Trevi Group’s North American subsidiary Treviicos has revealed that it was involved in the South Station Tower project in Boston, Massachusetts, US between September 2021 and February 2022.

The South Station Tower is a 51-storey mixed use skyscraper under construction atop Boston’s historic South Station complex as part of the South Station Air Rights project, which includes other upgrades and improvements of the historic South Station Transportation Center. Construction on Phase 1 of the project began in January 2020 and is expected to last four years.

New foundation elements were required for the 51-storey tower, and load bearing elements (LBEs) were selected as the most appropriate foundation system based on load support, constructability considerations and economics.

The new tower will be supported on eight super columns, which will in turn be supported by a continuous array of LBEs that are 3ft (0.91m) thick and extend over 120ft (36.58m) deep into bedrock. Treviicos’ scope of work included construction of 46 LBEs, which will serve as the main foundation for the tower.

Treviicos said that it was a challenging plan due to very tight site working conditions between the historic South Station headhouse and the active passenger platforms for Amtrak and MBTA commuter rail lines. The LBEs were installed through an operational personnel service tunnel.

The company has previously constructed LBEs for the 400 Summer Street project, also in Boston. In early 2021, it undertook foundation work for the North Washington Street Bridge Replacement project.

Treviicos stated that subsurface conditions at the South Station Tower project site consisted of fill, organic deposits, marine deposits, glacial deposits (glacial till, glaciomarine, glaciofluvial) and bedrock. The bedrock was classified as Cambridge Argillite with average and maximum compressive strengths of 4,034 psi (27,813kN/m2) and 5,485 psi (37,818kN/m2) respectively.

The LBEs were constructed with combination of a mechanical clamshell grab and a Soilmec Hydromill SC-135 cutter. The clamshell grab was typically used to excavate the overburden soils down to the top of the bedrock, while the Hydromill cutter was used to excavate the bedrock down to the designed LBE tip elevation.

Polymer slurry was successfully used to support the LBE trench excavations, and Treviicos found that it provided good trench stability throughout the LBE installation process from excavation to tremie concrete placement. All polymer slurry was processed through an on-site desanding plant to remove soil and rock cuttings, along with a centrifuge for additional solids separation.

The LBEs had an average concrete depth exceeding 36.58m and were socketed up to 31.7m into the bedrock. The total area of reinforced concreted LBEs is approximately 48,000ft2 (4,459m2), which includes a 5,800yd3 (4,434m3) total volume of tremie concrete placed, as well as 350 short tons (317.5t) of reinforcing steel.

In addition, 12 of the 46 LBEs included pairs of W14 structural columns within reinforcing steel cages to connect them to the tower base foundation above ground. An additional three LBEs were constructed for two tower crane foundations. All reinforcing steel and structural columns were assembled at an off-site staging yard and delivered to site according to the installation schedule.

Treviicos stated that the project included strict quality control procedures, including the first time use of crosshole sonic logging (CSL) in the Boston area to evaluate concrete quality and verify that no anomalies exist that could affect the load carrying capacity of LBEs. All CSL testing was performed by a third-party geotechnical engineering services firm and all testing results met the specifications requirements.

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