GEOLOGIC CONSIDERATIONS IN THE PLANNING OF THE SECOND AVENUE SUBWAY ALIGNMENT IN NEW YORK CITY, NEW YORK

The proposed Second Avenue Subway project of New York City Transit (MTA) includes 8.5 miles of tunnels with 16 stations between 128th St. in the north to Hanover Square (Water Street) in the south, mostly along Second Avenue in Manhattan, New York City. The area is primarily comprised of the Manhattan Schist and Inwood Marble (dolomitic marble and calcareous schists) within the Manhattan Formation. The study area demonstrates the significant effects of two major structural features: the Cameron Thrust Fault to the east (outside project boundaries) and the Manhattanville-125th Street Fault, which obliquely cuts across Upper Manhattan (NW-SE), between 125th Street (west side) and 94th to 96th Streets (east side). The secondary fault/shear/fracture zones associated with these major ductile/brittle structural features have been revealed from recent subsurface investigations at many locations along the alignment. Pegmatitic instrusions between 12th and 57th Streets and amphibole schists (amphibolites) throughout the alignment may be more extensive than has previously been reported. The comprehensive subsurface investigation program that has been conducted involved a review of literature, archived boring logs, lineament analysis, rock-face mapping, test borings, borehole geophysics (acoustic televiewer), in-situ soil and rock testing, instrumentation monitoring, laboratory testing, and hydraulic conductivities within the soil and rock mass at selected locations. The assimilation of these investigation techniques has yielded significant insight into establishing several parameters of the rock/soil/water system(s) which will be incorporated as an integral part of the tunnel/station design and construction.