DOI: 10.4231/d3c53f17m
title: JDA01: Centrifuge Testing of Rocking Foundations on Saturated and Submerged Sand

description: 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.

In responding to these hurdles, the first testing series (JDA01) was performed on six identical “single-degree-of-freedom” (SDOF) models that were built upon loose-liquefiable and erosive soil ground. The 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 11 shakes at a centrifugal acceleration of 55g. The experiment was performed in a flexible laminar container box which could enable the interlayer shear strain.

The purpose of this first centrifuge test was to observe the structural and soil response of a rocking foundation system in fully saturated soil, which includes a liquefiable layer. The presence of water introduces interesting fluid-soil interaction issues, such as suction, erosion, and liquefaction, which could potentially be amplified by the dynamic cyclic motion of the rocking foundation.

In consideration of the potential excessive settlements imposed by liquefaction, one goal of this research is to consider the potential benefit of coupling the rocking footing with unconnected deep piles. The purpose of these piles would be to bypass the liquefiable or poor layer of soil to set in a more competent, dense layer. If settlements are in fact unreasonable, coupling the rocking footing with unconnected piles may reduce excessive settlements. Through the use of unconnected piles, the footings are able to rock freely ensuring the desired self centering effect and benefits of a rocking foundation system, while reducing settlements.

The orientation styles of the six structures were chosen to try and capture the different fluid failure mechanisms presented above. That is, the surface footings would be susceptible to the rocking induced erosion, the embedded footings to rocking induced suction, and the unconnected pile footings to represent an improved site condition. Two surface soils were used for comparison purposes, resulting in six different stations of varying footing orientation and grounding.