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On Dec. 31, It's Official: Boston's Big Dig Will Be Done

One was Frederick Salvucci, a city kid for whom the demolition of the old Central Artery became a lifelong quest.

"It was always a beautiful city, but it had this ugly scar through it," said Salvucci, state transportation secretary during the project's planning stages.

Rather than build a new elevated highway, Salvucci and others pushed a far more radical solution: burying it.

Easier said than done.

Those who would build the Big Dig would have to undertake the massive project in the cramped confines of Boston's narrow, winding streets, some dating to pre-Colonial days.

Of all the project's Rubik's Cube-like engineering challenges, none was more daunting than the first -- how to build a wider tunnel directly underneath a narrower existing elevated highway while preventing the overhead highway from collapsing.

To solve the problem, engineers created horizontal braces as wide as the new tunnel, then cut away the elevated highway's original metal struts and gently lowered them onto the braces -- even as cars crawled along overhead, their drivers oblivious to the work below.

It was just one of what would be referred to as the Big Dig's "engineering marvels."

The Big Dig's long history is also littered with wrong turns: some unavoidable, others self-inflicted.

One of the biggest occurred in 2004 when water started pouring through a wall of the recently opened Interstate 93 tunnel under downtown Boston. An investigation found that the leak was caused by a failure to clear debris that became caught in the concrete in the wall during construction. Hundreds of smaller drips, most near the ceiling, were also found.

Some delays were unrelated to construction.

The Leonard P. Zakim Bunker Hill Bridge, the project's signature element, went through dozens of revisions as designers labored to come up with the most practical and elegant way to cross the Charles River.

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