Engineering around Manhattan’s History to Construct a Cutting-Edge University Building: Excavation Support and Foundations for the 181 Mercer Street Project

New York University’s (NYU) 181 Mercer development is a part of NYU’s master plan to provide university programming in the historic Greenwich Village neighborhood of New York City. To provide the desired amount of programming space, NYU proposed a 9 to 26 story building with two cellar levels to the lot-lines of a 7,430 square-meter (80,000-square-foot) site. General excavation extended about 10.7 meters (35 feet) below the adjacent streets, and localized excavations for proposed mechanical space, a pool, and caisson caps extended up to 4.6 meters (15 feet) deeper. Past developments on the project site, adjacent streets, and adjacent structures had a paramount impact on below-grade construction, coupled with the site being next to an NYC Landmark District and a landmarked 33-story tower. The proposed cellar levels extended below part of the de-mapped Greene Street which presented many unknowns regarding existing utilities and other buried structures. The north and south sides of the excavation were supported by bracing the foundation walls of the former building that occupied the site and jet-grout-underpinning the walls. Double-channel soldier piles braced by tiebacks were designed to support the east and west perimeters of the excavation. External bracing was designed to be anchored below the mat foundation of the neighboring landmarked tower, to avoid cellars of the neighboring historic buildings, below a subway, and to avoid utilities. Soil-mixing was constructed to support localized excavations for the proposed pile caps, mechanical pits, and pool below the general subgrade that were up to 3.6 meters (12-feet) below groundwater. Excavation support for the new building and new utilities were adjusted as excavation exposed utilities and foundations of the former industrial buildings that occupied the site in the early 20^th century. The new 181 Mercer building is supported by drilled caissons socketed into bedrock. The caissons were installed using a down-the-hole hammer with an internal air-flow system. Neighboring structures were monitored throughout excavation and foundation construction to verify activities on the site did not cause adverse effects on the structures.