PEOPLE, PROJECTS AND EQUIPMENT Up-down or top-down-down construc- tion is a technique where deep foundation elements and a perimeter concrete diaphragm wall are installed from ground surface. During excavation, permanent below- grade floor slabs provide lateral bracing for the concrete diaphragm wall. Excavation continues below each completed floor slab through temporary openings in the slab, termed a glory hole, to remove the excavated material. Since the building’s foundations and column elements are already in place, work on the superstruc- ture above proceeds while the excavation continues. Generally, the cost of excavation is greater because large quantities of soils must be excavated with low-headroom equipment. However, there is typically an overall cost savings because the completed building can be delivered sooner. In addi- tion, the need for temporary lateral bracing can be reduced or sometimes eliminated. At ground level the Atlantic Wharf site Redeveloped historic Atlantic Wharf (© Anton Grassl/Esto) Up-Down Construction on Boston Wharf The Atlantic Wharf redevelopment included the construction of a 31-story mixed-use tower with 6 levels of below-grade parking along the historic Boston Harbor waterfront. The site, formerly known as “Russia Wharf,” was the headquarters for trading with Russia beginning in the mid to late 1700s, and in 1773 it was the site of the Boston Tea Party. From the early 18th century until the end of the 19th century it served as a wharf. The site consisted of landfill placed in the late 18th century and early 19th century that extended the shoreline toward the harbor as the urban population and land needs grew in Boston. For the recent redevelopment, the project team had to address relics from former waterfront structures (granite block seawalls, numerous timber piles and old pier structures), weak subsurface soil conditions, tidal groundwater levels, and contaminated soils resulting from previous site usage and filling. The foundation system consisted of heavily loaded drilled shafts and a concrete diaphragm, or slurry wal l . The team chose up-down construction to address these factors and to expedite the overall construction schedule. Additionally, as a prerequisite for development, the project was required to preserve portions of the former historic buildings façades, protect them during construction, and finally, incorporate them into the completed new structure. The façades were temporarily braced and supported prior to the demolition of the buildings and commencement of the below-grade construction. Below-grade construct ion was completed in March 2010, and the building was opened for tenant occupancy in January 2011. (Figure 1) is bordered by Atlantic Avenue and The Rose Kennedy Greenway to the west, Congress Street to the south, a Central Artery vent building and luxury hotel and condominiums to the north, and Fort Point Channel to the east. Significant below-grade structures at or adjacent to the site include: • MBTA Silver Line Tunnel – an under- ground bus tunnel, within 3 ft (0.9 m) • Central/Artery Tunnel (CA/T) and Vent Building – Interstate 93 • Adjacent three-level underground garage, within 7 ft (2.1 m) • Granite block seawall along the Fort Point Channel AUTHORS: Rebecca B. Higgins, P.E., Senior Project Manager, Mark X. Haley, Sr. Vice President, and Lee S. Vanzler, Senior Engineer, Haley & Aldrich, Inc Boston, MA DEEP FOUNDATIONS • SEPT/OCT 2012 • 73
DFI Sep Oct 2012
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