Seismic behaviour of structures built on soft soils is influenced by the soil properties and the foundation type, where the response is significantly different from the fixed base condition owing to the interaction between the ground and the structure. Soil-Structure Interaction (SSI) reduces the natural frequency of the system and increases the effective damping ratio of the system, for typical soils and foundations, in comparison with the fixed-base structure. This can considerably alter the response of the building frames under the seismic excitation by influencing the structural demand of the building as well as amplifying the lateral deflections and inter storey drifts of the superstructure. This amplification of lateral deformations due to SSI may change the performance level of buildings in the performance based design approach, which should be considered with great rigor accounting for the influence of SSI significantly influenced by the foundation type (i.e. shallow and deep foundation), in order to provide safe and cost effective design against the natural disasters such as earthquake.
In this study, in order to provide a benchmark to verify and calibrate the numerical model as well as experimentally investigate the influence of SSI on the seismic response of buildings, a series of shaking table tests on the soil-foundation-structure models are conducted at the University of Technology Sydney (UTS) structures laboratory. Different foundation types such as shallow foundation, floating pile foundation, end-bearing pile foundation as well as fixed base condition, excluding SSI interaction, are physically modelled. A laminar soil container is designed and constructed to simulate the free field soil response by minimising boundary effects. Simulating the superstructure as a multi-storey frame during the shaking table tests makes experimental data unique. Accordingly, in the current shaking table tests, by adopting the same soil properties, same superstructure, same input motions, and same test setup, a clear comparison is provided between the structural responses for different types of foundations. The experimental results indicate that soil-structure interaction amplifies the lateral deflections and inter-storey drifts of the structures supported by different types of foundations. However, the choice of the foundation type influences the structural performance significantly and should be addressed carefully in investigating the influence of SSI on the superstructure response during shaking excitations.
A fully nonlinear three-dimensional numerical model employing FLAC3D is developed to perform time-history analysis and simulate the performance of the superstructure considering the seismic soil-structure interaction. Hysteretic damping of the soil is implemented to represent the variation of the shear modulus reduction factor and the damping ratio of the soil with the cyclic shear strain. Free field boundary conditions are assigned to the numerical model and appropriate interface elements, capable of modelling sliding and separation between the pile and soil elements, are considered. The developed numerical model is verified and validated against the conducted shaking table results. Comparison of the numerical predictions and the experimental data shows a good agreement confirming the reliability of the numerical model. Consequently, the proposed numerical model is a reliable method of simulation which can be employed for further numerical investigations concerning the dynamic soil-structure interaction. Practicing engineers can adopt this verified numerical modelling procedure in the design to consider the effect of SSI.
Furthermore, in order to investigate the different characteristics of SSI and its influence on the seismic response of superstructures, parametric studies with respect to different types of foundations are conducted employing the previously verified three-dimensional numerical modelling procedure. A full scale fifteen storey structure (prototype) with four different types of foundations, namely, (i) fixed-base structure representing the situation excluding the soil-structure interaction, (ii) structure supported by a shallow foundation, (iii) structure supported by a pile-raft foundation in soft soil, and (iii) structure supported by a floating (frictional) pile foundation in soft soil, are simulated. According to the results of the numerical investigations, the properties of the in situ soil influence the characteristics of the excitation in terms of peak acceleration and frequency content. Moreover, the reduction ratio of the shear forces of superstructure due to SSI is a function of the foundation type, while the magnitude of this reduction is different for different levels in the superstructure. Accounting for the rocking-dissipation concept, results of this study can help the practicing engineers in selecting the proper foundation type for the structures. The foundation types experiencing considerable amount of rocking during an earthquake, dissipate significant amount of earthquake energy in comparison with the other types of foundations, and this rocking-dissipation in turn results in directing less shear forces to the superstructure and reducing the structural demand of the superstructure.