As part of New York City's redevelopment of the far west side of mid-town Manhattan, the No. 7 Subway Line Extension project will add 1.5 miles of twin bored tunnels to the existing subway line. The proposed alignment will extend west beneath 41st Street from the present Times Square Station turning south under Eleventh Avenue and terminate at 25th Street. Under the current design, the extension will incorporate a new station at 10th Avenue and 41st Street and a two track terminal station at 34th Street and Eleventh Avenue. The new subway tunnels and underground station structures will be primarily constructed within the Manhattan Schists and Pegmatitic rocks, and will be located in close proximity to a number of major existing structures, including railroad and vehicular tunnels. A comprehensive and phased geotechnical investigation program has been conducted along the project alignment to characterize the ground conditions for design and construction purposes. The exploration program included archive searches, rock exposure mapping, geophysical surveys, test borings in soil and rock, in-situ testing, groundwater monitoring, and laboratory testing of soil, rock and groundwater. Oriented coring and televiewer surveys have been undertaken in the borings to determine the orientation of the rock mass discontinuities. Geological mapping and stereophotogrametric surveys on surface rock exposures were conducted to correlate the borehole orientation data. This paper focuses on the philosophy and description of the methods of rock mass characterization undertaken to present a comprehensive textural, mineralogical and structural discontinuity reference and geologic model for design.
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