Civil Engineering Association

Analysis and Modeling of SSI in Bridge Support Structures (PhD Thesis)

ABSTRACT

The p-y method is an established analysis tool for lateral response of piles. Existing soil lateral load-displacement (or p-y) backbone curves have been calibrated with tests on small-diameter, linearly elastic piles. The first portion of this study is devoted to obtaining several new p-y curves by using data from three solitary reinforced concrete shafts embedded in stiff clay and tested to failure under lateral loading. These new curves are shown to differ from the standard curves, primarily in capacity: For a 6ft-diameter free-head specimen, the new curve reaches a load capacity that is 60% higher than the standard curve; for a 2ft-diameter free-head shaft, the new curve is 20% weaker; and for a 2ft-diameter fixed-head shaft, the new curve is 100% stronger than the existing standard.

In the second part of this study, the calibrated p-y model of the 2ft-diameter fixed-head specimen, and a validated finite element model of a group of nine piles are utilized to determine "group efficiency factors." These scaling factors are found to depend on the magnitude of lateral pile-cap displacement. The efficiencies are less than unity when the passively resisting soil wedges in front of the piles interfere with each other—for the same lateral displacement, a pile in the group generates a resisting force that is less than that of a solitary pile. Usually dubbed as the "shadowing effect," this behavior is observed up to intermediate levels of lateral pile-cap displacement. The group efficiencies tend to unity as the displacement increases.

The third part explores abutment-backfill interaction in bridges. This effect can significantly influence the seismic response of a bridge. Both log-spiral hyperbolic (LSH) and finite element models are validated using data from several abutment tests. Extensive parametric studies are carried out using the LSH model, which is more amenable for this task than the finite element model because of its computational efficiency. Results are used to devise hyperbolic equations to represent the lateral load-displacement backbone curves of abutments as an explicit function of wall height and the backfill soil's physical parameters. This physically parameterized hyperbolic formula is amenable for routine seismic response simulations of bridges.

Advisor: Professor Ertugrul Taciroglu

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