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It was the first major link constructed as part of Boston's Big Dig. It is constructed from ten steel sections fabricated in a Baltimore shipyard. These sections were then brought near the site and each was fitted with a large surrounding mass of concrete (so that the completed tunnel wasn't bouyant ). Using additional flotation, the tunnel sections were then floated into place, sunk, and joined to the other sections. At this point, the steel panels sealing the now-joined sections could be cut out and the finishing operations could commence.
Initially, only commercial traffic was allowed to use the TWT. Later, non-commercial traffic was allowed on weekends and holidays. Now, with the substantial completion of most of the Big Dig, the tunnel is open to all traffic at all times.