Sewer force main rehab

NASSCO Report – Rehabilitation of Sewer Force Mains: From Planning to Execution

Sewer force mains, like most underground pipelines, will require rehabilitation or replacement once their useful lifespan is reached. Selecting the best-fit method can be complex, with the need to consider not only the capabilities of the options to meet the operating conditions, but also cost, the impacts of construction on the environment, and residents impacted by the construction. The Triple Bottom Line (TBL) approach is an evaluation process that considers these three factors, and municipalities have been using this approach for some time in differing levels of sophistication.

Once the option has been vetted and selected, the focus turns to technical and execution evaluation for the design and installation aspects.

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A recently completed renovation of 4,920 ft of a 24-in. DR41 PVC force main provides insights into both the evaluation and selection of the preferred solution, as well as a unique opportunity during the tender phase to assess several liner configuration options to deliver the most cost-effective solution. The Shoreline Trail Sanitary Sewer Upgrades project in Port Moody, British Columbia, included the 24-in. force main, as well as a parallel 16-in. asbestos cement gravity sewer. The project was impacted by environmental considerations, social considerations and construction aspects that made it more complicated than most force main renovation projects.

This project used a form of the TBL concept to select the most appropriate trenchless technology for the unique project.

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The City engaged Kerr Wood Leidal associates Ltd. as its consultant for the planning, design and execution of the project. The City’s Design Intent defined their expectations to be considered in this environmentally sensitive, highly used park area. Key constraints:

Environmental

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  • Environmentally sensitive riparian areas, including wetlands
  • Crossing eight watercourses, two of which are fish bearing; limit impacts from construction during fisheries window from Aug. 2 through Sept. 15
  • Forested areas – trees are viewed as high value, limit impacts from construction footprint and activities
  • Wildlife traversing the park area
  • Great Blue Heron Colony – rookery with 13 nests required controlled construction during nesting period from Jan. 1 through September

Social

  • Archeological/historical area
  • 230,000 visitors/users per year; maintain best possible access during construction

Technical

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  • System hydraulics – contributions from three local pumpstations with large flow variability along alignment, with potential to impact inflow at connection points
  • Local terrain – Steep slopes adjacent to alignment and hills on alignment
  • Canadian Pacific Railway Right of way adjacent to the park and pipeline alignments limits flexibility of alignment and construction access

A variety of trenchless technologies were considered for the project, including CIPP, open-cut, sliplining, and pipe bursting. CIPP was ultimately selected for the project.
The project team developed a range of options to refine their assessment. Construction costs, as well as the impact on trees were estimated and quantified and used in the analysis of options.

  • Option 1 – 100 percent trenchless – $11.3 million, tree impact 6
  • Option 2 – 100 percent Open-cut – $17.2 million, tree impact 176 – 200
  • Options 3 – Realignment onto nearby roadway – $18.4 million, tree impact “High”
  • Option 4 – 3 combinations of trenchless and open-cut ranging from 40/60 to 75/25 – respective costs from $15.7 million to $13.8 million, and respective tree impacts from 75 to 25

Key benefits related to the CIPP option included GHG savings of 410 metric tonnes of CO2, or 140,000 L of diesel fuel saved, hundreds of trees saved, no disruption to existing waterways, and the park and trail could remain partially open during the construction. Additionally, the locations of access pits required for CIPP lining could be adjusted to minimize environmental impacts on water courses and large trees.
The project tendering process included contractor prequalification and other activities such as procurement of long-lead items and permits. They ultimately selected general contractor PW Trenchless Contracting, with Insituform Technologies Ltd. as the lining contractor, proposing a UV-cured lining option as an alternative to a felt/fiber reinforced CIPP system.

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The construction team evaluated three liner material configurations in preparing the successful bid. The UV cure solution was selected based on a range of considerations including liner thickness, project installation time optimization, reducing impacts on the local environment and costs.

The City’s approach to selecting the most effective solution to the project led to a result that allowed the project to be delivered within budget, and on schedule.

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George Bontus is director of engineering at Insituform Technologies Ltd. and a member of the NASSCO Pressure Pipe Committee.

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