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

PPT Presentation

Pdf Summary

The project involved constructing a 14-story building with three levels of underground parking in Boston’s Fenway neighborhood, requiring excavation up to 44 feet deep. The site’s dense urban setting included historic and newer buildings and nearby MBTA Green Line tracks. These structures were founded at varying depths, often above the excavation bottom, making ground movement control critical.<br /><br />Subsurface conditions included variable urban fill and organic deposits over marine sand and thick Boston Blue Clay, with groundwater about 8–11 feet below grade. The excavation support system was designed around a 3-foot-thick braced slurry wall, selected for its stiffness and groundwater cutoff capability. The excavation proceeded using a bottom-up method and was divided into phases.<br /><br />Lateral earth pressures and adjacent surcharges were evaluated using two-dimensional DeepEX analyses, with finite element analysis used to estimate wall deflections. Structural calculations established wall reinforcement and brace sizes and layouts. To address the soft clay and reduce the risk of inward wall-toe movement, the design also included three deep subgrade struts spanning the excavation. These were considered protective for nearby buildings and rail infrastructure, though they were not judged cost-effective in the second phase, where geometry and connections posed challenges.<br /><br />Slurry walls and subgrade struts were constructed using bentonite slurry for excavation support. A cement-bentonite panel was installed at the interface between the slurry wall and an intermediate sheet-pile wall to limit water or soil ingress.<br /><br />Extensive monitoring tracked potential movement, including inclinometers in the slurry wall and points on adjacent buildings, rail tracks and infrastructure, utilities, and the ground surface, alongside groundwater wells and a heave gauge. The presentation describes the monitoring program and reports on lateral wall, building, and rail movements, but the supplied text does not provide numerical performance results.

Keywords

Boston Fenway

14-story building

underground parking

deep excavation

slurry wall

Boston Blue Clay

groundwater control

lateral earth pressure

subgrade struts

construction monitoring