Investigation of complex geologic conditions for the Second Avenue Subway tunnel alignment in New York City, New York

The proposed Second Avenue Subway Project in New York City is approximately Smiles tong, up to a depth of 120 feet beneath grade and runs the length of the eastern side of the island of Manhattan from Harlem in the north to the financial district in the south. A comprehensive subsurface investigation has been carried out that included archive searches, rock exposure mapping, test borings, in situ soil and rock testing, instrumentation, monitoring and laboratory testing over the full length of the alignment. The exploration encountered a 1.3 Ba. geological history from Precambrian to Devonian tectonites from the creation of the continental USA, systematic brittle and ductile faulting and shearing from repeated orogenies, proto and post glacial erosion and culminating in Pleistocene and Holocene weathering and deposition. The geological structures created by these events present significant challenges for design and construction of this mega-project and their investigation is a critical part of the design process. This paper presents the geological techniques that were used to investigate soil-rock-groundwater systems in order to advance the understanding of the fundamental structural geology of central Manhattan.