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CHAPTER ni—DESIGN

Dam

11. Embankment. - O'Sullivan Dam, called Potholes Dam until the name was officially changed on September 28, 1948, is a zoned earth-fill structure with a length

of approximately 19,000 feet and a height ranging up to 160 feet above streambed (fig. 6). The crest elevation is 1061.0. The total embankment volume including riprap and gravel blanket approximates 8, 770, 000 cubic yards of material. The dam is protected by an open-channel spillway 500 feet wide, excavated in rock with a concrete crest. A 30-foot gravel roadway is constructed along the crest of the dam embankment and through adjoining rock cuts. That portion of the roadway which extends across the concrete spillway crest was designed for use as both a spillway and road.

The dam embankment was designed as a zoned-earth fill consisting of three zones. The central impervious zone (zone 1) is made up of selected sand, silt, and clay, spread and compacted by rolling to 6-inch layers. The slopes of this zone were allowed to vary somewhat but in general are 1-1/4 to 1 upstream and 3/4 to 1 downstream. Zone 2 is a transition zone of selected silt, sand, and gravel placed to serve as an inverted filter between the impervious inner zone and the pervious outer zone. Zone 2 slopes are in the order of 1-1/2 to 1 upstream and 1 to 1 downstream. The upstream slope of the pervious zone 3 consisting of selected sand and gravel is 3 to 1 between elevations 960 and 1047.5, 4 to 1 below elevation 960, and 2-1/2 to 1 above elevation 1047. 5. The downstream slope of zone 3 is 2 to 1 down to elevation 960, 4 to 1 between elevations 960 and 930, and 2 to 1 below elevation 930. Section 34 (d) discusses modifications made in top portion of the dam.

Protection for the slopes of the dam is provided by a blanket of rock 2 feet thick on the downstream face of the dam and 3 feet thick on the upstream face. The thickness was specified as the depth normal to the slope. The rock was placed against the zone 3 material and was required to follow the same slope as the outer slope of zone 3.

12. Cutoff Trench. - A cutoff trench in the foundation of the dam was used to act as a barrier to the flow of water through original ground under the dam. The trench was designed with a maximum width of 50 feet and with side slopes of 1-1/2 to 1. When foundation conditions became known, the resident engineer prescribed the exact location and the variation in bottom width with elevation. The bottom width was set at 15 feet from elevation 1060 (1 foot below the crest of the dam) to elevation 1045; from elevation 1045 to elevation 1010 the trench was widened 1 foot for each foot that the bottom of the trench was below elevation 1045; and below elevation 1010 the trench was 50 feet wide. It was also established that the top of the 1-1/2 to 1 slope of the upstream side of the trench and the upstream toe of the zone 1 material would be located the same distance at any one point from the dam axis.

Originally a concrete grout cap 3 feet wide and 3 feet in minimum depth below the surface along the centerline of the cutoff trench was designed to facilitate grouting. However, the character of the rock in the bottom of the trench was such that a grout cap was required only at each end of the closure section, where there was a layer of spongy interflow material. The grout caps were placed across these areas and keyed into good rock at the ends of each grout cap. Grouting of the rock foundation under pressure to insure against leakage was required as shown on the drawings or as directed by the contracting officer.

13. Crest Details. - The freeboard for the dam was established at 6 feet above maximum water surface elevation 1055.0, giving a crest elevation of 1061.0 as discussed in section 14. Camber was provided along the crest to ensure that the freeboard would not be diminished by foundation settlement or embankment consolidation. The selection of the amount of camber was somewhat arbitrary but also was based on the results of laboratory tests which were made to determine the amount of foundation settlement and embankment consolidation. The camber diagram is shown in figure 7.

The roadway constructed on the crest of the dam consisted of 6 inches of selected road surfacing material placed on top of 12 inches of selected gravelly material and meets local county road standards. Concrete guard rail posts conforming to state highway requirements were adopted to provide safe travel over the dam crest.

