The City of Portland, Oregon, has completed construction of a six mile long combined sewer overflow (CSO) tunnel on the east side of the Willamette River through downtown Portland. The tunnel is 24 feet in diameter at a depth of 80 to 100 feet below ground level, and was constructed using a tunnel boring machine (TBM). The preliminary tunnel alignment passed beneath a major elevated freeway interchange where it will negotiate a series of steel H-pile foundations that extend below the tunnel horizon. Given the soft soils at the site, it was anticipated that these H-piles may have deviated from vertical at the depth of the tunnel. If the TBM were to encounter a deviant H-pile considerable costs would be incurred for the tunneling operation, and the operation and integrity of the freeway interchange may be compromised.


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