The project involved the construction of elevator access to the MBTA subway station at Copley Station in downtown Boston, MA. The construction required 25± foot deep excavations adjacent to the historic Old South Church and the Boston Public Library. The role of one of Northeast Geotechnical’s principal engineers, as a member of the Project Conservator team, was to review the design and construction of the elevator excavations and assess the implementation and effectiveness of the mitigation measures proposed to protect adjacent cultural resources, including the Old South Church and the Boston Public Library, during construction. We consulted with the other Conservator team members, the MBTA, and their consultants to address the performance of the various mitigation measures and the cultural resources that the measures were designed to protect.
Mitigation measures included internally-braced excavations supported using drilled secant piles and jet grout columns for groundwater control along with preconstruction surveys of adjacent properties and structures. The survey of the Old South Church included excavations to expose and document the condition of the woodpiles and granite pile caps that support the church and a load test on one of the piles. A dewatering and recharge system was also designed and implemented to maintain ambient groundwater levels at the site.
The adjacent structures were monitored for vibrations and movement during construction using seismographs, deformation monitoring points, optical prisms, liquid level meters, extensometers, and crack gauges. Groundwater levels were monitored using observation wells and piezometers. Excavations were monitored for movement using inclinometers.
