Eisenhower Bridge North Abutment and Approach Settlement: A Case History of Timber Pile Downdrag and Comparative Downdrag Effect on Steel Piles

The north abutment and approach of the Eisenhower Bridge has settled significantly since completion in 1960 and is currently several feet lower as compared to the 1960 construction plan profile. The north approach originally consisted of a 25-foot-high embankment placed on a soil profile of loose/soft alluvial sands, silts, and clays (alluvium) underlain by a very dense layer of sands and gravels; the north abutment was supported by 26 timber piles driven through the embankment fill and alluvium to practical refusal bearing on very dense sands and gravels. Given the timber piles were driven to practical refusal in competent materials, the movement in the abutment appears to be a result of downdrag loads. As the approach embankment adjacent to the abutment settled from consolidation of the soft alluvium, drag load developed on the timber piles and induced excessive internal stress and end bearing pressure on the piles. It is believed the yield strength of the timber piles was exceeded in compression resulting in damaged or broken piles. The purpose of this study is to demonstrate how the drag loads on the timber pile foundation system caused excessive stress and movement of the abutment and provide comparative analyses to model the effect if steel piles had been selected in place of the timber piles.