DESIGN AND CONSTRUCTION OF A MULTIPLE-DRIFT SUBWAY UNDERPINNING STRUCTURE FOR BOSTON'S CENTRAL ARTERY/TUNNEL PROJECT

The Central Artery/Tunnel (CA/T) project in Boston is the largest current public works project in the U.S. This project requires major cut-and-cover excavation and tunneling for complete replacement of the interstate highway through the heart of Boston. A challenging component of the project is the underpinning of the Massachusetts Bay Transportation Authority (MBTA) Red Line rapid transit underground station at South Station. Constructed in 1913, the Red Line structure is critical to the movement of mass transit riders to and from downtown Boston, and must remain in service without interruption. The alignment of the CA/T Northbound roadway crosses beneath the alignment of the Red Line, necessitating that the CA/T structure be lowered to its deepest depth (34 m below grade), and built using mined construction methods to pass under the Red Line. A unique underpinning system was designed and constructed that enabled the contractor to create the 22-meter wide tunnel opening beneath the existing Red Line concrete structure.

Subsurface conditions imposed challenging design constraints on the underpinning structure. A layer of cohesive very stiff to hard, impervious, glacial till directly underlies the subway station. Beneath the cohesive till is a glacially derived deposit of very dense sand and gravel with frequent cobbles and boulders, which is quite pervious. Bedrock underlies the glacial deposits and is comprised of argillite with occasional diabase dikes. The static groundwater level in the granular till is typically 9m below grade.

Two grouting galleries were carefully mined directly below the subway structure and pre-grouting of the granular, pervious glacial soils and weathered bedrock was carried out from these galleries. The reinforced concrete walls of the underpinning structure were constructed within three stacked drifts that were sequentially mined below each grouting gallery. After 11 cross adits were mined between the grouting galleries, cast-in-place post-tensioned girders that spanned between the stacked-drift walls were constructed to support the subway station. The stacked-drift walls provided lateral support for the 22- by 29-m opening beneath the girders. The underpinning structure was successfully constructed beneath the subway station with negligible vertical settlement.