Reworking such a busy corridor without seriously restricting traffic flow required a number of state-of-the-art construction techniques. Because the old elevated highway (which remained in operation throughout the construction process) rested on pylons located throughout the designated dig area, engineers first utilized slurry-wall techniques to create 120 ft. deep concrete walls upon which the highway could rest. These concrete walls stabilized the sides of the site, preventing cave-ins during the excavation process.
Other challenges included an existing subway tunnel crossing the path of the underground highway. In order to build slurry walls past this tunnel, it was necessary to undermine the tunnel and build an underground concrete bridge to support the tunnel's weight.
The [Zakim Bunker Hill Bridge]?, designed by Swiss designer [Christian Menn]? represents the terminus of the project, connecting the underground highway with I-93 and Route 1. A distinctive suspension bridge, the Bridge is supported by two forked towers, which are connected to the span by cables and girders.
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