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PL6 Stormwater Storage Tank Tunnel

  • PL6 Stormwater Storage Tank Tunnel
  • PL6 Stormwater Storage Tank Tunnel
  • PL6 Stormwater Storage Tank Tunnel
  • PL6 Stormwater Storage Tank Tunnel

The City of Cambridge, Massachusetts’ Port neighborhood has experienced significant historical flooding and sewer backups, with recent assessments on climate change vulnerability showing the risk of flooding increasing over time. The Parking Lot No. 6 (PL6) Stormwater Storage Tank Project is Phase 1 of the City’s plan to alleviate historical flooding and sanitary sewage backup problems.

To capture stormwater and redirect the flow from an over-burdened Massachusetts Water Resource Authority (MWRA) combined sewer system to a previously installed storm drain on Massachusetts Avenue, Kleinfelder provided planning, design, permitting, and construction services for the project, which included utility coordination for new electric services for the tank and pump station, as well as new gas service for the stand-by generator. Kleinfelder also managed the community relations program during design.

Complex Connection

The complex connection between the 390,000-gallon, 119-by-36-foot cast-in-place stormwater tank located in PL6 and the 54-inch storm drain on Massachusetts Avenue required microtunneling a 72-inch Permalock steel casing pipe, carrying four 18-inch force mains, beneath the Massachusetts Bay Transportation Authority (MBTA) Red Line subway. Advancing the tunnel within difficult ground conditions due to weight-of-hammer (WOH) Boston blue clay was an added challenge. Design of the stormwater tank also needed to consider potential future development adjacent to the tank by the Cambridge Housing Authority.

Detailed Coordination Efforts

Several phases of subsurface explorations were undertaken as the design advanced and multiple alternatives were assessed. Geotechnical lab testing was conducted, and recommendations included deep foundation for support of the tank to mitigate settlement of the underlying clay due to variable loading conditions, temporary support of excavation alternatives for tank construction and tunneling access and receiving shafts, and geotechnical instrumentation program. Detailed coordination efforts were required during design and continued into construction with both the MBTA and a private developer constructing a new Market Central development project – including two towers, 6 and 19 stories high – surrounding PL6.

Project Success

After the successful completion of a complex design and construction process, the tank was operational in May 2021 and redirected more than 3,480,000 gallons of stormwater from The Port neighborhood during the Boston area’s third-wettest summer on record with over 19 inches of rain. The PL6 stormwater tank successfully reduced the surface flooding that impacted the neighborhood during similar-sized storm events prior to the tank installation.

Awards and Publications

Trenchless Technology Magazine, the leading publication covering the trenchless marketplace, recognized the project’s microtunnel operation as the 2020 Project of the Year Honorable Mention in the New Installation category, and the project received recognition through both the American Council of Engineering Companies (ACEC) Massachusetts chapter, as well as nationally at the 2022 Engineering Excellence Awards Gala. This project was also featured on the cover of the February 2022 issue of Stormwater Magazine.

Cambridge, Massachusetts

City of Cambridge