THREE DIMENSIONAL SEISMIC BEHAVIOUR OF SOIL-PILE INTERACTION WITH LIQUEFACTION

Abstract

The behaviour of pile-supported structures under strong ground motion is reasonably a complex problem. Catastrophic damages ofsome recent earthquakes such as North Ridge (1994), Kobe (1995), Kocaeli (1999), Chi-chi (1999) and Bhuj (2001) have raised concern about the current approaches used for the design of piles and pile-supported structures. Since the behaviour of soil during strong excitations is highly nonlinear, soil-structure interaction with due consideration of nonlinearity of soil is a necessity. The problem of soil-pile interaction gets further complicated when the piles are embedded in liquefiable soil medium. A three-dimensional finite element soil-pile model has been developed in MATLAB with proper radiation boundary conditions. Frequency dependent Kelvin elements have been used for simulating radiation conditions at infinity. Then Drucker-Prager perfectly plastic soil model without cap and perfectly plastic soil model with cap have been implemented for nonlinear soil-pile problem. The pore pressure generation for liquefaction is incorporated by two-parameter volume change model (Byrne model) established in the literature. Separation between the soil and pile is considered by using three-dimensional two-noded contact elements. Finally, a four storied portal frame has been considered as a superstructure. The finite element model and the algorithm developed have been verified with the literature. Effects of material nonlinearity and liquefaction of soil on the dynamic behaviour of pile groups are investigated. Excitations are applied for discrete frequencies varying over a wide range. It was observed that the soil plasticity reduces the real part of the dynamic stiffness significantly. However, the effect of plasticity on imaginary part is not as significant for cap model as observed without cap. It is also noticed that the plasticity of the soil medium reduces the group effect significantly. When liquefaction is considered in the analysis, both real and imaginary parts of the dynamic stiffness are further reduced. It was also observed that the soil plasticity has a significant influence on the seismic response of the soil-pile system. But the influence gets reduced in the high frequency range. Both translational and rotational seismic response increase significantly when liquefaction is incorporated. Kinematic interaction factors obtained for harmonic base accelerations are in good agreement with the responses obtained with real earthquake acceleration time histories. The importance of considering plasticity of the soil medium is demonstrated. It is in also shown that for design and analysis of piles in liquefying soil under seismic loading, both the translation and rotation is to be taken into account. The effect ofseparation on the real and imaginary parts ofthe dynamic stiffness ofthe soil and pile system considering nonlinearity of the soil medium is investigated for nonliquefiable as well as liquefiable conditions. For soil medium, without liquefaction capability, it is observed that the separation between soil and pile reduces the real part of the dynamic stiffness considering the elastic soil medium. But with the introduction ofthe plasticity of the soil medium, it is observed that the effect of separation on dynamic stiffness is not significant unlike it was the case with elastic soil medium. For translational kinematic interaction factor for seismic excitation, it is also observed that the effect of separation with plasticity of the soil medium is not dominant unlike it was the case with elastic soil medium. Thus the effect of separation is significant for elastic soil model but plasticity of soil overshadows the separation. Now when the liquefaction capability is incorporated in soil medium, it is observed that the separation between the soil and pile has little influence on the dynamic stiffness and seismic response of the soil-pile systems. The effect of inertial interaction on the response of an idealized structure is demonstrated. The effect of soil plasticity and liquefaction of the soil medium on the response of the structure is shown. It was observed that the nonlinearity of the soil medium has a definite influence on the response ofthe structure. However, when liquefaction is considered in the study, the effect on the response of the structure is more pronounced. Since the effect of nonlinearity and liquefaction on the dynamic behaviour of the soil-pile systems is very much dependent on the intensity of excitation, parametric studies are performed considering different amplitudes of excitation. Parametric studies are also conducted varying the elastic modulus of the soil to demonstrate the effect of nonlinearity and liquefaction on the soil-pile system for stiffer aswell as softer soil medium. The study has very much practical significance as in many structures piles pass through liquefiable soil layers.