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11. Specification Zoning. - The abundance of pervious materials within reasonable haul distances, and the necessity of removing oversize cobbles from the impervious borrow materials indicated a thin core-type dam would be the most economical. The zoning scheme adopted (fig. 4) resulted in stable embankment accommodating a maximum of oversize cobbles from borrow and rock from required excavation of the spillway. See section 5 for investigations of embankment materials. ,

12. Embankment Slope Protection. '- (a) Upstream Slope. --In preliminary designs, it was intended, that slope protection be obtained from talus deposits and required excavation. However, owing to an insufficient supply of talus, and the jointed and sheared condition of the rock in the immediate area of the dam, an adequate supply of suitable riprap was unavailable from required excavation. Consequently, a separate item for producing and placing riprap on the upstream slope above elevation 9965 was included in the specifications. Below elevation 9965, the crest of the permanent cofferdam, rockfill was permitted in lieu of riprap.

Instead of using the downstream rock source, the contractor requested and received permission to obtain riprap from an area located 100 feet upstream from the left abutment of the main dam, and a second area located approximately 600 feet west of the left abutment of the dike section.

(b) Downstream Slope. --Slope protection is obtained with heavy rockfills from required excavation, oversize rock from the zone 1 screening plant, and material from rock borrow areas. A fill below elevation 9900 between the downstream toe and the valve house has been added to provide a disposal area for waste material and an access and a parking area for the valve house. The fill utilizes required excavation unsuitable for the embankment proper and is designated as a "disposal area" on the specifications drawings. To facilitate drainage of the downstream toe, zone 3 rockfill is placed at the end of the disposal area. The rockfill is contained at the lower end by a retaining wall that extends from the left downstream corner of the valve house to the opposite side of the river channel. Additional drainage is provided by a 12-inch drain that begins at the 2-1/2 to 1 downstream slope, passes under the "disposal area", and extends through the retaining wall.

13. Embankment Design Details. - Studies in connection with designs for 1946 cost curves indicate a variation of less than 2 percent in embankment volume for various axis alinements in the immediate area. A dam with the axis beginning on the right abutment in the vicinity of station 0+00 of the specifications design and continuing as a single arc to a point on the left abutment 400 feet downstream from station 15+00 was found to contain the least yardage. However, this layout was abandoned in favor of a three-directional axis in order that the cutoff trench might avoid fault zones or cross them at approximately right angles. When the spillway was finally located in the ridge separating the main embankment from the saddle on the left, only minor adjustments were necessary.

The 35-foot crest width was considered to be suitable and in line with design for dams of comparable heights. Owing to the fact that a road around the reservoir was not expected to be constructed and no road existed that was to be relocated across the dam, crest treatment was restricted to a minimum. Twelve inches of selected gravelly material was used for roadway surfacing on the crest of the dam.

Original designs included a toe drain in the downstream rock toe below elevation 10,0257. On reviewing the preinvitation design, it was noted that the topography of the foundation area was such that it would contain and direct seepage into natural outlets and that artificial drains would require considerable rock excavation. Consequently, the anticipated costs of toe drains could not be justified and the drains were omitted from final plans except under the waste area.

14. Stability Studies. - The adopted design section, consisting of a comparatively thin impervious core flanked by pervious zones, combined with sound foundation conditions, proved to be stable under the usual design stress conditions of construction stage, steady state with full reservoir, and rapid drawdown. However, during the initial embankment construction season, the borrow area was found to contain moisture in excess of the upper limit of the desirable placement range. Although it contained considerable rock, the

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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.

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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 l 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.

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