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glacial till material was moderately compressible. Additional stability studies were made based on placement conditions approaching laboratory optimum. These studies indicated that construction pore pressure of serious magnitude might result if placement at optimum moisture conditions was permitted. As a means of reducing the effective moisture content the contractor was permitted to increase the spacing of the bars in his separation plant to produce zone 1 material with maximum-size rocks of 5 inches instead of 3 inches 3/. Increasing the size of rock in the zone 1 material, extending the borrow area into drier material, and the more favorable drying weather throughout the remainder of the construction period, resulted in acceptable placement conditions from the standpoint of stability without adversely affecting the permeability characteristics of the zone 1 material.

15. Test Apparatus. Since Platoro Dam was to be built with a thin impervious zone resting upon a firm rock foundation, no stability problem was expected during construction or during reservoir operation. However, embankment settlement markers were installed at five points along the axis on the crest of the dam (fig. 3). The locations of the markers, their original elevations, and their elevations two years later, are shown in the following tabulation:

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16. Field Control. (a) Impervious Material. --Field control of the impervious material was maintained throughout construction by standard Bureau procedures. The natural moisture condition in the borrow areas plus an extended rainy season in 1950 resulted in a mean placement moisture content near the laboratory optimum condition. At the start of construction, fill densities of the minus No. 4 fraction of the impervious material were high. However, with the separation plant opened to 5-inch size as discussed in section 14, the increased rock content in the zone 1 material resulted in lower fill densities of the minus No. 4 fraction, but the dry unit weight of the total material remained high. Percolation tests showed that with the minus No. 4 fraction placed at 95 percent of laboratory maximum density, the total material was still impervious.

A total of 304 representative field density tests were taken in the zone 1 portion of the embankment. The material contained an average of 39.3 percent of plus No. 4 rock. The mean placement moisture was 0.4 percent below the mean laboratory optimum; the mean fill density (minus No. 4 fraction) was 4.5 pounds per cubic foot below or about 96.5 percent of the mean laboratory maximum density.

(b) Zone 2. --Field control of the zone 2 pervious zones was based on the relativedensity concept of compaction control. On the basis of laboratory tests of similar materials a minimum relative density of 70 percent was set as the criteria of satisfactory control. A total of 115 density-in-place tests showed a mean relative density of 82.1 percent.

(c). Other Zones. --No field control tests were performed in the rockfill and riprap portions of the embankment. Visual inspection of these zones was relied upon for proper gradation and placement of these materials in accordance with the specifications.

17. Field Modifications. To improve the stability of the placed material (and incidentally increase the ease of separation), the contractor was allowed to widen the openings of his screening plant to permit separation on a 5-inch screen, as discussed in section 14 3/. The reduction in cobble yield from this operation, together with a low yield of slide rock from the specified rockfill borrow sources, necessitated modification of the zoning plan 25/.

In addition, because of the undesirable moisture content of the impervious material (section. 14), it was decided to reduce the zone 1 embankment to a minimum. This further reduced the cobble yield from this source so that additional rock borrow areas had to be located. In the meantime, an outcropping of andesite rock near the left abutment of the main embankment had been blast tested. Based on the results of this trial blast, the riprap source was at the contractors request changed to this new area.

In obtaining sufficient riprap of required gradation, considerable quantities of rock too small to meet riprap specifications was produced. This rock was utilized in the rockfill zones in lieu of slide rock from additional borrow areas.

B. Spillway and Outlet Works

18. Spillway.

Considerable study and investigation was made in an effort to utilize the Mix Lake area as a spillway location (fig. 3). Geological conditions, desired storage capacity, and flood control requirements, however, indicated the economics and advisability of placing the spillway on the left abutment of the main dam through a rock saddle. The spillway as constructed is an open-cut unlined channel 50 feet wide at the base with side slopes in the rock of 1/4 to 1. The only concrete involved in the spillway, except for the bridge, is a crestwall which serves as a sill at elevation 10, 034.0. The crestwall connects with adjacent grout caps to complete the grout curtain wall.

A 10-year hydrograph of Conejos River near Platoro Dam is shown in figure 5. The spillway design flood, which has a 15-day volume of 44, 500 acre-feet and a peak flow of 9,000 cubic feet per second, was routed through the spillway and outlet works. The maximum water surface in the reservoir thus determined for the design flood is elevation 10,041.0 and the spillway discharge is 3,000 cubic feet per second (fig. 6). Flood routings were made using discharge curves based on coefficients of 3.0 and 3.4. The difference in the results was negligible.

A reinforced concrete bridge 12 feet wide spans the spillway at the dam crest to provide access for maintenance and inspection (fig. 32). The bridge is designed for an H-20 truck load and was placed such that its width may be increased easily at a later date if desired.

19. Diversion Plan. - During construction of the outlet tunnel the river was permitted to flow in its natural channel. Flow was to be diverted through the outlet tunnel during construction of the dam, and the tunnel was designed to pass a 10-year flood (fig. 5). This flood has a 15-day volume of 28, 200 acre-feet and a peak flow of 1, 400 cubic feet per second. Several stages of outlet tunnel completion were considered for diversion, and flood routings were made to determine the required height of the cofferdam for each stage.

