Geology

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 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.

Portal Construction and Highway Connections

The tunnel portals would serve as the only access to this transportation link. In their final completion, the portals would also provide locations for support facilities and infrastructure for the operation and maintenance of the tunnel facility.

South Portal

The south portal would be located just to the north of the intersection of Route 135 (Seaford-Oyster Bay Expressway) and Route 25 (Jericho Turnpike) in Syosset, Nassau County, NY. Consistent with the original design of this interchange, the tunnel would extend Route 135 northward from the Route 135 stub that currently exists to the north of the interchange. This interchange’s close proximity to the existing intersection of Route 135 with I-495 (Long Island Expressway) to the south will ease east-west traffic flow along I-495 to the tunnel. Improvements would be undertaken to accommodate tunnel traffic flow as well as traffic to and from I-495 via Route 135. Any impacts of increased traffic at the Route 135/Route 25 junction would be mitigated by reconfigured design of the adjoining roadway network and improved traffic flow.

North Portal

The north portal and facilities would be located in the vicinity of the I-95 and I-287 interchange in Rye, Westchester County, NY. The engineering of the portal location and various connections to the interstate highways is more challenging than for the south portal due to the uneven topography and current interchange configuration. There are several alternative configurations to successfully achieving the required connections, each having advantages and disadvantages with respect to the level of construction complexity, temporary impacts to existing interstate and local traffic and property taking and land acquisition.

One alternative would be to maximize the use of available space within the existing interchange footprint when constructing the new approaches to and from the tunnel. Other alternatives would involve the creation of new tunnel approaches to the north of the existing interchange, with new ramps and a loop overpass built over I-95. The evaluation of these and other configurations would be the subject of extensive design engineering studies.

Tunnel Excavation and Lining

As is the case for all tunnel projects, the tunnel construction methods to be utilized will be tailored to the geologic conditions likely to be encountered along the tunnel alignment. Types of ground materials through which the tunnels would be driven will include glacial deposits (sands, gravels, cobbles and possibly boulders), lacustrine deposits (silts and clays) and bedrock (gneisses and schist).

“Pressurized face” tunnel boring machines will be designed to address the mixture of geological materials anticipated. As the tunnel is excavated, the tunnel opening would be supported by precast concrete segments that are bolted together to form a watertight ring immediately behind the tunneling machine. Each ring would engage and extend the tunnel lining incrementally as tunnel excavation proceeded. There is worldwide precedent for these types of tunnel boring machines. An animation of the lining erection process is shown below.

Aesthetics

Driver comfort in long tunnels can be enhanced in many ways. In order to maximize driver experience in long tunnels, the following aspects can be addressed to enhance driver safety and comfort:

• Interior tunnel geometry (tunnel width and headroom)
• Architectural finishes (reflective, multi-toned, varying texture)
• Tunnel lighting (variable to maintain interest and focus)

The use of a driving simulator would help evaluate the benefits of alternative tunnel configurations, finishes and lighting schemes.

Flood Control Measures

Potential flooding scenarios for subaqueous tunnels can be mitigated by implementing the following measures:

1. Main and backup in-tunnel drainage and pumping systems to collect and convey water out of the tunnel through either portal.  
    
2. Flood gates at the tunnel portals that would be closed upon notification of an on-land flooding event that could impact the tunnels.

Ventilation

A ventilation system would be provided that would deliver sufficient air exchange during periods of heavy traffic as well as effective smoke control in the event of an incident in the tunnels. While a number of different systems could provide the necessary protection, one common element would be the pressurization of the central service tunnel so that it would always provide a source of fresh air and a protected pathway for emergency access and egress.

Security

Because the Cross Sound Link Tunnel would become a critical transportation asset in the region, security of the tunnel would likely be considered a matter of national security policy. The approach to security of a tunnel under the Long Island Sound would start early in the planning phase and would be borne out through detailed vulnerability assessments and risk management evaluations. Evaluation of a security plan for the facility would include:

1. Identify the threats.
2. Assess a range of countermeasures, including permanent physical measures and response systems.

There is a range of countermeasures that could be deployed to enhance the security of such a critical transportation asset.