Subterranean Artery-Hardening Saves Time and Money
An innovative engineering method that rehabilitates deteriorating subterranean walls without the need for expensive and disruptive excavation at street level recently made its New York debut in Lower Manhattan. The venue for this tryout was the South Street Interceptor, a half-century old, 1,000-foot-long tunnel (eight feet wide by six feet tall) that stretches between Fulton and Dover Streets, 30 feet beneath the pilings that support the FDR Drive viaduct.
Interceptor lines are major sewer mains that accept water from a network of converging, smaller pipes and channel it toward a treatment facility. According to a spokesman for the City’s Department of Design and Construction (DDC), the problem with the South Street Interceptor, which handles 40 million gallons of water each day, is that “its concrete walls are wearing down, making it vulnerable to infiltration from the East River and therefore more likely to overflow.”
The traditional method for addressing this issue would have been to dig an open trench, three stories deep, and almost four times as long as the Brooklyn Bridge is tall, which would have meant closing South Street to traffic, possibly for several years. Such a project would also have encroached on dozens (perhaps hundreds) of other underground utility vaults, and possibly posed a danger to the FDR Drive.
So DDC decided to give a new technology, called “spiral wound lining,” its first New York tryout. This approach relies on a miles-long spool of polyvinyl chloride (PVC), a thermoplastic polymer that combines flexibility with strength and resistance to water. The spool is brought to an existing access point (such as a manhole) and slowly unwound.
Before that process could start however, DDC sent scuba divers into the South Street Interceptor, armed with a submersible pump, to empty the tunnel of most of its water. After the space was cleared, works crews lowered into the shaft the spiral lining installer, a piece of machinery that expands or contracts to the size of the tunnel. As the spool of PVC (positioned above, at street level) unrolled the liner down into the tunnel, the installer below molded it into a precision-customized, interlocking shape, forming a watertight seal, and affixing it to the tunnel walls.
“Workers feed the winding machine with PVC material and guide it as it rotates throughout the interceptor sewer,” the DDC spokesman explained. “The machine constructs the liner through a continuous winding process, as the profile edges form successive wraps of PVC. Space between the PVC liner and concrete wall of the sewer are then sealed with grout to help improve the structural integrity of the sewer.” With the new lining (which was installed at the rate of a few linear feet for each eight-hour shift), the tunnel is now four inches narrower on each side, but much stronger.
As the South Street Interceptor rehabilitation (which has been in progress since last autumn) nears completion, DDC estimates that it saved roughly $12 million by not having to excavate an open trench along South Street. The $29-million budget also translated into finishing the project in a fraction of the time that would have otherwise been required.
“Once the South Street project is complete,” the DDC spokesman says, “1,000 feet of deteriorated sewer will be reinforced with four inches of PVC material. With the interceptor now impermeable to water, it will be less prone to potential overflow, which reduces the risk of flooding for the surrounding community. The finished project will also extend the overall lifespan of the interceptor sewer and prevent further damage from root intrusion.”