Abstract:
The recent renovation of the Boston City Hall Plaza, which took place between 2020 and 2022, involved the transformation of a historically underutilized public space into a vibrant and active destination. One of the challenging aspects of the construction was the presence of three tunnels running beneath the site; two of which are some of Boston’ s oldest tunnels constructed circa 1900. These tunnels presented unique challenges throughout the course of the design and significant logistical issues that had to be carefully managed during the construction process.
Given the age and condition of the tunnels beneath the site, it was necessary to transfer loads associated with the new construction below the tunnels. This was accomplished by essentially constructing a below-grade bridge spanning over the tunnels to transfer new loads to micropile-supported pile caps on either side of the tunnel. Prior to constructing the below grade bridge, the pre-existing concrete “ void space structure, ” above the tunnels needed to be removed. Large crawler cranes with significant ground pressures were utilized to remove the structure and construct the below-grade bridge. The selection of crane equipment and the positioning of the cranes on the constricted site had to be carefully analyzed during construction to limit the lateral loads imposed on tunnel to tolerable levels.
Despite these challenges, the renovation of the Boston City Hall Plaza has been a major success, and the renovated plaza now features a range of amenities and attractions, including a water feature, playground, and ample seating areas. The new design also incorporated sustainable infrastructure to meet resiliency standards. This included installation of permeable surfaces collect stormwater for irrigation purposes, 100 new trees to provide shade and improve air quality, and the use of recycled materials (which included unearthed remnants of old foundations from a hotel that previously existed on site). The new design has been well-received by both residents and visitors, and the plaza has become a popular destination for a variety of events and activities.