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Prime consultant: Jacobs Associates
Client/owner: J.W. Fowler Co., city of Portland鈥檚 Bureau of Environmental Services.

The city of Portland originally planned to build the $74 million Balch Consolidated Conduit using a traditional design-bid-build model. But a review of the risks of the micro-tunneling project, plus the penalties the city faced if the work wasn’t completed by a federally mandated deadline, led the city to pick an alternative method.

The project consisted of installing about 6,700 feet of reinforced concrete pipeline and digging six 75-foot-deep shafts. The project area, located in Portland’s northwest industrial area, was a mix of conditions, from soft silt and flowing sands to cobbles and boulders. One area of the project site even featured a man-made landfill. Adding to the complexity of the project was the fact that soil and groundwater in the area was contaminated, the result of past industrial activities on some of the land.

The project team, which included general contractor James W. Fowler Co. and its prime consultant Jacobs Associates, decided to use a cutter soil mixing (CSM) method to construct the tunnel shafts. The method creates “soilcrete” panels by mixing water and cement, and is faster and more affordable than traditional construction methods. For the conduit project, rows of the panels were used to support the tunnel machine and pipe through soft alluvial soils between shafts.

The project was the first time CSM was used in Oregon. It also represents the first known use of the method for ground improvement, according to Jacobs Associates.