Borehole GPR to Detect and Map Deviated H‐Pile Foundations

Abstract

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. Several geophysical methods were considered to detect potential deviated H‐piles along the tunnel alignment. A borehole ground penetrating radar (GPR) reflection survey was selected as the most appropriate, coupled with a borehole magnetic gradiometer survey to confirm the GPR interpretations. Initially, the GPR and magnetic gradiometer surveys were run in 16 borings to investigate H‐piles from 3 bents. One deviated H‐pile was mapped which conflicted with the proposed tunnel alignment. The alignment was subsequently moved approximately 100 feet to the west, to pass between a different set of bridge piers. Nineteen (19) additional borings were drilled and surveyed with GPR, finding no H‐piles which might intersect the new alignment.