Successful Innovative Technology
In retrospect, it's amazing how much went right after construction started in 1991. Over 14 years, Massachusetts, its consultants, and its contractors carved canyons under Boston while the city hummed above. They designed and built seven and a half miles of highway - 161 miles of separate lanes - more than half of them in tunnels. They built six interchanges and 200 bridges.
One of the innovations that made this work possible was the slurry wall, also used at New York's World Trade Center. As Salvucci puts it, the slurry wall allowed the state to build huge underground tunnels "arthroscopically," without pock marking Boston with huge uncovered holes. The slurry wall, in effect, let Boston dig itself up without shutting itself down. The Big Dig's general reliance on new technology was a major factor in one of the project officials' biggest decisions: choosing a consortium made up of Bechtel and Parsons Brinckerhoff as the "management consultant."
Parsons was an expert in innovative urban tunneling dating to the early twentieth century, when it built New York City's subways. Bechtel had constructed the Hoover Dam as well as most of modern Saudi Arabia, and had fabled political connections to accompany its engineering and logistics mystique. Caspar Weinberger and George Shultz, both in Reagan's cabinet, were Bechtel men - though that connection hadn't helped Salvucci at veto time.
One of the creative, audacious, and potentially disastrous things that Bechtel and Parsons, as well as the state's contractors, did right on the Big Dig was jacking up the Central Artery. This required replacing the half-million-ton highway's 69 support columns with temporary "underpinnings" so that contractors could remove the columns and make way for the tunnels. "Pinning the Artery" wasn't the Big Dig's only never-before-done feat. The striking Zakim Bridge - the one redesigned after community outrage - is the world's widest cable-stayed asymmetrical bridge, expanding from eight lanes to ten to account for complicated traffic flow.
Elsewhere, underneath active railroad tracks, engineers froze ground too soft to withstand construction and pushed pre-built tunnel sections through. They erected a huge temporary dam to dry out a basin in which to construct tunnel segments - and then flooded the basin, floating the segments and re-sinking them in their new underwater homes. They built underground bridges to hold up subways, threading highway tunnels above and below mass transit. And though insurance tables had predicted 40 serious accidents during the project, the Big Dig suffered only a quarter of that total, and three construction deaths. That showed how much things had changed since, say, the 1870s, when raising the Brooklyn Bridge took 27 lives.