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Figure 18. --Stripping activities on the foundation of the river section of the main dam.

5-SL-744

52. Valve House Structure. · Initial excavation for this structure (figs. 14 and 33) was begun on June 21, 1949, in order to open the tunnel face for drilling operations, and excavation was completed May 22, 1950. Excavation was necessarily discontinued during tunnel excavation and concrete placement operations and during the winter months. Excavation methods were similar to those described for the trashrack structure. Some delays were incurred during the spring of 1950, because additional excavation was required in a fault zone and because of river water seeping into the excavation.

B. Borrow Areas

53. Impervious Borrow Areas. Three borrow areas were considered for impervious (zone 1) material (fig. 19). These included borrow area 6, located about 1 mile northwest of the dam site and 500 feet above the design crest elevation; an extension of borrow area 6, located about 4, 600 feet northwest and 300 feet above the crest elevation; and borrow area 13, located on the mountain slope immediately above borrow area 6. Borrow area 6 provided 93, 600 cubic yards of material, although it was too wet and required mixing and scarifying to dry it on the fill. The extension of borrow area 6 contained suitable material that was 1 or 2 percent dry of optimum. This pit was opened up on September 11, 1950, and used instead of borrow area 6 to provide impervious material for completing zone 1 of the dam. The material from this pit proved much more workable and was separated with much less difficulty than the material from borrow area 6. Borrow area 13 was not used.

54. Semi-Impervious Borrow Area. - Gravel area No. 2, used for obtaining semiimpervious materials for zones 2 and 4, is located in the stream bed and flood plain of the Conejos River, beginning approximately 600 feet upstream from the dam site (fig. 19).

Preconstruction investigations of this area indicated an average of about 10 feet of available material for zones 2 and 4 of the dam. Upon developing the pit, however, there was found to be only approximately 50 percent of the estimated depth of suitable material. This required working the area farther upstream than was originally anticipated.

Since the water table in this area varied from 2 to 12 feet below the ground surface, it was necessary to use a dragline equipped with an underwater drag-bucket for excavation. Stripping operations were performed just sufficiently in advance of excavation to prevent any delay in embankment placing.

55. Rock Borrow Areas. - Seven slide rock borrow areas for production of pervious (zone 3) material were considered. These areas are located around outcroppings of andesite rock situated on various slopes in the vicinity of the dam site. The material consists of loose, exposed accumulations of talus and is hard, angular, and well graded.

Actual quantities of this material were considerably lower than those estimated, requiring the use of rock spalls from the riprap borrow area (sec. 56) to augment the short supply. Material was excavated by the use of power shovels and transported in end-dump trucks and bottom-dump trailers.

56. Riprap Borrow Areas. - Two borrow areas, rock borrow area 2 and rock borrow area 4, were used for riprap materials. The former area is located in an outcropping approximately 100 feet upstream from the left abutment of the main dam and the latter area is located just north of the left abutment of the dike section. The rock in both areas is a blocky andesite known to be a suitable riprap material, but the rock in rock borrow area 2 is weathered at the surface. After blasting, excavation was performed by power shovel and transported in end-dump trucks and bottom-dump trailers.

C. Embankment

57. General. The first placement made in the dam embankment was in the downstream zone 3 section on June 29, 1949. Zone 3 materials were then placed intermittently during the summer months. A small amount of zone 4 material was placed before operations were suspended for the season because of freezing weather November 10, 1949.

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Figure 19. --Vicinity map showing borrow areas with test holes and profile lines.

5.1-E-16

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Figure 20. --A 2-1/2-cubic-yard shovel used to work a 30-foot face in borrow area 6. Material was carried to separation plant by 17-cubic-yard bottom-dump trailers. 5-SL-760, August 2, 1950

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Figure 21. --Loading operations in gravel area No. 2. The dragline with 2-cubic-yard

bucket was used to load 11-cubic-yard end-dump trucks.

September 26, 1950

5-SL-840,

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