The assessment of the pipelines in Salem Harbor found areas of advanced corrosion with an imminent need of replacement to avoid potentially devastating environmental consequences and service disruption. The project marked the first major subsea sewer in New England installed by the “float and sink” method.

 


Location

  • Marblehead, Massachusetts, USA

Sector

Service

  • Water and Wastewater Treatment and Conveyance
  • Water Risk Management
  • Water Supply and Reuse
  • (View all)

Client

  • Salem and Marblehead, MA

Project Status

  • Completed in May 2016


Protecting an Active Harbor

Following the rupture and emergency repair of one of the cross-harbor pressure sewers running under Salem Harbor, the South Essex Sewerage District (District) retained WSP USA to conduct a condition assessment study. The study included the condition assessment of 12,000-feet of 20-inch and 24-inch pressure sewers that convey up to 20 CFS of wastewater beneath Salem Harbor from the town of Marblehead to the District’s wastewater treatment facility in Salem, Massachussetts. Based on direct underwater observations of the existing pipelines, ultrasonic thickness testing and a soils corrosivity analysis, our team documented that the 37-year-old ductile iron pipes were severely corroded and in imminent danger of failure.

 

img-Marble-Sewer-Replacement

Implementing Innovative Solutions

Following the assessment, WSP evaluated various alternatives for immediate replacement of the pipelines and completed the design and permitting of two new high-density polyethylene (HDPE) replacement pipelines. Using the float-and-sink approach, the replacement pipelines were installed in a shallow trench dug 6,000 feet across the harbor, offset from the existing active pressure mains to protect these deteriorated pipes. Because the existing mains had no redundancy, a 6,000-foot temporary bypass was installed and connected to the upstream sewer and treatment plant before the permanent connection of the new mains to the system was made.

Design innovations included careful selection of materials to provide long service life in a corrosive marine environment, as well as provisions to monitor and protect the environment and public safety throughout construction in this historic and popular recreational harbor. Construction was completed with no adverse environmental impact, as demonstrated by monitoring of sensitive resources, and included restoration at the Marblehead and Salem shorelines to protect a recreational beach and reconstruct a seawall. The new pipes provide improve capacity and redundancy, and are expected to provide service for well over 50 years.