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

Create Account

Availability

On-Demand

Cost

Member: $20.00

Non-Member: $40.00

Credit Offered

No Credit Offered

Abstract: This project in Boston's Fenway neighborhood involved constructing a 16-story mat-supported building with three below-grade parking levels spanning a 65, 000-square-foot footprint. The site adjoins a 17-story steel-framed building founded on 240– 250-foot micropiles to bedrock and an 8-story historic brick building on spread footings in marine sand at 16 feet depth. Excavations reached depths of 35– 44 feet to accommodate the below-grade levels.

Subsurface conditions generally consisted of 5 to 20 feet of urban fill, 1 to 18 feet of organic silt and peat, 5 to 17 feet of marine sand, and 135 to 173 feet of Boston Blue Clay. Subsequent glacial deposits and bedrock underlie the clay deposit. The Boston Blue Clay deposit had a stiff overconsolidated crust transitioning from soft to very soft, normally consolidated clay with depth. Undrained shear strengths of the clay deposit varied from 2.5 kips per square foot near the clay crust, down to approximately 1 kip per square foot near the bottom of the excavation.

To address the proximity of adjacent foundations and the sensitivity of the underlying clay deposit, a robust support of excavation system was required to construct the below-grade levels and mat foundation. A slurry wall system was selected to function as a groundwater cut-off during the excavation, provide stiff lateral support, and serve as the building’ s perimeter foundation. The walls were supported in the excavated state by three levels of internal steel bracing and integrated steel reinforcement wales within the slurry wall panels. Since the excavation terminated below the stiff crust of the Boston Blue Clay, an innovative “ subgrade strut” approach was employed. Slurry wall and bracing installation were carefully sequenced to minimize wall movements and coordination with mat placement was critical.

During and after the building's construction, extensive geotechnical instrumentation was performed to monitor the movement of the slurry wall and adjacent building and utility settlement. Based on observed data, the excavation system support met project requirements under complex conditions.

Dylan Locke; Collin Sweeney

Annual Conference Proceedings