Long-term monitoring of a pile abutment for force and settlement

The soil profile at the 1960 Eisenhower Bridge consisted of 10 m of loose sand and 15 m of soft, compressible clay overlying dense sand and gravel on sandstone. The bridge suffered drag force that exceeded the structural strength of the foundation wood piles, which effectively removed pile bearing and caused the piles to be dragged down by long-term soil settlement. Eventually, the bridge was replaced. Wick drains were installed for the replacement bridge embankments to accelerate settlements and pore pressure dissipation. Settlement, pore pressures and pile axial forces were monitored during and after construction. The piles terminating in the sand were found to carry a smaller load than piles terminating in the sandstone. An equilibrium plane developed that was deeper for the piles terminating on the sandstone. The consolidation of the clay layer took almost a full year. The new approach embankment settled about 400 mm, while the new H-piles settled only due to elastic compression and a small toe penetration. The development of drag force, downdrag and pile settlement was analysed according to the method expressed in the recently proposed revision of the American Association of State Highway and Transportation Officials (AASHTO) specifications.