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volcanics are now known as the Conejos Andesites and Breccias and the Treasure Mountain Quartz Latite. Subsequent erosion again leveled this area creating the San Juan Peneplain. Still later in the Tertiary Era there were regular basaltic extrusions and the area was again elevated and tipped slightly to the east, bringing on a new cycle of erosion in which the present canyons and valleys were created.

4. Geology of Dam Site and Reservoir. - There was very little overburden material in the dam foundation area at the time of construction and the relatively thin accumulations of talus and soil were easily excavated from the dam and dike foundation area. A relatively light stripping of weathered and fractured rock was required. Figure 2 is a geologic map of the dam site area.

Much of the exposed rock throughout the dam site area appears severely fractured, and there are apparent localized shear zones and faults which contain highly stained or mineralized vein material. The quartz vein filling contains vugs or openings along old fracture planes but these openings are believed to be discontinuous or unconnected. However, except for these localized zones, explorations showed that the rock is generally fresh and unweathered, and the joints and fractures are remarkably well healed within 2 or 3 feet from the surface exposures. It is believed that seepage will be effectively controlled by the grouting performed, as the few unhealed or open fractures were found to be generally clean and free from mud or other disintegrated material.

Whether or not the visible faulted or mineralized zones will in the future transmit appreciable quantities of water remains to be seen. The angle drill holes which were intended to explore these features, in most cases either missed the disintegrated rock or crossed the fault too near the surface to obtain good core recovery or good percolation tests. These holes did, however, prove the zones affected by the faults to be relatively narrow. Rock alteration or intense fracturing is limited to within a few feet of the vein material.

For the convenience of description, the principal shear zones have been designated as faults 1, 2, and 3 (fig. 2). Fault 1 crosses the river beneath the upstream toe of the dam and enters the draw into which water from the spillway will be discharged. The affected portion of the left abutment is a zone about 4 feet or less in width. Iron-stained quartz and clay gouge were excavated from this zone. The fault is believed to be insignificant as far as leakage or stability are concerned and too distant from the proposed spillway structure on the ridge to induce any serious problems. Holes drilled in the vicinity of the spillway draw were in good rock and it is believed that water can be discharged into the unlined channel below the toe of the dam. The exact location of the fault on the taluscovered slope on the right side of the river is uncertain, but it is believed that it lies in the vicinity of the inlet of the outlet tunnel. Fault 2 is downstream and nearly parallel with fault 1. It is exposed by a series of prospect holes in the quartz vein material and fault gouge and shattered rock. Fault 3 is not clearly defined but has been designated as afault to account for the sheared and jointed rock more or less parallel to the river near the outlet tunnel line. The upstream extension of this sheared zone is uncertain. Other minor faulted zones probably exist but are not apparent.

Two holes were drilled to locate the top of sound rock for the inlet of the diversion tunnel. The tunnel outlet was located in a steep bluff of exposed rock, the character of which could be discerned without drilling. A slight shifting of exploration holes would have shown wide or varied results as the rock in the area is severely jointed and sheared.

In the vicinity of the spillway, located on the ridge between the dam and dike, sound rock is exposed over virtually the entire ridge and through most of the draw leading to the river below the dam. Except as may be affected by the localized faulted zones, it is anticipated that the rock will be resistive to any appreciable scour by spills of water discharged a safe distance from the dam. For this reason a lined stilling pool for the spillway was not warranted.

5. Embankment Materials. - (a) Investigations. --The preliminary search for embankment construction materials began in 1940. Investigations continued intermittently until 1950. Preliminary investigations of a reconnaissance nature located 11 possible sources

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2 miles downstream of the dam axis, were originally numbered from 1 to 11. 13/, 14/, _1_/, 17/- Of these areas, area 5, located about 1, 600 feet northwest of the dam site, was considered to be the most promising source of impervious materials.

Subsequent investigations for impervious material were concentrated in areas 5 and 6, and an additional area 13. Areas 6 and 13 are located about 4, 000 feet northwest of the dam site and about 500 feet above the crest elevation. The investigations showed the material to be a glacial fill deposit and that separation would be necessary in all three areas. Moisture conditions indicated that the material in areas 6 and 13 might process more easily than that from area 5. For this reason area 5 was eliminated from final consideration as a source of impervious material.

