On January 29, 2008, three speakers from Parsons Brinckerhoff discussed some of the significant geotechnical and design challenges associated with the construction of large rock caverns for four major rail and subway infrastructure projects in Manhattan. The technical lecture was held at the Polytechnic Institute of NYU and focused on the underground caverns for the East Side Access, No. 7 Subway Extension, Trans-Hudson Express Tunnel (Access to the Region's Core), and Second Avenue Subway projects.
Mala Ciancia first provided an overview of geologic history in the New York City area and discussed how Manhattan has a tremendous variety of rock types for such a small area—including Mica Schist, Granite, Pegmatite, Gneiss, Marble, Serpentinite, Mylonite, Amphibolite, and Talc Schist—primarily due to its unique location at the junction of three physiographic provinces: the Atlantic Coastal Plain, New England Upland, and Piedmont Province. She emphasized how understanding the site geology is critical to the planning of any underground project and that the investigation for caverns is an iterative process that includes a review of existing data, a field testing program, and a laboratory testing program. She also spoke about faults, foliation, fracturing, and compared the key geotechnical parameters between tunnels oriented in a north-south direction versus those oriented in an east-west direction.
Some of the special challenges associated with conducting subsurface investigations in an active urban area such as Manhattan are the dense network of underground utilities, special requirements for drilling near active subway lines, and existing vehicular and pedestrian traffic at the street level. These factors make it a challenge to find a clear space to drill a boring. Ms. Ciancia showed examples of down-hole acoustic televiewer (ATV) logs and vibrating wire piezometer records and mentioned how the definition of rock mass properties is most important for underground caverns, whereas intact rock properties are of greatest interest for tunnel boring machine (TBM)-drilled tunnels.
Timothy Smirnoff next spoke about cavern support and construction, pointing out how ground is not a homogeneous material and its properties can vary along the entire length of a cavern. Ground stabilization is important to ensure durability and provide a long-lasting structure. Some of the unique technical issues involved with constructing underground caverns in New York City include a small contractor pool, limited experience with very large caverns (the exception being the valve chamber below Van Cortlandt Park), limited experience with the Sequential Excavation Method (SEM)/New Austrian Tunneling Method (NATM), noise and vibration restrictions, and impacts to surface and near-surface infrastructure. Another complexity in constructing large caverns in Manhattan is that they must be located along public rights of way (e.g., wide thoroughfares such as 34th Street).
The technical lecture concluded with a discussion by Sanja Zlatnic of the structural analysis and design of large span caverns. She described some of the ways in which caverns are designed to reduce maintenance and operation costs over their service life, providing fire protection, waterproofing, and corrosion control. Two dimensional and three dimensional finite element modeling are used in the design of the caverns, accounting for normal loading conditions (such as the live load, water load, rock load, and temperature load) and seismic and other extreme conditions (such as fire loads, blast loads, and loads from future overbuilds). Ms. Zlatnic also discussed her involvement with the design and construction of another rock cavern across the Hudson River, the Bergenline Avenue Station in the Hudson-Bergen Light Rail system, which first opened in February 2006.