For bridges and other structures subject to lateral loadings, passive earth pressure contributes significantly to overall stability. While the maximum passive pressure acting on pile caps and abutment walls can readily be predicted, the issue of how passive pressure develops as a function of deflection is more problematic. Furthermore, essentially all of the load-displacement relationships currently used are derived from static or extremely slow loadings. Under seismic loading conditions, both dynamic and cyclic effects are present which alter the load-deflection relationship. While cyclic loading effects typically reduce the strength and stiffness of the soil, dynamic loading effects tend to produce an apparent increase in soil strength and stiffness due to material and radiation damping. Faced with the lack of well-defined load-displacement relationships which address the effects of both stiffness degradation and increased damping, the engineering community has often applied static load-deflection relationships in seismic design situations. The degree to which this approach is conservative or non-conservative is uncertain. What is certain, however, is that soil stiffness at abutment contacts is a critical factor in overall bridge performance.
Our vision for this project is to develop load-displacement relationships for typical soils adjacent to pile caps and abutments based on full-scale testing, which account for both dynamic and cyclic effects. Incorporation of dynamic effects into passive earth pressure relationships would create a validated definition of dynamic and cyclic soil response that has previously been unavailable. Although shake table and centrifuge models can provide important guidance regarding these issues, a reasonable number of full-scale load tests are necessary to provide ground truth information. This research would fulfill this need. In terms of intellectual merit, the proposed testing and analysis will provide practitioners with dynamic stiffness and damping factors based on full-scale pile cap tests for a variety of soil types and densities, as well as for a range of vibration frequencies and displacement levels. In addition, the test results, archived in the NEES database, will become important benchmarks for researchers interested in calibrating/verifying new computer codes or numerical models.
To obtain the data needed to develop dynamic, passive load-displacement relationships, a field testing program will be conducted. This program will consist of laterally loading a concrete pile cap without backfill and then reloading after backfilling with four different soils type. Since the resistance provided by the backfill is a function of density, each backfill will be compacted to two different densities and then tested. Loading will be accomplished by a combined use of a hydraulic load actuator and NEES-UCLA's eccentric mass shakers. Initially, a hydraulic load shaker will load the cap to a target deflection, at which point the actuator length will be fixed. Next, eccentric shakers mounted atop the pile cap will be activated to produce a dynamic loading. The combined use of actuator and shaker will provide levels and rates of strain previously unobtainable in full-scale testing. The frequency of the shakers will be varied to obtain a range of loading frequencies. This loading process will then be repeated at increasing levels of deflection. Subsequent data analysis will quantify the observed passive earth pressure relationships in a series of curves and corresponding equations. Additional analytical work will determine dynamic impedances of the backfill based on spring and dashpot models.
The broader impacts of this project include improved design of structural components through a better definition of the geotechnical component of soil-structure interaction. Additional impacts include mentored educational experiences for participating university students as well as increased awareness of earthquake engineering issues by junior high and high school students participating in an outreach program centered on the research performed.