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Figure 6. --General plan and sections. (Sheet 1 of 2). --From Drawing No. 222-D-10120

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Figure 6. --General plan and sections. (Sheet 2 of 2). --From Drawing No. 222-D-10120

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B. Spillway

14. Hydrologic Requirements. - A tentative design flood inflow hydrograph was furnished by the Hydrology division. Because of the topography and subsurface conditions, a 500-foot-wide overflow spillway (fig. 8) seemed to have adequate capacity and be the most economical. Flood routing studies showed that the spillway would discharge a maximum of 20, 000 cubic feet per second and the water surface would rise 6 feet, giving a maximum water surface elevation of 1058.0. The crest of the dam was set at elevation 1062. 5, providing 4-1/2 feet of freeboard. A revised hydrograph was later furnished reflecting drainage area boundary changes and reconsideration of other pertinent data; and upon receipt of this hydrograph the spillway was redesigned for a maximum discharge capacity of 7, 500 cubic feet per second. The spillway width of 500 feet was retained and the maximum water surface elevation was calculated as 1055.0. After consideration of the factors influencing wave height and wave rideup, including maximum wind velocity, its direction and duration, offshore reservoir depths, fetch, slope of dam and shore, and the nature of the surface protection, the freeboard was increased to 6 feet. The crest of the dam was finally set at elevation 1061. 0 and the spillway crest at elevation 1052. 0.

15. General. - One possible location for the spillway was in the saddle at the right abutment An examination of the topography revealed that the proposed spillway would dump water into a pothole with no suitable outlet. To provide an outlet, a large amount of excavation would be necessary for the required 4, 000-foot channel and this location was abandoned. Another possible location, in dense basalt, was in the vicinity of axis station 145+80. This site was chosen. The downstream toes of the embankments would be protected by confining the flow to a channel, and it was thought that the spillway excavation materials would provide the rock required in the dam construction. The centerline length of this spillway is about 925 feet.

The spillway discharge was computed as for a broad-crested weir with C ■ 2.65. The depth of the water throughout the channel was calculated. The calculated maximum depths at channel grade indicated the flow would probably be in rock throughout the length of the channel. The crest of the spillway was constructed of concrete.

C. Outlet Works

16. General. - The contributory watershed area for Potholes reservoir is 4,000 square miles. There are many small lakes and depressions which tend to retard the runoff. During the summer, streamflows come mostly from waste water and the return flow from irrigated lands upstream. No hydrograph for a 5-year or 10-year flood for diversion studies was provided. It was assumed that a 7-1/2-foot-diameter tunnel would have adequate capacity for stream diversion during construction of the dam. Accordingly, an outlet tunnel of that size was provided, to serve also as a diversion tunnel during construction for which purpose it proved to have adequate capacity.

Two outlets were provided downstream from the gate chamber (fig. 9), one consisting of the 7-1/2-foot-diameter tunnel controlled by a 4- by 4-foot high-pressure gate, and the other consisting of a 22-inch-diameter pipe beneath the tunnel, controlled by an 18-inch pivot valve. The former is used periodically for sluicing the silt from the trashrack structure and the latter is used for supplying existing water rights downstream. The impounded water is distributed for irrigation purposes through canal systems with headworks at other locations. Both outlets, while their discharge is independently controlled, utilize the same trashrack structure, section of tunnel from trashracks to gate chamber, and stilling basin.

Because of the excellent grade of rock existing at the dam site, a tunnel-type outlet works seemed most desirable (fig. 9). Owing to the concave conformation of the left abutment, it was necessary to aline the tunnel with a sharp bend upstream from the gate chamber in order to keep the tunnel excavation in solid rock. The outlet works structures were designed to be constructed from concrete and to act as continuous frames in resisting loads.

17. Design Details. - The outlet works consist of a trashrack structure, tunnel, gate chamber, valve house, outlet pipe, and stilling basin. (See section 18 for discussion of the outlet pipe.)

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