Plaza Renovation Over Boston’s Oldest Tunnels-Paper

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Boston City Hall Plaza, originally built in the 1960s, was renovated to replace its difficult stepped layout and barren concrete expanse with an accessible, welcoming public space. The redesign added an ADA-compliant sloped promenade, trees, play areas, sustainable stormwater features, and a Civic Pavilion.<br /><br />Construction took place above three MBTA subway tunnels, including tunnels dating to 1898 and 1902. Because assessments found that the aging tunnels could support only limited additional loads, the design used lightweight fill and deep foundations. More than 80 drilled micropiles supported a below-grade bridge over parts of the Hanover Street Tunnel and the Civic Pavilion. The building was also partly cantilevered over the tunnel to avoid loading it.<br /><br />Construction logistics were especially challenging: vehicles and heavy cranes had to be carefully positioned to protect the tunnels, and sections of heavy structures above the tunnel had to be removed. Engineers analyzed crane locations, ground pressures, temporary slopes, and tunnel loading. Instrumentation monitored settlement, vibration, tilt, cracking, and wall movement throughout construction. A temporary bridge provided access for construction vehicles without crossing restricted tunnel areas.<br /><br />The project was completed successfully through close coordination among the City, contractor, and engineering team. The renovated plaza is now accessible to all and serves as a popular venue for public events and activities.

Keywords

Boston City Hall Plaza

accessible public space

ADA-compliant

micropiles

MBTA subway tunnels

stormwater features

Civic Pavilion

construction logistics

tunnel protection

public events