Investigation of foundation rocking behavior considering nonlinearity of soil-foundation-structure interaction during earthquake

Abstract

Earthquake causes loss of human lives and huge damage to infrastructures leading to both immediate and long-term impacts on the society. As critical infrastructures such as bridges, ports, and pipelines are important for proper functioning of society, the stability of these infrastructures is paramount after a big earthquake event. Conventional seismic design for the shallow foundation usually prohibits failure at the foundation during an earthquake, so that the dynamic behavior of the shallow foundation should be within the elastic range. However, the conventional capacity design for the shallow foundation does not ensure adequate safety for the infrastructure during an unexpected strong earthquake. In conventional capacity design, the superstructure column usually collapses during a strong earthquake because the seismic energy mostly dissipates at the structure not the soil-foundation level. To safeguard from strong earthquakes, a necessity for a new paradigm in seismic design has been highlighted. The main purpose of the new design philosophy is to conduct safe, accurate, and economical seismic design. Hence, the effect of soil-foundation system on the whole foundation-structure system behavior has to be considered for a sophisticated seismic design

this is referred as dynamic soil-foundation-structure interaction (SFSI). Furthermore, the new seismic design methodology allows the use of the nonlinear plastic behavior of foundation during a strong earthquake. Especially, the rocking behavior of the foundation has been introduced as a useful phenomenon to reduce the ductility demand and seismic load of the superstructure during a strong earthquake

the new design philosophy is referred to as “rocking foundation”. However, previous studies have not considered two major problems related to the SFSI and the rocking foundation. Firstly, although the nonlinearity of the SFSI caused by both the soil nonlinearity and the system nonlinearity occurs during a strong earthquake, the previous studies have only evaluated the response of foundation-structure system within the elastic range. Secondly, since the rocking behavior of the foundation have been investigated by assuming the superstructure as a rigid body, the effects of structural inertial motion cannot be reflected by assuming the superstructure as a rigid structure. Consequently, a comprehensive evaluation of the foundation rocking behavior considering nonlinearity of SFSI is needed. The main objective of this research is to evaluate the foundation rocking behavior considering the nonlinearity of the SFSI during earthquakes. This study is divided into two major parts. Firstly, this study aims to evaluate the mechanism and the effect of the nonlinearity of SFSI on the dynamic behavior of the structure and the foundation using both analytical methods and centrifuge tests. Since the previous studies had only described the linear elastic characteristics of the SFSI, period lengthening which is a representative phenomenon of the SFSI was evaluated considering both soil and system nonlinearity. The current study investigated the dynamic behavior of the foundation affected by the structural inertial motion considering the nonlinearity of SFSI. In addition, the rocking behavior of the foundation considering structural inertial motion was evaluated and the difference between the cyclic and the dynamic rocking behavior of the foundation caused by the structural inertial motion was also discussed. Secondly, to apply the rocking foundation in practice, estimation and minimization of permanent deformations of the foundation are necessary. Therefore, a simplified method to predict the maximum foundation rotation during earthquakes was suggested using Housner’s rocking model. On the other hand, to minimize the permanent settlement and rotation, disconnected piled raft (DPR) has been suggested for the design of rocking foundation. The seismic response of the DPR from geotechnical point of view was evaluated in terms of the dynamic bending moment of the DPR and the seismic response of the foundation on DPR. Finally, in order to suggest an economical and effective design for the rocking foundation, an optimized design for the rocking foundation using disconnected short piles was suggested and the evaluated using slow cyclic and dynamic centrifuge tests.