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Portland Back Cove CSO

Client
City of Portland
Location
Portland, Maine
Services Provided
Construction of a 4-Tank, 3.5-Million Gallon Underground CSO Facility
Market
Alternate Procurement

Project Overview

Sargent’s design-build team created the plans for this four-tank underground CSO storage facility located in Back Cove Park, which is capable of storing 3.5 million gallons. We constructed the tanks to include more than 17,000 cubic yards of cast-in-place concrete, along with eight stainless steel tipping buckets and four chopper pumps.

The tanks connect to the city’s gravity sewer system and the East End Wastewater Treatment Facility through a force main. The project collects CSO from three city outfalls and directs it to the tanks through new 36″-72″ pipelines. We also installed a new 8’x6′ concrete box culvert at Exit 7 on I-295. Our teams used a multi-level internally braced shoring system and a sheet pile to support the excavation for the tanks. There was also a significant dewatering need while excavating, due to the fact that the tanks and pipe reach depths exceeding 25 feet below sea level.

We encountered several challenges on this project, including dealing with poor underlying soils, including clay and bay mud. There were environmental issues as well, due to contaminated soils, including urban fill, sand, and bay mud. We mitigated this concern by obtaining a beneficial-use solid waste license from MDEP. Finally, pedestrian safety was a concern as well, as the project encompassed a large section of the popular Back Cove Trail.

Scope of Work