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Full Scale Cyclic Large Deflection Testing of Foundation Support Systems for Highway Bridges. Part I: Drilled Shaft Foundations

  • Author(s): Stewart, Jonathan P
  • Taciroglu, Ertugrul
  • Wallace, John W
  • Ahlberg, Eric R.
  • Lemnitzer, Anne
  • Rha, Changsoon
  • Tehrani, Payman
  • Keowen, Steve
  • Nigbor, Robert L
  • Salamanca, Alberto
  • et al.
Abstract

This research involved analysis and field testing of several foundation support components for highway bridges. Two classes of components were tested - cast-in-drilled-hole (CIDH) reinforced concrete piles (drilled shafts) and an abutment backwall. The emphasis of this document (Part I of the full report) is CIDH shafts.

CIDH shafts are among the most common support structures in highway construction. Typically, drilled shafts have simple, prismatic geometries; yet, they display a complex, inelastic response under applied loading. The two major factors that affect their behavior are the interaction between the shaft and surrounding soil media, and the material inelasticity of the shaft itself. In this report we document the results of two single shaft tests and one shaft group test. All specimens are two-feet diameter reinforced concrete drilled shafts that extend approximately 24ft below ground line. The single shaft specimens include one in a flagpole configuration extending 13.3ft above ground line and the other capped at the surface in a fixed-head configuration. The group test specimen had 9 individual shafts in a 3 by 3 configuration anchored at the ground surface (with a moment connection) in a reinforced concrete cap. The test site consists primarily of low plasticity alluvial clay that is expected to exhibit an undrained response to the cyclic lateral loading. The quasi static loading was applied with a hydraulic control system in displacement-control mode, with the full suite of loading taking several days to complete for each test. The test data have been reduced to provide complete load-deflection backbone curves for loading in both directions, curvature profiles at pre-yield deflection levels, hysteresis curves documenting the cyclic behavior of the shaft soil system at pre-yield displacements, p-y curves for the single shaft specimens, and group interaction factors for the group specimen.

Pre-test response predictions of the CIDH specimens were obtained via (1) a three dimensional finite element model, (2) a macro-element model, developed at UCLA, and (3) the so-called strain wedge model adopted from the literature. Simulation results were compared with each other and with field measurements. It was observed that all of the three numerical approaches yielded reasonably accurate predictions for these small diameter shafts. We provide p-y curves in the API format calibrated to the test data and show that those curves improve the accuracy of predictions relative to generic p-y curves in commonly used design guidelines published by the American Petroleum Institute (API).

The p-y curves obtained from the experiments are shown to differ from what would be predicted using standard API models, with the data indicating a stronger and stiffer response at shallow depths where the shaft-soil interaction is most pronounced. We also compare results of various tests to evaluate head fixity effects on p-y curves and the adequacy of the diameter effect built into API p-y guidelines.

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