Publication Number: FHWA-RD-01-065
Date: July 2001

Big Lessons from the Big Dig

One-hundred and sixty-one lane miles of highway. More than three million cubic yards of concrete. Five miles of steel-reinforced concrete slurry tunnel walls. And that's just for starters. The Central Artery/Tunnel Project in Boston, Massachusetts, better known as the Big Dig, is one of the Nation's largest and most complex highway projects. The 14-year endeavor, scheduled for completion in 2004, will replace an existing 6-lane elevated viaduct structure with a 10-lane underground expressway. During the course of construction, the project has produced a number of firsts, from the largest application of slurry walls to the widest cable-stayed bridge in the world to the most extensive highway tunnel ventilation system ever constructed. A number of these technological advancements are now being highlighted in the Federal Highway Administration's (FHWA) Innovations and Advancements Program, which consists of a series of instructional presentations that can be scheduled by highway agencies.

"There's a lot that has been learned from this project, and everyone can benefit from it," says Daniel C. Wood of FHWA. The presentation modules focusing on technology innovations include Urban Applications of Slurry Wall Technology, Soils Improvement Techniques, Intelligent Transportation System (ITS)/Traffic Simulation Model, and Tunnel Fireproofing and Ventilation Systems.

The Slurry Wall Module describes this technique for tunnel wall construction and stabilization, which allows excavation work to be performed in a confined space while still maintaining continuous traffic movement above. The Soil Improvement Techniques module looks at methods used to stabilize soil. One such method is soil freezing, which enabled engineers to construct a tunnel big enough for an Interstate without disrupting the nine active railroad tracks traveling just above the tunnel space.

Instead of digging a tunnel, hydraulic jacks were used to push tunnel boxes up to 380 feet long under the railroad tracks. Due to soft soil conditions under the tracks, ground freezing was necessary to stabilize the soil before performing the tunnel jacking, so that the tracks would not settle and endanger rail operations. This was accomplished by pumping a brine mixture through hundreds of pipes installed between the tracks. Over the course of several weeks, the circulating brine drew the heat from the soil and froze the ground outward from the pipes.

The tunnel fireproofing and ventilation system module details the project's use of the largest mechanical ventilation system in the world. It also covers how the project conducted a full-scale tunnel fire test program to improve tunnel safety. The Big Dig's Integrated Project Control System, meanwhile, is described in the ITS/Traffic Simulation Model workshop. The system will use more than 35,000 data collection points to manage traffic and incidents, as well as such things as ventilation, lighting, security, and air quality. It will also collect data on traffic speed, volume, and congestion and distribute information to motorists through such means as electronic message boards and lane control signs.

One State that has already hosted a workshop is Connecticut. "After seeing the presentation on tunnel jacking, they changed one of their projects to use the tunnel jacking technique," says Wood.

For more information on scheduling a workshop, contact Wood at 617-494-2462 (email: daniel.c.wood@fhwa.dot.gov). Information on the Big Dig's use of innovative technologies can also be found on the Web at http://bigdig.com.

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