On behalf of the New York City Department of Environmental Protection (NYCDEP) and the Port Authority of New York and New Jersey (PANYNJ), the New York City Economic Development Corporation (NYCEDC) recently embarked on a project to replace two existing water siphons crossing the New York Harbor between Brooklyn and Staten Island. The existing pipelines require replacement to accommodate a dredging program within the harbor. As part of this project, an assessment of alternative methods for construction of the new siphon was performed. Several alternatives were considered, broadly classified into dredged trench and tunnel methods. The recommended alternative for the new siphon was a new 9,500-linear-foot, 72-inch-diameter pipeline installed within a 12-foot diameter tunnel. A pressurized face tunnel boring machine will be used to bore through the predominantly soft ground and to construct a tunnel lined with gasketed, precast concrete segments. Following completion of the tunnel drive, a welded steel pipeline will be installed. While NYCDEP has extensive experience and comprehensive design standards for both rock tunnels and steel water pipelines, the installation of a steel pipeline in a soft-ground tunnel is an uncommon project in New York City. While it is well known in the tunnelling business that a new rail tunnel under the Hudson River is now being planned and designed, the water siphon tunnel is scheduled to be the first tunnel built under a river or harbor in many decades. This paper discusses the various challenges faced by the design team in the first project of its scope and magnitude to be constructed in New York.
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