JDA02: Centrifuge Testing of Rocking Foundations on Saturated and Submerged Sand

Many researchers have studied rocking shallow foundations for various structures using different experimental tools. Several consecutive series of centrifuge model tests characterized the static and dynamic behavior of rocking shallow foundations for rigid shear walls or for highway bridges. Additionally, the behavior of rocking foundations has been experimentally investigated using shaking table and in-situ model tests.

Current research has indicated that shallow rocking foundations on competent soils can reduce seismic ductility demand on columns and improve the performance of the system. If the moment capacity of the footing is designed so that it is smaller than the moment capacity of the column, rocking of the structure about the base of the footing will be initiated. In the conventional fixed-base foundation condition, almost all energy must be dissipated through bending of the column. The rocking of the foundation results in a ductility demand transfer from the column to the footing and surrounding soil. This reduces the magnitude of differential displacements imposed along the column in comparison to a column supported on a fixed foundation.

Although the beneficial effects of rocking foundations on bridge systems have been widely explored, some hurdles impeding the use of rocking shallow foundations in bridge design still exist. One of these hurdles is the excessive displacement and collapse potential due to foundations rocking on poor, liquefiable soils. The overall goals of this research are to (1) delineate the soil conditions under which rocking foundations would not be acceptable and (2) explore the viability of rocking foundations in poor soil conditions if the foundations are supported on unconnected piles.

The JDA02 test was completed at the Center for Geotechnical Modeling Facility at the University of California at Davis on the 9 meter radius centrifuge. The test series was comprised of 21 shakes at a centrifugal acceleration of 55g. After reviewing the data from the JDA01 test, the purpose of the second centrifuge test (JDA02) in the series was to observe the structural and soil response of the same rocking foundation system in fully saturated soil with a thinner liquefiable layer. Also, alternative pile connection designs were implemented for two structures. After observing significant sliding of the footings from the unattached piles in JDA01, shear keys were fixed to the base of two of the footings in JDA02 to add an explicit lateral connection from the footing to the pile. Additionally, an increased super structure mass was also tested with one of the structures to observe effects of bearing pressure.