Award of Merit - Transit

Westport Station Pedestrian Underpass

Sometimes great things come in small packages. That's true at the Westport Station pedestrian underpass, a 75-ft.-long, 14- by 14-ft. tunnel that employed cutting-edge tunneling technologies never used before anywhere in the country.

The tunnel's $2.5 million price tag looks minuscule next to the $100 million price tags associated with urban infrastructure projects, but the client - the Connecticut Department of Transportation - got a lot of bang for its buck.

The project team had to design and construct an upgrade compliant with the Americans with Disabilities Act for the Westport Commuter Rail Station. That upgrade also could not take any of the four active Metro-North Railroad tracks that run overhead out of service at any time.

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"It was executed under a working railroad," said one juror. "That's not easy."

Several factors set the table for the design team. First, hybrid soils - which ranged from a mix of relatively weak and soft deposits on one side of the tracks to hard bedrock on the other - made conventional tunneling impractical. Another project demand was avoiding a complicated system with deep access stairs and elevator connections. Had the team employed existing soil stabilization techniques during the short nighttime windows when trains were not active, the project schedule could have dragged out several years. Rerouting trains via a temporary bypass wasn't a realistic option, either. The design team had to think outside the box.

Its solution called for the tunnel to be jacked under the tracks, with the roof of the tunnel passing just 2 ft. under the ties supporting rails that carry hundreds of Metro North and Amtrak trains a day. The jacked tunnel used a grillage, a technique never attempted in North America. The grillage consisted of a series of preinstalled interconnected orthogonal beams running parallel and perpendicular to the rail tracks. The design spaced the main beams at approximately 7-ft. centers.

The design called for the project team to progressively transfer the loads of the tracks from the ties, which rest on the ground, to the grillage and then to the roof of the advancing tunnel. The design also ensured the grillage could safely support the tracks in all situations, including ground failure in the area ahead of the advancing tunnel.

The team installed small steel box beams parallel to the track along the length of the tunnel. Construction crews went in at night to excavate small 2-ft.-deep, 12-in.-wide trenches and then installed the 90-ft. long steel beams that, once connected with the box beams, created the steel framework of the grillage.

At the end of the project when the tunnel was in place, the crews placed and compacted ballast between the underside of the ties and the top of the tunnel roof. It then removed the grillage beams, finally transferring the load of the tracks to the top of the tunnel.

The tunnel itself was cast in a joint-free box pushed under the grillage at a rate of 2 to 3 ft. per day. The entire tunnel project took about six months, but it took less than 30 days to jack the tunnel through the ground despite the need to navigate a large boulder encountered in softer soils under the tracks. The project team halted tunnel installation for several days to drill, split, and remove the boulder.

The significance of the Westport project lies far beyond the creation of a pedestrian underpass at a busy station. The conditions required creative thinking, and that process showcased a new technology available to engineers and contractors. In its submission, Boston-based Hatch Mott McDonald noted that since the Westport project, the company has fielded requests to use the tunnel-jacking technique in various other locations.