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Meeting a deadline for a wastewater tunnel

Mott MacDonald designed the new force main in 11 months, successfully meeting a year-end deadline.


After 30 years of service, the Sayreville Relief Force Main in New Jersey, more than eight feet (2.4 meters) in diameter, ruptured and released millions of gallons of untreated wastewater into the Raritan River. The Arsenal Force Main had been in continuous service even longer.


Recognizing the need for improved and backup facilities, Mott MacDonald was retained by the Middlesex County Utilities Authority to evaluate a proposed new 4,000-foot (1.2-kilometer) force main under the Raritan River. We considered ways to connect the new force main to the pump station and treatment plant without disrupting service, and ways to inspect or rehabilitate the Arsenal Force Main after the new force main became operational.

We recommended the abandonment of the Arsenal Force Main and the construction of a new multipurpose tunnel that could accommodate two new force mains five feet (1.5 meters) in diameter.


The tunnel headspace above the grouted force mains functions as a utility corridor or utilidor, providing a passage for electric power, communications, and gas.

The final design was completed in 11 months, meeting a year-end deadline set by an Administrative Consent Order. Earth pressure balance (EPB) tunneling — a New Jersey first — was used to drive the tunnel through soil materials of little to no shear strength.

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