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The four lunnels would serve another purpose when their use as diversion tunnels \vas completed. The two outer tunnels would become outlets for the huge spillways. The inner tunnels would be utilized for installation of penstocks to convey water from the intake towers in the reservoir to the power plant or to the outlet valves below the dam. The dispatch with which the diversion tunnels were driven forecast the efficiency and speed which was to predominate the ,,,hole construction schedule. Tunnel excavation was started in June 1931 and completed in N avember 1933. The canyon walls were attacked first by batteries of compre5sed-air drills. \Vhen the drills had bitten 10 to 20 feet into solid rock a ton of dynamite was loaded into the holes. The electrically fired blast shook the walls of the can van. The resulting loose rock and debris was loaded into trucks. The tru;ks roared away to dump their loads in side canyons. An average blasting round broke 1,000 cubic yards of rock and advanced the heading 17 feet. A total length of 256 feet of tunnel was driven in 24 hours and 6,848 feet in one month. Removal of the million and a half yards of rock in the four tunnels required 3,561,000 pounds of dynamite, or 2.38 pounds per cubic yard. Each of the four tunnels was holed out to 56-foot diameter and then lined with a 3.foot thickness of concrete. The combined length of the four tunnels was approximately 3 miles. I I , i Ii Th,e Enginperl' Turn the Rh'cr With the completion of the two tunnels on the Arizona side of the river, steps were taken to begin actual diversion of the river's flow. A temporary dam of earth and rock (cofferdam) was thrown across the river just below the inlets of the two tunnels. Within 24 hours, after startin1Z this work, a dam was formed of sufficient height to block the channel and foree the water through the tunnels. Meanwhile, downstream below the dam site, another cofferdam was pushed across the river's channel just above the tunnel outlets. This prevented the river from backing into the construction area. The Colorado River had been diverted. With the cofferdams in place and the water diverted around the construction site, excavation for the foundation of the dam and power plant proceeded swiftly. Manning huge power shovels, drag lines, and other equipment, men labored 24 hours a day digging down through the mud and silt of the channel to solid bedrock. In some instances it was necessary to dig 135 feet below the channel before reaching solid rock. More than half a million cubic yards of muck ''las removed. Work also went ahead on the removal of loose and projecting rock from the canyon walls. To reach the desired spots, the "high scalers" either climbed up ropes or were suspended from anchors sunk in the canyon wall. They swung in safety belts or bosun chairs pendulum fashion hundreds of 2Q n '762 feet above the river and gouged at weak spots or drilled blasting holes. Nearly a million cubic yards of rock were dislodged from the walls of the canyon. "1 Plati' St4?el Fabricatinfl Plant Erected at aU! l)aln Site A major problem confronted the engineers in trying to devise means of obtaining the steel pipe for the penstock system. Designs of the penstocks called for 2% miles of plate steel pipe nearly 3 inches in thickness whose gross weight exceeded 44,000 tons. Header lines were designed with diame- ters of 30 and 25 feet, with individual sections weighing more than 175 tons. Standard railroad cars were not designed to carry that much weight. Nor would loads of those dimensions pass through a normal railroad tunnel. It was obvious that the fabricated pipe-sections could not be shipped from eastern plants. The situation appeared critical. The engineers conferred. They hit upon the solution. They would construct a plant at the dam site and fabricate the great pipe on the spot. Accordingly, Babcock & Wilcox, the contractors, erected a plant along the construction railroad a mile and a half from the top of the darn on the Nevada side of the canyon. Flat plates of steel were shipped to the plant and there fabricated into the required sections. Special equipment was req'uired for both fabricating and transporting the finished pipe sections to the dam site. Planers, rollers, presses, electrical equipment for welding the plates, and X.ray equipment for examining the welds were installed in the plant. A 200-ton trailer, pulled and controlled by I i View looking downstream from crest of dam showing downstream section 0/ power plant, canyon wall outlet works and downstream cofferdam excavation.