Big Dig - Obstacles

Obstacles

In addition to political and financial difficulties, the project faced several environmental and engineering obstacles.

The downtown area through which the tunnels were to be dug was largely landfill, and included existing Red Line and Blue Line subway tunnels as well as innumerable pipes and utility lines that would have to be replaced or moved. Tunnel workers encountered many unexpected geological and archaeological barriers, ranging from glacial debris to foundations of buried houses and a number of sunken ships lying within the reclaimed land.

The project received approval from state environmental agencies in 1991, after satisfying concerns including release of toxins by the excavation and the possibility of disrupting the homes of millions of rats, causing them to roam the streets of Boston in search of new housing. By the time the federal environmental clearances were delivered in 1994, the process had taken some seven years, during which time inflation greatly increased the project's original cost estimates.

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-foot-deep (37 m) concrete walls upon which the highway could rest. These concrete walls also stabilized the sides of the site, preventing cave-ins during the continued excavation process.

The multi-lane interstate highway lanes also had to pass under South Station's seven railway tracks, which carried over 40,000 commuters and 400 trains per day. To avoid multiple relocations of train lines while the tunneling advanced, as had been initially planned, a specially designed jack was constructed to support the ground and tracks to allow the excavation to take place below. Construction crews also used ground freezing (an artificial induction of permafrost) to help stabilize surrounding ground as they excavated the tunnel. This was the largest tunneling project undertaken beneath railway lines anywhere in the world. The ground freezing enabled safer, more efficient excavation, and also assisted in environmental issues, as less contaminated fill needed to be exported than if a traditional cut-and-cover method had been applied.

Other challenges included existing subway tunnels crossing the path of the underground highway. To build slurry walls past these tunnels, it was necessary to dig beneath the tunnels and to build an underground concrete bridge to support the tunnels' weight, without interrupting rail service.