Gilboa Dam Schoharie Reservoir Low Level Outlet


gilboa sunrise over pipe

Pipe Specifications:

2,140’ of 110.75” OD x 1.375” wall T-7 Permalok®

160’ of 108” OD x .879” wall T-7 Permalok®

160’ of 78” OD x .750” wall T-7 Permalok®




New York City Department of Environmental Protection

Engineer Firm:

Hazen & Sawyer


Southland Renda JV

Formed in the 1920’s with the construction of the Gilboa Dam, the Schoharie Reservoir on Schoharie Creek is one of the major reservoirs that supplies New York City with a large portion of its potable water. The New York City Department of Environmental Protection realized the aging infrastructure was in need of significant repairs and began a complete overhaul in 2011.

As part of this project, the city selected Southland Renda JV to build a new output facility with both a land leg and a water leg of the output. The project team selected Permalok® steel pipe as an optimum material to support the tunnel operations for the new Low Level Outlet and was installed in both legs of the tunnel.

In most trenchless applications, steel pipe is structural pipe element with other utilities being installed within the casing. In this case, the project team determined that the casing pipe would also be the carrier pipe with a necessary pressure rating of 5bar.

Northwest Pipe manufactured multiple sizes of Permalok pipe for this project including some sections of 108” Internal diameter, which is the largest use of Permalok® as a carrier pipe manufactured to date. The pipe utilized a T-7 joint and Polyurethane lining with testing to C-200. Due to the high water pressure expected, the contractor applied a 1/8” seal pass weld to the joints at installation.

The project faced many installation challenges including critical line and grade requirements, multiple mixed geological conditions between the two legs of the tunnel drive, and the depth of the tunnel at 150 feet below the surface of the reservoir. The team used a ‘wet retrieval’ of the microtunneling boring machine (MTBM), meaning they were working under the reservoir to retrieve the output boring machinery.

The successful installation of the pipe ahead of schedule demonstrates the importance of project team collaboration and emphasizes Northwest Pipe’s unique ability both above and below ground.