CHAPTER IV--CONSTRUCTION 11/

45. Diversion and Care of the River 9/. The diversion plan is discussed in section 19. Contrary to expectations the contractor had the entire outlet structure installed, including the 56-inch-diameter outlet pipe and the 4- by 5-foot high-pressure gate, by July 1950, the time planned for diversion. At the contractor's request the river was therefore diverted through the completed outlet works rather than through the tunnel itself (fig. 7). On July 12, 1950, the contractor began constructing a cofferdam just below the inlet channel and approximately 55 feet from the upstream toe of the dam.

The cofferdam (figs. 12 and 13) was a temporary earth structure 160 feet long and 8 feet high above the riverbed, with crest at elevation 9920. Approximately 2, 420 cubic yards of material was placed in the structure. The estimated flow in the river at the time construction began was about 25 cubic feet per second. Initially, the contractor used four 11-cubic-yard end-dump truck to haul material to the west bank of the river where it was dumped and spread by a bulldozer. The material was too wet, however, and dry material was hauled to the site and stockpiled. A dragline was used to excavate a small cutoff trench to solid rock across the river channel and the stockpiled material was bulldozed into the river. Eight end-dump trucks were then used to haul pit-run material from borrow area 6. This material was dumped onto the west abutment of the cofferdam and bulldozed across the river.

The river offered little resistance to closure until the channel had been narrowed to 12 or 15 feet in width. In order to prevent washing out a second bulldozer was placed in the channel to protect the end of the cofferdam (fig. 13). As material was received, it was bulldozed in front of the second bulldozer.

The river began flowing into the outlet tunnel at 9:25 a. m. and diversion of the Conejos River was completed at 11:30 a. m., July 17, 1950. As soon as the entire flow of the river was flowing through the diversion tunnel (fig. 14), the contractor began rolling the material with a sheepsfoot roller. From this level to the crest of the cofferdam the material was placed in 10-inch lifts and rolled.

46. Clearing Operations. - Clearing operations at the dam site were intitiated by the prime contractor in May 1949. Trees were felled by means of two-man cross-cut saws and small gasoline-driven chain saws; stumps were removed by means of a bulldozer and the brush, logs, and stumps were burned. Clearing of borrow areas 2, 16, and 13 was also required and was performed during the summer and fall of 1949.

Clearing of the reservoir area was accomplished by separate contract, as noted in section 32. The severe climatic conditions at the site restricted clearing and all construction activities to about a six-month period from May to November each year. Reservoir clearing began on August 30, 1949, and all clearing work was completed on August 13, 1951.

Two portable sawmills were used throughout the 1949 working season and one portable sawmill was used during the 1950 season, producing dimensioned lumber. All sawmill operations were discontinued at the end of the 1950 working season. A portion of the logs were sold on the site and hauled out by private individuals and logging firms.

Burning of cleared material was performed intermittently throughout the 1949 and 1950 work seasons. Burning operations were terminated on July 4, 1951, on order from the U.S. Forest Service and were not resumed until July 23. Because of prevalent winds during the day and in order to take advantage of the almost nightly fall of dew, practically all burning operations were confined to late evening and early morning hours. Only one fire of any consequence spread to the standing timber adjacent to the reservoir area. This incident occurred on May 28, 1951. Prompt action and the use of all Bureau personnel plus that of the prime contractor brought the fire under control with very slight damage.

Figure 12. --Dumping material during initial construction of temporary cofferdam.

Eleven-cubic-yard end-dump trucks were used to haul material from borrow area No. 6 and another area upstream from the site. 5-SL-723

Figure 13. --Two large bulldozers were teamed together to construct the cofferdam. One was used to push earth material to extend the end of the cofferdam while the other blocked the flow of the river and prevented the end from washing away. 5-SL-728

A. Excavation

47. Cutoff Trench. Excavation for the cutoff trench was performed along with stripping of the entire area occupied by the main dam and dike. The trench was excavated to firm rock. Because of the irregularity of the foundation, considerable hand labor was required to clean the cutoff trench. In the maximum section of the dike, the cutoff trench was excavated from the axis to 75 feet upstream. In the maximum section of the main dam, the cutoff trench was excavated from 140 feet left to 60 feet right of the dam axis.

48. Dam Foundation. Stripping and excavation of the dam foundation were started June 23, 1949. Objectionable material from the dike section was wasted in a spoil area located upstream from the dike section. The slide rock excavated along the left abutment of the dike section was placed in the upstream and downstream zone 3 sections of the dike, or stockpiled.

Excavation of the foundation in the main dam section was somewhat retarded by the spongy condition and type of material encountered, making it necessary to strip the foundation to bedrock in all zones adjacent to the river. Objectionable material from the main dam section was wasted in the spoil area upstream from the dam. Materials suitable for zone 4 were placed in the dam embankment and materials suitable for zone 3 were stockpiled.

Equipment used consisted of one 1-1/2-cubic-yard dragline, one 2-1/2-cubic-yard shovel, one truck-mounted crane with a 1/2-cubic-yard dragline bucket attached, two tractor-drawn scrapers, and one bulldozer. Some cleanup was accomplished by hand methods.

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

Excavation for the dam foundation was dicontinued during the winter months. At the beginning of the 1950 work season, efforts were concentrated on stripping of the dike sections. A large pothole was encountered in the dike section adjacent to the left abutment between stations 13+80 and 14+70, from 10 to 70 feet right of the axis of the dam; it was 25 to 75 feet in width and 18 feet deep. Indications are that at one time the pothole had formed part of the old river channel and that it had become filled with alluvial material. Placing of impervious materials in the dike section was delayed until this hole was cleaned down to firm rock.

Excavation in the riverbed was commenced immediately after diversion of the Conejos River. Excavation was performed by hand labor, dragline, backhoe, and shovel. Most of the material was stockpiled for use in the dam embankment, and the objectionable material was wasted in the spoil area upstream from the dam. Considerable hand stripping was involved because of the irregularity of the foundation. An old river channel was uncovered downstream from the axis of the dam. The bottom of this channel varied from 15 to 25 feet below the normal river flow line.

Except for the usual winter shutdown excavation and stripping operations continued to completion in 1951 in advance of the fill placement. During the 1951 construction season, excavation consisted principally of hand stripping on the abutments of the main dam and dike.

A grout cap trench (sec. 73 and fig. 17) was excavated in the cutoff trench and a grout cap placed to provide a stable base for the grout pipes and seal off the upper surface so that grouting would be effective. The grout cap trench was excavated to a minimum depth and width of 3 feet, a total of 848 cubic yards of rock being removed and placed in the dam embankment. The work was performed between July and November 1949.

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 over break 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 partly completed. 5-SL-734, July 17, 1950

Figure 15. --Looking upstream at trashrack structure and conduit. Temporary cofferdam with haul road across top is at right of photograph. 5-SL-739, August 2, 1950