Engineering for the Geotechnical, Geostructural, and Environmental Elements of the Boston City Hall Plaza
Weston & Sampson conducted environmental and geotechnical investigations in addition to geotechnical and structural engineering for the 7-acre $70M City Hall Plaza Renovation as part of the Sasaki Associates team working for the City of Boston Public Facilities Department. Three MBTA Green Line subway tunnels and associated vent structures located under the plaza complicated redevelopment of the site, as did the need for on-site stormwater management. Proposed loads from the plaza on the tunnels (constructed in 1898) and potential associated differential settlement required creative solutions for the proposed improvements.
The geotechnical/structural investigation included a review of as-built MBTA record drawings, as well as inspection and rating reports, to determine load capacities of the subsurface structures. After determining that the tunnels had not been load-rated, Weston & Sampson inspected the MBTA tunnels and plenums to evaluate their structural capacities. We identified seven significantly compromised columns in the 1898 tunnel, designed repairs for each on behalf of the MBTA, and inspected the repairs during summer construction efforts in 2020.
To ensure that the new loads on the Government Center Loop and Washington Tunnels did not exceed the tunnel’s available structural capacity, soils above the tunnel were over-excavated and replaced with lightweight fill, including geofoam and foam glass aggregate. Weston & Sampson’s design of the plaza supporting structure over the Hanover Tunnel included tying the new support structure into the tunnel walls. This approach required additional up-front investigation and analysis, but ultimately saved $6M in construction and more than six months in project duration. Our team designed large spans around vent structures, which included micropiles extending to weathered rock to support new elements of the tunnel.
Weston & Sampson’s design efforts included geotechnical engineering for a new public building with one below-grade level on three sides and daylighting to street level on the other side, as well as over-hanging the MBTA tunnel with a cantilevered slab supported on micropiles. Weston & Sampson’s geotechnical engineers also conducted lateral pile analyses for the building.
In addition, Weston & Sampson provided Licensed Site Professional (LSP) environmental services, including on-site testing to develop soil and groundwater management strategies per the Massachusetts Contingency Plan (MCP).
Weston & Sampson is currently conducting full-time construction observation, including instrumentation and movement monitoring of the tunnel/area structures. We are also supporting Shawmut’s construction operations, including the design of temporary bridging structures and providing crane support during construction.
Despite the additional tunnel work, Weston & Sampson completed design efforts in less than 10 months and met the city’s July 2020 construction start date requirement.
Key Project Features:
- soil and groundwater management
- environmental engineering, including MCP compliance
- geotechnical engineering
- structural engineering