It was not expected that the outlet gate and outlet pipe would be installed before diversion. But during actual construction, on February 13, 1950, the contractor asked for and received permission to install the emergency gate in the gate chamber and the 56-inch steel pipe including the wye-branch prior to diverting water from the river channel through the outlet tunnel. The 10-year flood routing showed that the water surface would rise to elevation 9965.

Periods of complete flow shutoff were limited to the nonirrigation season, October 15 through March 31. Continuous flow for fish preservation was not required. The size of any feature of the outlet works was not determined by the diversion requirements. The actual diversion operations are discussed in section 45.

20. Outlet Works. The outlet works consists of a tunnel through the right abutment of the dam, a gate chamber and high-pressure emergency gate located downstream from the axis of the dam, and a valve house with control valves located at the downstream end of the tunnel (figs. 7, 8, and 33).

It was required to discharge a minimum of 600 cubic feet per second through the outlet works with the water surface in the reservoir at elevation 9963.0. The volume in the reservoir at this elevation is 10, 000 acre-feet. It was also necessary to completely drain

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Figure 5. --Hydrograph of Conejos River below Platoro Dam site.

the reservoir (sec. 8). The outlet discharge at maximum reservoir level, elevation 10,041.0, is about 875 cubic feet per second (fig. 6).

The outlet works, as shown in figure 7, includes a low bear-cage trashrack structure which is set out into the reservoir away from a rock slide and talus slope area. The outlet conduit extends from the trashrack to the inlet tunnel portal and has an inside diameter of 6 feet 6 inches, the same as the tunnel to which it connects. At station 7+24 the tunnel connects with the gate chamber which houses a 4- by 5-foot high-pressure emergency gate. The emergency gate connects to a 56-inch-diameter steel pipe which extends to the discharge valves. The steel pipe is protected by an 8-foot 6-inch horseshoe tunnel which also provides access to the gate chamber from the valve house. The 56-inch steel pipe bifurcates into 48-inch sections at station 11+37 and terminates at two 48-inch butterfly valves which regulate the flow into the existing river channel.

Various studies made during the outlet works design include the following:

(1) The possible use of a conduit rather than a tunnel.

(2) The use of jet-flow gates for regulation at the gate chamber and a free-flow downstream tunnel.

(3) Hollow-jet valves for regulation.

(4) The possibility of diverting through the installed emergency gate.

Some aspects and conclusions of each of the above considerations are listed below:

(a) Consideration was given early to the possibility of using a conduit rather than a tunnel; in fact, the field data included a statement that a conduit should be used. Layouts were made with a conduit on each abutment near the river channel. It was impossible to get a suitable alinement without excessive cuts near the downstream toe of the dam. A comparative estimate showed that the conduit would cost about $50, 000 more than the adopted tunnel layout. Dam foundation stripping later revealed a poor foundation condition in the vicinity of the proposed conduit alinement. If the conduit layout had been adopted the construction complexity and cost would have materially increased.

(b) The tunnel was excavated with little difficulty from one heading at the downstream portal. Steel liner plates were not required and a relatively few 6- by 6-inch H-20 structural steel supports were used. Some supports were placed at the portals and some through a fault zone section near the gate chamber.

(c) Information in the Denver office to the effect that access to a control house located near the toe of the dam would be very difficult made it desirable to consider an outlet gate which could be controlled from the crest of the dam.

(d) A jet-flow gate was suggested; in fact, a model study was made to observe the flow pattern and air requirements for the free-flow downstream tunnel. After considerable model testing and discussion of available time for design and delivery of the new jet-flow gate, it was decided to adopt a more conventional-type gate. Additional information indicated rather easy access to a valve house at the downstream toe of the dam, and the standard layout shown in specifications No. 2594 was adopted. It should be noted that the valve house access road crosses the spillway discharge channel and may require maintenance and repair following the infrequent use of the spillway.

(e) Just prior to issuance of the specifications it was decided to substitute the hollowjet regulation valves with butterfly valves. With the invert of the tunnel placed at streambed a certain amount of sand, mud, and debris may be carried into the outlet pipe and operating mechanism of the hollow-jet valves. A butterfly valve would not be damaged seriously by mud and debris collecting against it. The discharge flow pattern is somewhat erratic but believed to be satisfactory.

(f) When specification drawings were being prepared it was believed that the 4- by 5foot high-pressure emergency gate would not be at the dam site when the contractor wished

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OUTLET DISCHARGE AND DIVERSION DISCHARGE IN HUNDREDS OF C. F. S. 3

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SPILLWAY DISCHARGE IN THOUSANDS OF C.F.S.

AREA-CAPACITY-DISCHARGE CURVES
* 56" Dia. pipe not installed

RESERVOIR DATA

Superstorage 7800 a.f.-El. 10,034 to El. 10,042
Conservation storage 60,000 a.f.-El. 9911.5 to 10,034

Figure 6. --Area-capacity-discharge curves for Platoro reservoir, spillway, and outlets. From drawing 253-D-294

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