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CHAPTER II--DESIGN

8. General. - Preliminary layouts and estimates for dams of three different heights were prepared and assembled in a San Luis Valley report dated January 5, 1940. After World War II, in December 1946, alternative estimates were again made for dams impounding 77, 000, 55, 000, and 30, 000 acre-feet, respectively, with a dead storage of about 10 percent of the total. These estimates included provision for a fish screen at the inlet to the outlet works.

Study of the 1946 designs and estimates resulted in a decision to eliminate the fish screen and to set the reservoir capacity at an intermediate value of 67,000 acre-feet. In harmony with the Rio Grande Compact, which stipulates that provisions must be made to completely drain any reservoir tributary to the Rio Grande River, such provisions were planned for Platoro Dam.

As actually constructed the total capacity of the reservoir is 67, 800 acre-feet. Conservation storage is 60, 000 acre-feet at spillway crest elevation 10, 034.0, with 7,800 acre-feet of superstorage between the crest and maximum water surface elevation 10, 042.0. There is no dead storage.

A. Earth Dam

9. Detailed Description. - Platoro Dam is a rolled-earth and rockfill structure with a volume of 909, 884 cubic yards. The dam consists of two sections separated by a rock knoll--the main dam across the present river channel and a smaller dike across an older channel of the river (fig. 3). The main dam has a crest length of 885 feet, a crest width of 35 feet, a crest elevation of 10, 048.0., and a maximum height of 165 feet. The upstream slope is 2-1/2 to 1 from the crest to a 20-foot berm at elevation 9965, and a 5 to 1 slope from the berm to the riverbed. The downstream slope is 2 to 1 to elevation 9950 and 2-1/2 to 1 from that elevation to the disposal area at elevation 9900. The smaller section of the dam, referred to as the dike section, has a crest length of 590 feet and a crest width of 35 feet. The upstream slope of the dike section is 2-1/2 to 1 from the crest to ground surface and the downstream slope is 2 to 1 from crest elevation to ground surface. The specifications requirement for construction of a small dike at the end of Mix Lake as shown on figure 3 was eliminated by order for changes No. 5, dated August 7, 1951, and the contractor was ordered to construct a haul road at the upper end of Mix Lake which will serve in lieu of the dike.

10. Foundation Design. - The foundation of the dam is a massive andesite flow which has been eroded by stream action and glacial scouring. The damn foundation is fractured throughout the area and there are apparent localized shear zones and faults which contain highly stained and mineralized veins. With the exception of localized zones, the joints and fractures are well healed within 2 or 3 feet from the surface exposures. The area was covered with very little overburden and, under the zone 1, the foundation was stripped to firm rock. It was required that, whenever a projection or pothole was encountered, special effort was to be made to prepare a suitable foundation surface.

During excavation operations an abandoned mine tunnel was uncovered on the right abutment of the dam a few feet downstream from the axis approximately at elevation 9907. The tunnel, which extended into the abutment a distance of 50 feet, was cleaned and backfilled with lean concrete. After the concrete was placed, the remaining voids were filled with grout.

Excavation of the cutoff trench, which extends the full length of the dam embankment, was accomplished in conjunction with other required foundation excavation. The cutoff trench, as excavated, had a maximum width of 200 feet in the main dam section and 75 feet in the dike section. The depth ranged from 3 to 10 feet.

The concrete grout cap was placed in a trench excavated in rock along the centerline of the cutoff trench. Material excavated from the dam foundation was either wasted, stockpiled for future placement, or placed in zones 3 or 4 of the dam embankment. A total of 73, 026 cubic yards of material was excavated, of which approximately 26, 500 cubic yards were utilized in zones 3 and 4.

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Figure 3.--General plan and sections for Platoro Dam. (Sheet 1 of 2).

drawing 253-D-294.

From

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Figure 3.--General plan and sections for Platoro Dam. (Sheet 2 of 2). From drawing 253-D-294.

10' crs. as directed -3'Min, 3'-6"Av.

CONCRETE GROUT CAP

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 10,025. 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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Figure 4. --Material distribution chart. Chart does not show Zone 1 slope changes discussed in section 58.

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