The City of Portland Bureau of Environmental Services has completed the design of the final segment of their West Side Combined Sewer Overflow (CSO) system, part of the City's overall CSO program. This project includes 6,900 feet (2,103 m) of 84-inch (2,134 mm) microtunneling in five drives and 950 feet (290 m) of 54-inch (1,372 mm) microtunneling in a single drive. The City chose to use an alternative contracting mechanism for project delivery that included choosing a contractor, based on qualifications, at the 60% design level. From the 60% to 100% design completion level, the contractor participated in final design decisions, made suggestions for modifications to existing design features and provided input to refine cost parameters. This mechanism was previously coined "The Portland Method" on the West and East Side CSO projects. There were several challenging aspects to the project, including soils that varied widely from extremely soft silts that would not provide adequate bearing capacity for the microtunneling machine without ground improvement to extremely abrasive open-graded gravels. An extensive multiple-phase geotechnical investigation was conducted to carefully evaluate the soil conditions along the alignment. The Design Team worked together to analyze the soils, and from this information the City worked with the contractor to procure a new microtunneling machine for the project. Shaft depths ranged from 35 to 75 feet deep (11 to 23 m) in soil zones with high groundwater where dewatering had to be minimized to prevent the movement of contamination plumes. All microtunnel drives were a minimum of 1,100 feet (335 m), with the longest drive on the project of 1,690 feet (515 m) traversing beneath heavily traveled U.S. Highway 30.
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