogee face slab on the downstream side of the dam, terminating in a concrete apron extending 50 feet below the dam, for stream-bed protection. A gatehouse is provided over the spillway gates to house the hoisting machinery and to furnish supporting walls for a traveling crane. A concrete walkway on top of the dam extends in both directions from the spillway gatehouse to the two ends of the dam. The spillway is located in the creek channel and the needle-valve outlets at about the center of the dam, north of the creek channel.

DESIGN DATA

Overturning is not a criterion in the design of an Ambursen dam, as the resultant of all forces at any elevation inherently passes close to the center of the base. This condition results in a practically uniform distribution of load on each buttress footing and in approximately equal compressive stresses through the length of the buttress.

In Stony Gorge Dam a limiting value of 0.65 was as

sumed for the sliding factor, and the actual values vary from a maximum of 0.63 at average height of the dam to a minimum of 0.45 at a height of 12 feet. At the maximum height of the dam the sliding factor is 0.60.

A limiting value of 100 pounds per square inch was assumed for horizontal shear in the buttresses; and the thickness of the buttresses at all elevations was controlled by this assumption, except near the top of the dam, where a minimum buttress thickness of 18 inches was adopted for the top lift of the dam. This minimum thickness was increased by 2 inches at every succeeding 12-foot lift, until the limiting value of 100 pounds per square inch was reached. Below this point the increase in buttress thickness was made just sufficient to maintain the assumed value for horizontal shear.

Studies and estimates were made of comparative costs for the finished structure, with buttress spacings of 16, 18. 20, 22, and 24 feet, and with face slabs of corresponding thicknesses; also for heights of dam varying by 12-foot lifts up to 120 feet. The results of the estimates indicated that the usual buttress spacing of 18 feet was the most economical for this structure.

The lower side of the face slab makes an angle of 45° with the horizontal, and the upper side tapers uniformly from 15 inches in thickness at the top to 50 inches at a height of 120 feet, the taper being 3% inches in 12 feet, vertically. Four different alternative systems for reinforcing the face slab were studied, and quantity estimates made as follows:

(1) Size of bar variable, spacing variable.

(2) Size of bar constant, spacing constant.

(3) Size of bar variable, spacing constant.

(4) Size of bar constant, spacing variable.

The first and last systems listed gave the smallest quantities, while system no. 2 gave the largest, and system no. 3 the next to the largest quantities. System no. 4 was adopted, using 1-inch round bars spaced 12 inches on centers in the top lift, 7 inches in the second lift, 5)4 inches in the third lift, 4U inches in the fourth lift, and 4 inches in the fifth to tenth lifts, inclusive. The resulting stresses in the steel varied from 12,500 pounds per square inch in the top lift to a maximum of 17,000 pounds per square inch in the eighth and ninth lifts. The corresponding stresses in the concrete vary from 380 to 624 pounds per square inch. The maximum shear in the face slab, at the edge of the buttress corbel, is 78 pounds per square inch. Every third bar in the face slab is bent up near the edge of the support for shear reinforcement.

The buttresses were first designed with %-inch round bars, 18 inches on centers, placed diagonally and parallel to the face slab in both sides of the buttress; and with %-inch round bars 3 feet on centers, placed vertically in both sides of the buttress. After several of the largest buttresses were started, vertical shrinkage cracks began to appear, and it was decided to add horizontal reinforcement in all buttresses

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