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49. Spillway. - Drilling operations in the spillway were started on May 20, 1950, and completed in September 1951. Jackhammers were used to drill all holes, which varied in depth from 2 to 20 feet. Drilling usually followed a grid pattern with spacing between holes varying from 2 to 6 feet. The general practice was to drill the deeper holes in the center of the spillway and spring these holes before blasting. The deeper holes near the slopes were not sprung. Successively shorter holes were drilled up the slope and were bottomed about 2 feet inside the slope. All the holes near the slopes were loaded lightly without springing and were fired with delay exploders. The first blasting was done May 29, 1950, using 150 pounds of 40-percent strength stick dynamite. After each blast the rock was bulldozed to the upstream and downstream ends of the spillway. It was then loaded by a 2-1/2-cubic-yard power shovel and two 10-cubic-yard end-dump trucks and hauled to the downstream zone 3 sections of the dam.
The rock extending above the crest of the dam on each side of the spillway was pattern drilled and removed down to elevation 10,047. Because of seams and faults running at various angles to the spillway axis it was difficult to maintain uniform 1/4 to 1 side slopes in the excavation.
The contractor's plan for excavation was to drill and blast the spillway in three stages. The first and second stage holes were drilled to a maximum depth of 20 feet, leaving a 2-foot cushion or rock for the third stage. The holes in the third stage were loaded lightly to present overshooting.
50. Tunnel. - Preliminary excavation for driving the outlet tunnel was initiated on June 21, 1949, with drilling and blasting operations in the valve house area preparatory to "holing in" the tunnel at the downstream portal. An access road and temporary bridge were also constructed across the Conejos River. Drilling of the tunnel proper began on July 7, 1949, at station 11+26 and was continued on a three-shift basis, six days a week, until completion.
A fault, running at a slight angle to the centerline, which was encountered at the portal of the tunnel, necessitated the placement of steel support rings for the first 40 feet. A total of 80 support rings were used in soft and shattered areas throughout the tunnel (fig. 29). Generally the support rings were placed on 4- to 5-foot centers in areas requiring support. Rock encountered, although occasionally massive, was generally blocky andesite with solid calcite-filled seams and occasional fractures and fault zones filled with calcite, fragmented andesite, and/or gouge material.
Drilling of the tunnel was performed on a normal pattern for this type of work, with approximately 35 to 40 holes drilled in each round in the horseshoe section and 28 to 34 holes in the high-pressure section. Holes were drilled to an average depth of 6 feet and were loaded, depending upon the condition of the material, with 220 to 480 sticks of powder to pull an average of 6 feet. Considerable overbreak was caused by the blocky characteristics of the rock, localized shattered areas, and occasional overloading. Equipment used in this operation consisted of a drill jumbo, a mucking machine, two diesel locomotives with five 3-1/2-cubic-yard dump cars, and related power and supporting equipment.
Although there were the normal number of minor delays, derailment of hauling equipment, and machinery failures, there were no major delays and the tunnel was "holed through" at station 1+80 on August 19, 44 days after the first round was shot July 7, 1949. Excavation for the dome section in the gate chamber was deferred while driving the tunnel and was completed during excavation for the trashrack structure.
51. Trashrack Structure and Inlet Channel. - Excavation for the trashrack structure (fig. 15) was started July 18 and was continued intermittently until completion on August 23, 1949. The method of excavation was by drilling and blasting, the material being moved by conventional construction equipment. Excavated material that was considered suitable for zone 3 was stockpiled for future placement, while the remaining material was wasted upstream from the dam on the east side of the Conejos River. A portion of the inlet channel was excavated during the same period and was completed during diversion operations.
Figure 14. -- The entire flow of Conejos River was diverted through the outlet pipe to
permit construction of Platoro Dam Valve house structure is only
Figure 15. --Looking upstream at trashrack structure and conduit. Temporary
cofferdam with haul road across top is at right of photograph. 5-SL-739,
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 streambed 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.
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.