OCR Text |
Show First excavation for each tank started at a point on the ground surface, high above the vault's center, with .1 shaft which measured 4x6 feet inside the timbers. Driven downward 80 to 100 feet to the top of the dome, it was enlarged at that point to 12x 12, feet and driven that size 250 feet to the bottom of the future vault. Into the walls of this shaft horizontal holes were drilled 12 feet deep on three -foot centers to be blasted later in enlarging the shaft to its final circumference. Meanwhile, an upper access tunnel was driven from; an outer portal at the level of the base of the dome, to run between the two rows of vaults, A connecting cross tunnel was run to each dome on a. 45-degree angle, and thence in a circle around the base of the dome. From this ring tunnel, excavation for the shell of the dome was stoped out in raises up to the crown. A steel liner plate was welded in place as the inside form of the dome, supported by eight-inch H beams. The dome concrete was poured from the ground surface above, through a 12-inch pipe inside the access shaft, and was distributed through 12 rounds of articulated pipe. Thickness of the dome concrete graduated from four feet at the crown to eight feet at the base, the extra bottom thickness providing a bearing of five feet to sustain the weight of the dome and. overburden until the wall below was completed. At the bottom of the vault, in the center of the work shaft, a steel hopper was built with an apron-type feeder. Then the shaft timbering was removed, the network of holes previously bored, loaded and shot, enlarging the shaft, and the muck removed by the bottom hopper. Then excavation all by controlled blasting -- started in the top of the dome and was carried down in 10-foot lifts, extending the 30-foot shaft to a circular shaft roughly 106 feet in diameter. The muck was drawn off through the bottom hopper as work progressed, the hopper feeding onto a 48-inch conveyor belt in a lower cross tunnel, which transferred material onto another 48-inch, belt running through a lower access tunnel to a 48x60-inch jaw crusher. As excavation progressed below the come the vertical walls were covered with six inches of gunite to support the unstable pockets of volcanic cinders. When the downward excavation reached 20 feet, a circular catwalk was suspended from 24 cables anchored into the dome concrete, with 24 hand hoists to raise or lower it. All work was done from this platform, which carried water and air lines. After a vault was completely excavated, a steel construction tower was built from the bottom of the tank to the top of the dome with four booms suspended from it that were used to install liner plate for the concrete form and to concrete the vault walls. The liner plate was placed in five-foot lifts with welded seams. Concrete around the steel |