Influence of geologic conditions on excavation methodology

摘要

The existing MTA-New York City Transit's (MTA-NYCT) No. 7 Subway was built in the early 1900s and currently terminates at Times Square near 41st Street and Seventh Avenue. As part of the City's redevelopment of the west side of Manhattan, the MTA-NYCT's No. 7 Subway extension project will add approximately 1.5-miles of twin tunnel alignment to the current line. The proposed alignment will extend from the existing Times Square Station west beneath 41st Street then turn south under 11th Avenue and terminate between 24th and 25th Streets. The project will include two new stations, a two-track line station near 41st Street and 10th Avenue and a three-track terminal station at 34th Street and 11th Avenue. The alignment lies in close proximity to several major underground structures. The tunnels and station caverns will be constructed in the Manhattan Schist, which has been extensively intruded by pegmatitic and aplitic veins along and across the foliations. The lithology of the bedrock as well as the orientation and condition of the rock mass discontinuities are critical to the design of the tunnel boring machines. This paper will focus on the identification and influence of geology on the selection of excavation methodologies that minimize impact to the adjacent underground structures.