Building on Liquefiable and Compressible Soil for a Major Transit Project in Salem, Massachusetts

The Massachusetts Bay Transportation Authority (MBTA) is addressing accessibility throughout their facilities. Improvements at the Commuter Rail Station in Salem, Massachusetts include a five-level parking garage, a pedestrian bridge, two accessible elevators, and a full-length accessible high-level platform, as well as a passenger drop-off/pick-up area, secure bicycle parking, and improved traffic flow patterns for buses and taxis. The project site is bound by North River to the north, and is retained by a 12 feet high seawall along the river. Subsurface conditions generally consist of fill overlying loose, potentially liquefiable saturated sands, overlying very soft to soft marine clay deposits extending to till and rock at about 60 to 80 feet depth. The site was classified as a seismic Site Class F, which called for a site-specific site response analysis and structurally robust parking garage building. Deep foundations in the form of drilled shafts bearing on rock were recommended for structural support of the garage. Through cooperation of the structural and geotechnical teams and evaluations of many options, ground improvement in the form of vibratory stone columns (VSCs) was recommended to address the liquefaction and lateral spreading potential, and improve the seismic site classification to Site Class E. The project team evaluated value-engineering options. The final design consisted of the installation of controlled modulus columns (CMCs) below the garage and bridge footings, and VSCs below the parking garage and platform. The project involved developing detailed specifications for ground improvement and a verification program to assess the effectiveness of the VSCs. The unique site conditions required additional coordination between the geotechnical and structural design teams, as well as the construction team during the design phase of the project. The collaboration between the teams was integral in allowing the project to move forward, ultimately saving the client millions in construction costs.