Design, Construction, and Performance of a Sensitive Braced Excavation in Boston Blue Clay-Paper

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The paper describes the design and performance of a three-level basement excavation for a 14-story building in Boston’s Fenway neighborhood. Excavation reached approximately 44 feet (13.4 m) into sensitive Boston Blue Clay, close to a historic brick building, other occupied buildings, and MBTA Green Line infrastructure.<br /><br />To control groundwater and limit movement, the project used a perimeter slurry wall supported by three levels of internal steel bracing. Construction proceeded in two phases to maintain access to existing loading docks. Because the excavation extended into soft clay, where wall toes were expected to move most, the team added three unreinforced concrete subgrade struts beneath the excavation near the historic building. Modeling predicted these would reduce settlement there from 1.3 to 0.9 inches (33 to 23 mm). The excavation also included an intermediate sheet-pile wall and a watertight connection between the two phases.<br /><br />Instrumentation tracked wall movement, building settlement, and track displacement during and after construction. Maximum measured slurry-wall displacement was 2.0 inches (51 mm), with reduced toe movement where subgrade struts were installed. Settlement of adjacent buildings was lower in areas supported by the struts, and Phase 2 added only about 0.1 inch (2.5 mm) of settlement at nearby monitoring points. Maximum MBTA track movement was 0.125 inches (3.2 mm) vertically and 0.25 inches (6.4 mm) in each horizontal direction, remaining within project criteria.<br /><br />Overall, the braced slurry-wall system and targeted subgrade struts successfully limited movement and enabled basement construction under challenging urban and geotechnical conditions.

Keywords

basement excavation

slurry wall

Boston Blue Clay

internal steel bracing

subgrade struts

groundwater control

building settlement

wall displacement

MBTA Green Line

urban geotechnical engineering