by Abigail Cory
In the heart of Downtown Boston, the newly transformed City Hall Plaza has reemerged as an iconic and visible civic anchor. No longer a stark, brick and concrete expanse, it is now a dynamic “civic front yard” distinguished by lush greenery, universal accessibility, and thoughtful public amenities. Adorned with playscapes, terraces for art, a striking civic pavilion, and generous seating shaded by new trees, the plaza gracefully sustains large-scale gatherings while inviting everyday visitors to linger, explore, and connect.
A critical aspect of these exciting renovations was the need to improve accessibility and provide designated playground and gathering spaces in the plaza. Led by the design firm Sasaki, Weston & Sampson served as the geo-structural subconsultant and faced various complex design challenges. These included active 19th and 20th century MBTA Green Line tunnels directly below the plaza and a proposed playground. We devised a below-ground concrete bridging system combined with lightweight fill materials to reduce and divert construction – and ultimate project loads – away from the existing tunnel roof beams.
Due to age, unknown current conditions, and limited as-built data, our design needed to reduce additional loads from the proposed aboveground conditions as much as possible to prevent overstressing the existing subsurface structures. Compounding the challenges was that extensive renovations to the existing tunnel structure were not feasible and minimal disruption to MBTA Green Line service was required.
The subway tunnels were inspected to assess existing conditions, and in response to visible deterioration, we implemented localized column repairs and completed a load rating analysis to determine the structure’s ability to support the anticipated loads. As a result, areas with an inadequate rating for proposed new loads were over-excavated and replaced with a new lightweight fill to reduce dead load.
Due to the age of portions of the existing tunnel structure, we used a bridging system to divert the load to the tunnel walls, regardless of the existing roof beam conditions. This bridging system included precast, prestressed concrete beams spanning over the tunnel with a waterproofed, cast-in-place concrete slab. This slab was built over and wrapped down the outside edges of the precast beams to prevent moisture infiltration. At several locations, existing vent structures over the tunnels presented an additional challenge for diverting loads. Precast beams, some post-tensioned, were designed to span over the existing vent structures, supported by deep foundations of micropiles and cast-in-place concrete pile caps. The deep foundations transfer loads directly to the ground, avoiding the existing vent structures altogether.
And finally, we used lightweight fill solutions along with the bridging system to reduce loads under the proposed playground area. Low-density materials, including expanded polystyrene fill (geofoam) and foamed glass aggregate fill, allowed for the same volume of fill required to form the sloped play surface while dramatically reducing the weight compared to traditional fill and concrete materials.
Our design allowed for the seamless integration of new programming and landscape elements while protecting the existing critical below-ground infrastructure to provide a revitalized civic space. These extensive new structural and geotechnical renovations will support the newly renovated plaza for many decades to come. They also went a long way toward transforming Boston’s City Hall Plaza from the bleak windswept expanse it has historically been known as into a safe, welcoming, and resilient public resource.
Abigail Cory
Abigail Cory, PE, is a senior transportation project engineer in Weston & Sampson’s Boston office.
