| Environmental Review Impact on Portal Communities and Our Environment While the realization of the transportation link would bring substantial long-term benefits to the public and the environment, construction impacts, mostly temporary, would be limited. The challenge is to identify and mitigate these impacts to the maximum degree possible. Impact assessments would be the subjects of further studies as the project progresses. An important objective throughout the course of design and construction would be to identify, refine and implement smart solutions that mitigate and minimize identified impacts. Integral with planning and design would be compliance with regulatory standards, environmental programs and community outreach. The geology of the Long Island Sound area consists of bedrock and overburden soils that have been laid down in association with coastal plain, glacial and postglacial deposition. The regional geology has been the subject of numerous studies over the years, yielding a good working understanding of the nature of the subsurface conditions likely to be encountered during construction. The crystalline bedrock that exists on the Westchester side of the Sound is readily apparent in various road cuts associated with the I-287/I-95 interchange and rock outcrops in adjoining residential communities. The bedrock slopes downward to the east beneath the coastal plain of Long Island. The southward movement and subsequent northern regression of glaciers resulted in numerous glacial deposits in the New York City region, including moraines (e.g., Staten Island), till and stratified materials. The glacial deposits were subsequently covered by a variety of more recent sediments, including river alluvium, freshwater peat, salt-marsh deposits and estuarine deposits. As a result, the tunneling machines would need to traverse through layered mixtures of sands, gravels, silts and clays for the majority of their alignments. In the vicinity of the North Shore, the tunnels would encounter crystalline bedrock. Based on available subsurface information, a geologic profile illustrating the anticipated ground conditions along the proposed project alignment is shown below. Prior to constructing the tunnels, a detailed subsurface exploration program would be carried out to confirm these preliminary judgments. Long Island Sound Geology along Proposed Tunnel Alignment The tunnel crown could be as much as 100 feet below the seafloor across the entire route. The tunnel excavation would be carried out using custom-built tunnel boring machines similar to those used to mine other subaqueous tunnels around the world. These “pressurized face” tunneling machines are designed the mine through unstable, saturated soils without destabilizing the ground above the tunnel. As the tunnel is excavated, the tunnel opening is supported by pre-cast concrete segments that are bolted together to form a watertight lining immediately behind the tunneling machine. An animation of the lining erection process is shown below. By using specialized tunneling techniques that have been utilized previously around the world, no water or ground materials would leak into the tunnel and the tunneling process would, therefore, not disturb the seafloor environment or ecosystem of the Long Island Sound basin. Reducing auto congestion would have a direct and positive impact on the region’s airshed. Tunnel Operations and Equipment All major mechanical, electrical and plumbing equipment would be contained within the limits of the tunnel right of way or within dedicated spaces in the service tunnel. Bulky equipment such as substations or fan plants would be designed and constructed in a manner that would be least intrusive to the environment. Equipment and technology are available for reducing or removing a percentage of the vehicle emissions from the air extracted from the tunnel. Such equipment has been incorporated into existing tunnel ventilation systems, notably roadway tunnels in Japan and Norway. Recent examples include the Lærdal Tunnel and six-lane, urban road Oslo Tunnel in Norway and the M30 Calle ring road tunnels in Madrid. There are more than 20 road tunnels in Japan that have electrostatic filters. The use of this technology would be considered during the design process. The number of road tunnels equipped with electrostatic precipitators for the removal of particulate emissions is increasing and much valuable operational experience would be gained during the design phase of the project. The discharge points for the air extracted from the road tunnels would not necessarily be at the portals. For example, no emissions are permitted from any newly constructed roadway tunnel portals in Australia. Rather, fans create a negative pressure that diverts exhaust through vertical stacks. Rather than vertical stacks above the tunnels, exhaust tunnels could divert exhausted air to a more environmentally acceptable location in the proximity of the north and south portals. These exhaust tunnels could be designed in concert with air cleaning equipment. Mitigation measures that would prevent flooding of the facility are discussed in Engineering. |
