Transcript for Challenge at Glen Canyon, segment 07 of 11
Meanwhile, at the Engineering and Research Center of the Bureau of Reclamation in Denver, scientists and engineers worked quickly to redesign the spillways. A model of the Glen Canyon spillways was used to design notches called air slots. Since cavitation can never be completely eliminated, a cushion of air bubbles can be introduced that will absorb the shock of the collapsing vapor cavities. This will prevent damage to the concrete. A notch of precise size and shape cut into a spillway turns this water white with air bubbles that foam into the rushing water.
The technology of air slots was unknown when Glen Canyon Dam was built in the early nineteen sixties. Up to this time, it was believed that the only solution for cavitation damage on tunnel-type spillways was to provide smooth surfaces. But in time, even the smoothest surface roughens with calcium deposits or small cracks in the concrete.
A number of reclamation dams with tunnel spillways were spared damage from cavitation, only because the spillways were rarely, if ever, used and then only for small amounts of water. After the effectiveness of air slots was demonstrated in laboratory research, they were installed in the spillway at Yellowtail Dam in Montana and in the spillway at Flaming Gorge Dam in Utah. But neither Yellowtail nor Flaming Gorge had the magnitude or the particular design features of Glen Canyon Dam.
A series of laboratory investigations determined that the air slots at Glen Canyon had to be four feet wide, four feet deep, and had to be located two hundred fifty-three feet below the spillway gates, a long way down into a steep, dark tunnel where seepage water poured in eternal rains. A platform hung from cables served as a space station for the drillers, who pierced the old concrete lining almost three thousand five hundred times.
From the rim of the canyon, concrete for the air slots was then fed through a long pipeline into the spillway.
The forms were then removed to reveal the completed air slot.