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Transcript for Hoover Dam Construction, segment 05 of 17
The boring of four diversion tunnels to carry the stream around the dam site during construction, two on each side of the river, fifty-six feet in diameter and averaging four thousand feet in length, constituted the first major construction operation. The drilling jumbos used on this job were mounted on motor trucks to facilitate handling and were capable of driving from twenty-four to thirty powder holes into the heading simultaneously by means of drifter drills. The tunnels were excavated through the rock simultaneously from four headings, one at either end and two boring in opposite directions from a river level auxiliary tunnel located about midway on the main bore. A pioneer drift was drilled at top line and closely followed by the excavation for the complete fifty-six foot bore. Thousands of tons of drilled steel were used in this work and the sharpening shops were kept working at top speed night and day to maintain a steady supply.
After the powder holes had been drilled and the rocks blasted, power shovels and trucks moved into the tunnels for the purpose of removing the shattered material. An unbroken parade of heavy duty trucks, each handling from eight to ten tons of rock, labored up over the steep roads cut into the canyon walls to dispose of the material in the gulches adjacent to the dam site. This phase of the task, which entailed the excavation and the handling of over one and a half million cubic yards of material, was completed within a period of thirteen months and was considered the most grueling portion of the work for both men and machinery.
After the tunnels had been excavated, they were lined with concrete, three feet in thickness. Due to the unprecedented size of the bores, special equipment was designed to facilitate this task. The tunnels were lined in segmental sections, the invert or base being the first in which the concrete was placed. A gantry crane operating through the tunnel itself handled the concrete throughout this operation. The side walls were next lined behind movable steel forms, which traveled through the excavated section on rails laid from portal to portal. The top arch was placed with the use of a concrete gun operated by compressed air.