Strength of Untreated Douglas-Fir Foundation Piles After Some 70 Years’ Service

Twenty-three untreated Douglas-fir piles contained for approximately 70 years in fill and bay mud below the permanent water table near the San Francisco waterfront were investigated to determine their residual strength. Freshly cut cross sections showed an outer, “punky” zone of about 1-inch width that possibly coincided with the sapwood, an adjacent zone of about 1-inch width stained a greenish color, and a core which appeared to be normal. Small, clear specimens taken serially from bark to pith were tested in compression parallel to grain, static bending, and toughness. The results showed losses in strength and stiffness in the punky zone ranging from 14 to 50 percent depending on kind of property and height of pile; stained wood and wood in the core appeared to be normal. Deterioration in the outer, punky zone extended from butt to tip of the piles but was somewhat more severe at the butt. Full-diameter compression tests of pile sections showed them to be essentially equivalent to new piles in load-carrying capacity. However, the presence of compression failures indicated that some of the piles may have been damaged in driving. Knots and other defects appeared to have had only a minor effect on pile compressive strength.

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