Evolution of the Dragload Behavior during Construction of Four H-Piles with Different Top Load and End Fixity Conditions at a Bridge Abutment with Embankment Fill

In 2017, the 57-year-old Eisenhower Bridge over the Mississippi River in Red Wing, Minnesota, was replaced by a new bridge with a longer span and taller, wider approach embankments. The north abutment embankment grade was raised over 20 ft; the new north abutment is supported by H-piles in an area of thick, soft alluvial deposits. The added embankment loading was expected to compress layers of underlying soft soils and result in undesirable downdrag conditions. The pile structural integrity was evaluated, and a nominal dragload of 350 t was estimated during the design phase, which was within the established structural limit. To evaluate actual downdrag response and compare measured values with predictions, four HP 14×117 steel piles were instrumented with a series of strain gauges. Two piles were located within the bridge footprint, and two were located outside the footing within the approach embankment fill. For performance comparison, two instrumented piles were driven to the top of bedrock with the aid of PDA, and two instrumented piles were driven to an elevation 5 ft above bedrock. A group of earth pressure cells was placed around the piles to measure overburden stress relative to pile strain. As of 2020, the bridge is complete, and the embankment fill had been in place for more than 2 years. An assessment of the pile strain profile indicates that negative skin friction progressively increased, then gradually stabilized during various stages of the construction. Maximum compressive force in the piles was around 300 t, and embankment fill overburden stress was 2 tsf. The neutral plane location became increasingly distinct at a position about 30%–50% of the pile length from the toe.