Abstract:
Boston is famous for tall buildings and deep holes in challenging subsurface conditions (Boston Blue Clay). This case study will present how below grade construction risks were managed during the construction of the 3rd tallest (62-story) office and residential tower with 4 levels of underground space extending approximately 58 ft below ground surface in the heart of the densely developed downtown Boston business district. Adjacent structures included three high-rise buildings and one historic building all very sensitive to settlement and differential movement. Excavations for the new tower was planned to extend below the bottom of the foundations for all the adjacent buildings except one. Significant explorations, field and laboratory testing, and detailed design was undertaken to understand the potential impact to the abutters that will be discussed. To minimize the risk of impacts to the abutting buildings the up-down construction method was utilized in conjunction with a stiff, 3 and 4 ft thick, reinforced concrete diaphragm walls (slurry walls) installed around the site perimeter. The structure was supported on load bearing elements (LBE's) to depths of over 170 ft. With the sensitivity of the abutters an extensive geotechnical monitoring program during foundation construction was developed and implemented to measure the system and adjacent building performance. This information was used to compare back to the finite element models during construction to understand system performance as construction proceeded. To further manage risk the owner initiated an independent geotechnical peer review, rare for the area, and worked closely with design and construction teams who all participated in meetings with the abutters. The presentation will discuss the unique design, construction, geotechnical monitoring, abutter engagement, and incorporation of a peer review that resulted in a successful project.