As the justifiably famous Caldecott Tunnel nears completion of its latest expansion, take this nice tour of the geology of the project from Oakland Geology's Andrew Alden: Rocks of the Caldecott Tunnel
It’s pretty crummy rock, and the tunneling was quite slow there. Every couple of meters, the tunnelers drilled a fan of holes over the roof of the upcoming dig segment, each one reinforced and grouted. That reinforcement allowed the tunnel roof to stay up long enough to spray it with shotcrete, which in turn held up the roof until the real concrete tunnel lining could be emplaced. The tunnel’s final shaft is round, as strong as an eggshell in its resistance to the earth’s weight. (To see what I mean, try gripping a raw egg firmly in one hand, fingers on all sides, and attempt to crush it. You can’t do it.)
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