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LIST OF FIGURES (Continued) Figure Title Page'

27. Equipment used at outlet portal of tunnel for concreting

operations 55

28. Pumping concrete into grout cap trench of the right

abutment by means of a pumpcrete machine 55

29. Structural steel used to support sections of the horseshoe

portion of the outlet tunnel 57

30. Placement of reinforcement steel in the high-pressure

section of the outlet tunnel 57

31. Workmen placing reinforcement steel in spillway bridge 58

32. Final excavation of spillway channel and completed spillway bridge 58

33. 48-inch butterfly control valve with improved operating mechanism,

scheduled for installation in the fall of 1954 63

34. Completed outlet valve house, with both butterfly valves (initial

installation) discharging at full opening and 57-foot head 64


1. General Description. - Platoro Dam (fig. 1) is located on the Conejos River

1 mile west of the town of Platoro, Colo., at an elevation of 10, 000 feet above sea level. It is the principal feature of the Conejos division of the San Luis Valley project; it will provide supplemental water to 85, 150 irrigable acres in the San Luis valley and will alleviate flood damage in the flood plain of the Conejos River.

The reservoir formed by the dam has an area of about 950 acres and contains about 60,000 acre-feet of water at full capacity (not including superstorage). The stored water is released back into the river through outlet works as needed for diversion to district lands about 40 miles downstream.

, The dam (fig. 3) is an earth-fill structure 1,540 feet long, with a maximum height of 165 feet. The spillway section consists of an open rock cut between the dam and the dike; the section is 50 feet long, with a crest elevation of 10, 034. 0 feet, and has a capacity of 3, 000 cubic feet per second with the reservoir surface at elevation 10, 041. 0.

The outlet works is on the right abutment. Progressing downstream, it consists of a trashrack structure, a concrete-lined high-pressure tunnel, a gate chamber, a concretelined horseshoe tunnel housing a 56-inch-diameter steel pipe, and a valve house structure. Flow is controlled by a 4- by 5-foot high-pressure emergency gate in the gate chamber and two 48-inch butterfly valves in the valve house. The invert elevation of the tunnel inlet is 9911. 5; this is sufficiently low to allow the reservoir to be completely emptied when required, and there is therefore no dead storage.

Platoro reservoir is the highest of any formed by a Bureau Dam. In fact it is believed to be the highest reservoir of its size in the world.

2. Site Investigations. - After the adoption of the RioGrande Compact on March 18, 1938, which paved the way for development of the waters of the Rio Grande River above Fort Quitman, Texas, the United States Corps of Engineers and later the Bureau of Reclamation investigated sites for irrigation and flood control dams on the Conejos River in the San Luis Valley project. During these and earlier investigations a total of 12 sites were investigated.

Early planning compared the advantages of a single main-stream (Counsellor) reservoir against those of a two-reservoir (Platoro-Mogote) plan of development. By July 10, 1946, exhaustive studies had eliminated the main-stream reservoir scheme from consideration 15/, and the object of later planning was to determine the most economical arrangement of Platoro and Mogote reservoirs. A summary of the status of investigations at these sites was made on July 29, 1946 16/. Up to this time, the Platoro Dam site is frequently referred to as "Upper Conejos Dam Site No. 6."

By April 30, 1947, cost studies had determined that, in view of current construction cost, Platoro Dam, with a capacity of 60,000 acre-feet, was the only feature of the project that could be economically justified 5/. Accordingly, designs and estimates were prepared for its construction.

3. Regional Geology. - Platoro Dam is located in the upper reaches of the Conejos River in the San Juan Mountains. The regional geology of the area is best described by listing the major stages in its history. The basic pre-Cambrian complex of igneous and metamorphic rocks was covered with great thicknesses of Paleozoic and Mesozoic sediments consisting of limestone, sandstones, and shales. At the end of the Cretaceous Era a broad dome was elevated in the present general position of the San Juan Mountains of today. Several cycles of erosion and elevation took place culminating in a widespread outwash deposit locally known as the Blanco Basin formation of Oligocene Age. During the Miocene Age, great thicknesses of volcanic rock were extruded over the area. These 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.

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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 a fault 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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