Port of Miami tunnel on the cutting edge

"On Watson Island, the ground has been readied and the massive tunnel boring machine (or TBM, in industry-speak) is almost good to go." From The Miami Herald here.

Looking like a cross between a rocket and a locomotive, it rests on an inclined concrete “launch pad” at the bottom of a 50-foot pit at the center of the island in Biscayne Bay.

The boring machine’s disc-like cutter head is aimed squarely at a wall marked with a 43-foot-wide, roughly drawn circle — marking what will be the tunnel entry portal. In early November, hydraulic thrusters behind the slowly rotating cutter head will push off against a giant steel brace anchored to the concrete slab, and the enormous train — all 410 feet of it — will begin to inch forward, grinding its way into the subsurface.

Right at that spot in the middle of the MacArthur Causeway, if all goes as planned, the first truck will enter the new Port of Miami tunnel in May 2014. It will dip down to 120 feet below the bottom of Government Cut before emerging on Dodge Island from an underground trip of less than half a mile.

Scores of such tunnels have been built around the world to convey everything from cars to trains to petroleum, and Miami’s — at less than two miles in total length — is no great shakes. The massive boring machine is the biggest of its type to be used in the United States.

Yet it’s still a complex task, and Harriet is an engineering marvel, designed and built at a cost of $45 million by Herrenknecht, a German firm, specifically for the Miami job, which requires digging in porous and potentially unstable limestone. . . .