Exploration in areas 6 and 13 included 12 test pits excavated in each area by a small dragline. For specifications purposes, the two areas were combined into one borrow area designated 'borrow area A. ' Vegetation in the impervious area consisted of light scattered stands of timber, low shrubs, and grasses. Overburden unsuitable for use in the embankment ranged from 6 to 8 inches in depth.

The glacial till in the impervious borrow areas occurred on a steep mountainside and consisted of a heterogeneous mixture of boulders, cobbles, gravel, sand, silt, and clay. The water table was about 12 feet below the ground surface. The material contained from 4 to 9 percent of plus 5-inch rock and from 8 to 15 percent plus 3-inch rock. The moderately plastic fines above the water table were at approximately optimum water content.

The preliminary investigations indicated gravel deposits both upstream and downstream from the dam site. Test pits were excavated in various river bars upstream from the dam for a distance of approximately 2 miles. The material was found to consist of wellgraded, free-draining, river-deposited sand and gravel, with only a small amount of fines. Maximum size of the gravel was about 2 inches. Because the depth of exploration holes was restricted by high water table, quantity estimates were uncertain, and no particular areas were outlined in the specifications. Free-haul distance, however, was set at 7, 500 feet. The principal source of the sand and gravel embankment material was finally developed in a flat, river-bottom area of about 55 acres, located approximately 1,600 feet upstream from the dam 8/. Bulldozer trenches, excavated in 1949, supplemented earlier hand-dug test pits. Vegetation consisted of low shrubs and grasses, and the overburden was light, generally less than 1 foot.

For'additional pervious material, attention was focused on numerous slide rock areas. These areas were located around outcroppings of andesite rock found on various mountain slopes in the vicinity of the dam. The materials in these areas were accumulations of hard, angular, and fairly well graded talus rock. Seven such areas were selected as sources of rockfill material for specifications purposes.

The rock borrow area selected for specifications purposes as a source of riprap material was an andesite outcrop on the left side of the river immediately downstream and across the river from the downstream portal of the outlet works.

(b) Laboratory_Testing. --Samples from the 14 test pits excavated during the initial reconnaissance investigation were tested for standard properties and gradation in the project laboratory at Vallecito Dam 13/. Samples from proposed pervious borrow areas and impervious areas 5, 6, and 13 were tested in the Denver laboratory in 1948 7/. Materials from the pervious areas were tested only for gradation. Visual classification was made for all samples from the impervious areas, and standard properties tests were made on selected samples representative of each area.

In 1946 a sample of andesite from a slide rock area proposed for Usp as riprap was submitted to Denver, and standard riprap tests were performed 6/. Initial excavation of a slide rock area produced a material of questionable permeability because of an excess of fines. A sample of this material was submitted to the Denver laboratories for 6.

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Cost Summary.- Platoro Dam was constructed in accordance with specifications No. 2594"! Minor work was done in accordance with various other specifications. Payments to the prime contractor totaled $2, 447,755. 02 and payments to the minor contractors $73, 348. 31, making total contract payments $2, 521, 755. 02. Following is a summary of the principal cost items for construction of Platoro Dam as of February 1, 1955. The item "Government Materials Furnished, " includes gates, valves, outlet pipe, and certain mechanical equipment. A breakdown of cost by pay items for work performed under specifications No. 2594 is given in appendix A.

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7. Special Problems. - Prior to construction of Platoro Dam and reservoir it was necessary to obtain title to various mining claims at the dam site and in the reservoir area. These claims are located in W 1/2 sec. 21, SW 1/4 sec. 22, NW 1/4 sec. 27, and NE 1/4 sec. 28, T. 36 N., R. 4 E., New Mexico Principal Meridian.

A survey of the archeological resources of Platoro reservoir site 3/ was conducted during July and August of 1949 by Mr. Harry C. Meyers of the Department of Anthropology, University of Denver. Mr. Meyers made the survey under the general direction of Mr. Arnold M. Withers of the Department of Anthropology as a voluntary contribution to the work of the River Basin Surveys.

1/Revision, October 4, 1956.

27 Includes $2, 940. 08 estimated to complete.

3"/ Preliminary appraisal of the Archeological Resources of Platoro Reservoir site,

Platoro, Colo., October 1950. Prepared by River Basin Surveys, Smithsonian



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 steam action and glacial scouring. The dam 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

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