The Portsmouth Force Main, one of the final major elements in the City of Portland’s CSO program, conveys flows from the Swan Island Pump Station to the existing Portsmouth tunnel. The project comprises 16,000 feet (4,877 m) of wastewater conveyance pipeline, an installation by open cut inside a large-diameter microtunnel, and a soft ground digger shield tunnel.
Delve Underground performed preliminary and final design as a subconsultant. As part of the preliminary design work, an extensive alignment analysis process considering different construction methods was used to arrive at the preferred alternative. The final design included tunnel and trenchless design, shaft design, cost estimating, geotechnical investigation program management, and geotechnical baseline report preparation.
A public outreach component was vital to this analysis process. It was used to gather input from key stakeholders in the project area and identify traffic concerns for some of the area’s more significant businesses. Because of the extensive input provided by these crucial stakeholders, an additional alignment alternative with trenchless and tunneling segments was identified during this outreach process.
This alternative was reviewed and later adopted as part of the preferred alignment, which resulted in lower project costs while minimizing impacts on businesses and traffic in the most congested portions of the project area.