TUNNELING THROUGH A BUILDING'S FOUNDATIONS

The Massachusetts Bay Transportation Authority (MBTA) is currently constructing the portion of the Silver Line Route that runs from South Station to the World Trade Center in Boston. A 100-meter-long segment of the project involves constructing a shallow, binocular-shaped tunnel approximately 8-m high and 12-m wide by NATM methods through the pile foundations of two seven-story, historic buildings with steel frames and brick facades. Concurrent with the MBTA work, deep, large open excavations for Boston's Central Artery/Tunnel Project are being constructed along the west and north sides of the buildings, approximately 3 m and 8 m away, respectively. The challenge is to construct the tunnel while keeping the buildings safely in service and preserving their historic attributes.

The geology of the project area is varied and complex because the tunnel is located at the edge of the Shawmut Peninsula where Boston's shoreline and mudflats were filled in successive stages during the 17th, 18th and 19th centuries to create more land and waterfront facilities. The building foundations are timber piles with granite block pile caps. The piles extend through fill, organic silts and Boston Blue Clay to a dense glacial till at the west end of the NATM tunnel. To the east, the glacial till drops off rapidly resulting in the building being underlain by a greater depth of soft clay as well as remnants of several timber bulkhead and cribbing structures from the various waterfront expansions prior to construction of the buildings. Provisions to protect the buildings include: a) ground freezing to create about a 2-m-thick frozen soil arch between the tunnel crown and the pile caps beneath the building; b) permanent underpinning to transfer pile loads around the tunnel; c) temporarily underpinned columns for adjusting columns up or down during construction, if the need arises; and d) extensive monitoring of the building for settlement, heave, lateral movement, distortion and damage.

This presentation will examine the planning and the implementation of the various measures used to construct this tunnel in a congested urban setting with complex natural and man-made geologic conditions while maintaining the use and historic fabric of the structures above it.