In 1818, Marc Brunel, a French engineer, invented a device that enabled workers to tunnel under rivers without having mud and water ruin their efforts. His "tunnel shield" was a rectangular cast iron wall with dozens of small shutters. Workers opened the shutters one at a time and dug out a few inches of dirt. Then the whole shield was pushed forward using screw jacks. As the shield lurched ahead inches at a time, the workers behind it constructed a thick, brick lining that became the shell of the tunnel. It took nine years to finish a 1,200-foot passage below the Thames River in London-the first underwater tunnel in the world-but Brunel was knighted for his feat and engineers around the world began adopting his idea.
Variations on the tunnel shield helped create some of the greatest underwater passages in the 20th century, including New York's Holland and Lincoln tunnels. To counter the intense pressure above the tunnels during construction-which caused flooding and structural collapses-engineers installed a system of airtight seals to keep the air pressure as high as possible under the river. Workers spent much of their day just compressing and decompressing. Though many suffered from decompression sickness, overall worker safety was greatly improved.
The Ted Williams Tunnel, which connects South Boston to Logan Airport, illustrates the most modern method of underwater tunneling-the immersed tube. First, workers dredged a 50-foot trench along the floor of Boston Harbor. Then, 12 giant steel tubes, each 325 feet long and already containing roads, were dropped into the water. Once the tubes had been connected on the harbor floor, the tunnel was buried in a 5-foot protective layer of rock. Finally, workers removed the steel bulkheads and linked the roads. (For more information on the Ted Williams Tunnel, part of the 15-year Central Artery/Tunnel project, see "The Big Dig," June '01.)