Chapter 3: Description of Construction Methods and Activities A. INTRODUCTION
Chapter 3: Description of Construction Methods and Activities A. INTRODUCTION This chapter describes the construction process for the Second Avenue Subway. Potential envi- ronmental impacts that could result from its construction, as well as mitigation measures to lessen their effects, are discussed in subsequent technical chapters. A preliminary sequencing plan for the proposed construction activities is also identified, although this plan could still change as engineering evolves and depending on the availability of funding. At this time, design of the Second Avenue Subway is still ongoing, and will continue to evolve over the next year. Consequently, this Final Environmental Impact Statement (FEIS) assesses the range of construction methods and activities that may be required, using a reasonable worst case approach throughout to describe potential impacts. In other words, where a variety of construction techniques could reasonably be used to build a particular project element, the method that would result in the worst overall impacts is the one that has been selected for analysis. The Second Avenue Subway would consist largely of twin tunnels with outside diameters of up to 23½ feet. (The tunnels described in the SDEIS would have had outside diameters of 21 feet.) Each tunnel would be approximately 8.5 miles long, running from East Harlem to Lower Manhattan. Sixteen new stations, numerous fan plants and ventilation cooling facilities, pumping stations, electrical power substations, new train storage yards, and various other elements would also be built. As described later in this chapter, where possible, construction would take place underground to minimize disruptions at the surface. Between approximately 92nd and 4th Streets (instead of 6th Street as described in the SDEIS), and again from Maiden Lane south, where Manhattan’s hard bedrock is relatively close to the surface, tunnels and stations would mostly be constructed underground in the rock, by one of several mining techniques.
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