Untreated Submerged Timber Pile Foundations: Part 2 – Estimating Remaining Service Life

As discussed previously in Untreated Submerged Timber Pile Foundations — Part 1 (STRUCTURE magazine, December 2013), deterioration of pile tops exposed above groundwater levels is a well-known problem. It is less known that submerged portions of timber piles can also deteriorate with time, albeit at a slower rate, due to bacterial attack. This may become critical when considering underpinning methods aimed at extending service life of structures supported on timber piles. Historically, timber-pile supported structures have been underpinned by the cut-and-post method, where the top portion of the timber piles is cut and replaced with concrete posts or concrete-encased steel posts. Although the cut-and-post method appears to be relatively straight forward and simple to execute, it remains an expensive undertaking due to accessibility issues, required temporary shoring and bracing, dewatering, and labor costs. Klaassen (2008-1) reports that, in the Netherlands, foundation replacement or repair sometimes involves up to 50% of the total renovation costs for a structure. The authors’ experience in the Boston, Massachusetts area indicates cut-and-post underpinning of a typical downtown row house costs approximately $200,000 to $250,000. Bacterial attack in the remaining, submerged portion of the timber piles, however, may limit the effectiveness of the cut-and-post method, as well as the estimated remaining service life of the piles.

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