Evaluation of the seismic rocking behavior for embedded shallow foundation via dynamic centrifuge test

Abstract

In conventional seismic design, the shallow foundation is designed very conservatively avoiding plastic hinge in soil-foundation system, leading not only to an uneconomical design of the oversized foundation, but to structure designs that are non-resistant to an intense earthquake loads. As an emerging design paradigm, nowadays, many researchers have studied the concept considering the nonlinear soil-foundation-structure interaction (SFSI), especially on ‘'rocking foundation'. The new design philosophy, which allows the uplift and sliding behavior of foundation and the transient yielding of soil-foundation system while ensuring the residual deformation are acceptable, has the benefit of the economical foundation design and safe structural behavior to extreme earthquake. So, allowing rocking motion for foundation has been proposed by several researchers as an effective method of seismic isolation and has been applied in the special design of modern bridge (Rion Antirion Bridge). Foundation rocking behavior by the overturning moment of structure during earthquake can be determined by the rocking stiffness and damping ratio. These parameters vary with the safety factor of soil-foundation system and are very important for the seismic design of rocking foundation. Up to a recent date, many researches about the influencing parameters of seismic rocking behavior have been performed, and displacement-based-seismic design approaches were introduced to adopt the rocking foundation concept. Then seismic design code of ASCE 41-13 provides some of those method such as backbone curve for rocking behavior. However, additional investigation about various condition of shallow foundation is still required, because the most of research to validate the design parameters focused on the surface foundation with slow cyclic loading condition. Most of shallow foundation in the real field are buried on the soil ground, and the embedded shallow foundation can have the relatively high safety factors and the effects of surrounding soil can be increased. And the earthquake motion have high frequency characteristics than the slow cyclic loading, it is necessary to evaluate the seismic rocking behavior including the seismic loads of superstructure on the embedded shallow foundation. Although seismic rocking behavior has clear advantages in the reduction of seismic loads of structure, the vertical settlements is still problem to use in the region of soft soil site. So the disconnected pile foundation can be the complementary method to take advantage of shallow rocking foundation and pile foundation. In order to understand the seismic performance of disconnected pile, it is crucial to evaluate the seismic behavior of various type of foundations in terms of seismic loads of structure and systems settlement during the earthquake. Motivated by these perception, main objective of this study is to evaluate seismic rocking behavior of embedded shallow foundation, and to suggest the improved approaches for using rocking concept. To achieve the purpose, a series of dynamic centrifuge tests were performed about test specimens composed of soil, foundation and single-degree-of-freedom structure. And parameters such as size and mass of foundation, periods of structure and intensity of earthquake were changed to characterize the effects. In the first theme, the relationship between mobilized moment and rotation angle for embedded shallow foundation were determined and corresponding rocking stiffness and damping ratio were compared from the centrifuge test results. And the seismic loads of SDOF structure were observed for the investigation of beneficial effects to limit seismic loads by rocking effects. In the second theme, the experimental evaluation for seismic behavior of the disconnected pile foundation was conducted using the dynamic centrifuge tests. By comparison of seismic loads and settlements for different types of foundation types such as shallow and connected pile foundation, the performance of disconnected pile was validated and considerations for design were suggested. In the third theme, all of dynamic centrifuge test results were unified for construction of database and parameters of rocking design in ASCE 41-13 for embedded shallow foundation were evaluated. The representative values like initial stiffness, yield rotation angle and re-centering ratio for each test condition were discussed for the embedded shallow foundation. In addition, the modified parameters are suggested and compared with previous research to investigate reliability.