Engineering Inc. - July/August 2007 - 16

KEy PLAyErS By the Numbers n Amount of dirt excavated: 15 million cubic tons. n Concrete used to replace the dirt: 3.8 million cubic feet. n Steel used: 26,000 linear feet. n Traffic management system: 400 video cameras, 130 electronic message signs, 30 infrared height detectors, six emergency response systems. n Reduction in air pollution due to central artery project: 12 percent citywide. n Maximum number of construction workers on project: approximately 5,000. n Number of construction contracts executed: 109. n Underground utilities relocated: 29 miles of gas, electric, water, sewer, telephone and other lines managed and maintained by 31 companies. n Deepest point: 120 feet at Dewey Square, where it passes beneath the red Line subway. n New parkland: 300 acres (27 acres reclaimed from the elevated highway). n Toxic contaminants hauled away: 435,000 cubic yards of soil and other debris. process of transporting them, and balancing their weight within the water, even more complex. Using remote control cameras, engineers positioned the pieces and connected them like giant Lego blocks. Constructing the last link between the ITT and I-90 created another challenge. Engineers had to dig under commuter rail lines carrying more than 150 trains and 100,000 passengers a day, without disrupting the tracks. Once again, workers faced the problem of soft soil that couldn’t support tunneling. Any attempt to remove the soft soil under the tracks risked derailing the entire project. That’s when one contractor proposed a new and radical solution: Freeze the soil during the tunneling process using 2,000 pipes located eight feet apart, each filled with a brine refrigerant solution chilled to minus 30 degrees Fahrenheit. “It was like creating a manmade permafrost,” Bertoulin says. Workers mined 18 inches into the frozen soil, creating a void using the tunneljacking process. The tunnel jacking required 10,000 PSI to move the 35,000-ton sections at a rate of three feet per day until they were in place. The process took three years to complete, but left the structures on the surface untouched. In September 2001, a massive leak occurred in the section where the land and sea tunnels met. Rapidly rising water inundated expensive cranes and other equipment and made the tunnel impassable. Although crews began working on the problem immediately, it took divers nearly eight weeks to identify and plug the leak. By then, the opening of the tunnel was delayed by four months. Driving Results FReD SALvuCCI An engineer and former Massachusetts transportation secretary (1983–1990), he is credited with first envisioning the concept of the Big Dig and generating the project’s initial financial and political support. SALLy PeRRIn Former deputy project manager for the Bechtel/ Parsons Brinckerhoff team, she oversaw environmental mitigation, real estate and traffic management as they prepared for construction. JoHn KenneDy Senior principal at vanasse Hangen Brustlin, Inc., he designed the traffic management plan that maintained safe and efficient traffic operations during the 15-year Big Dig construction period. PAuL HARRIngTon Director of structural division at Fay Spofford & Thorndike, Inc., he developed the underpinning (foundation support) system that allowed tunnel construction to proceed without affecting existing transit/rail systems or surface structures. SenA KuMARASenA HnTB associate vice president, he served as project engineer and project manager during the design and construction of the Leonard P. Zakim Bunker Hill Bridge, the widest cablestayed bridge in the world. The old elevated High Bridge over the Charles River is demolished in early 2004. Massachusetts turnpike authority The final piece of the project was the engineering of the tunnel beneath I-93. To clear space for the underground highway, engineers had to build corridors to relocate pipes, cables and other infrastructure that fills virtually every square inch of subterranean Boston. At one point, they had to dig underneath three levels of subterranean transportation systems—subway lines, heavy rail and a bus station—and create a tunnel space only 36 inches beneath the subway. “Essentially, we had to create a parallel set of walls with huge crossbeams beneath the existing raised structure in order to build the tunnel. The ground had to support both the existing steel and concrete structure and the tunnel box,” says Frank Leathers, a geotechnical engineer and president of GEI Consultants, Inc., which designed one section of the slurry walls for the project. Prior to the Big Dig, Leathers says, the slurry wall module building method had never been attempted on such a large scale. Project planners also recognized the need to replace a deteriorating three-lane double-decker bridge over the Charles River used to connect the tunnel to the interstate. Further complicating the project, engineers decided it was necessary to build around the existing bridge structure so that traffic continued to flow during construction. Today, the $100 million cable-stayed Leonard P. Zakim Bunker Hill Bridge, the widest asymmetrical bridge ever built, carries 10 lanes of traffic—eight through the legs of twin towers 16 ENGINEERING INC. July / AuGust 2007

Engineering Inc. - July/August 2007

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