Portland CSO TBMs arrive

Two Herrenknecht TBMs will arrive in the US-city of Portland, Oregon this month, ready to bore the 6km long West Side Willamette River CSO Project (also known as the West Side Big Pipe). The estimated US$260M project includes a new pumping station and new sewers to help control combined sewer overflows into the Willamette River.

The two Mixshield TBMs (5.05m o.d. and 5.14m o.d.) will both be launched from the shaft at northwest Nicolai Street. One will bore south to Clay shaft, while the other one will travel north, crossing under the Willamette River, to the new pump station on Swan Island. They will erect a bolted pre-cast concrete segmental lining behind them.

The tunnel alignment runs up to 40m below ground, before dropping to 46m under the pumping station. The geology is made up of layers: sand/silt alluvium, with lenses of gravel; gravel alluvium, containing cobbles, boulders (up to 1m diameter) and lenses of sand; and the Troutdale Formation, which is medium to course gravel within a sandy silt matrix, containing open-graded zones with very high ground water flows. Cobbles and boulders are also found in the Troutdale Formation.

The Mixshield machines will face pressures of up to 3.5 bar, and potentially ravelling, running and flowing ground. Particular attention will be made under the river, where there is shallow cover between the river-bed and the tunnel crown. Soil reinforcement under some of the bridges, and attention to annulus grouting should help limit settlement.

The client, the City of Oregon's Bureau of Environmental Services (BES) awarded the tunnel construction contract to the Impregilo/SA Healy JV, which saw off competition from the Kiewit/ Kenny JV, and the Traylor/ Obayashi/JF Shea JV. Impregilo/SA Healy was brought on board soon after its appointment at the end of 2001 to assist the design team, a group led by Parsons Brinckerhoff with Carollo Engineers, CH2M Hill, MWH, Tetrotech/KCM, and URS.

All construction work should be complete by 2006, paving the way to begin work on a similar project on the east side of the river.

The West Side Willamette River CSO Project is part of a wider US$1bn programme, which began in 1991, and is scheduled to end in 2011. So far it has already reduced annual overflows by more than 50% from 23Mm³ to 11Mm³.

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Big Pipe