Numerical Modeling of Pile and Liquefied Soil Interaction using Non-Linear Spring Method

Decrease in the strength and stiffness of soil duo to liquefaction may cause large bending moments and lateral deformations in piles located in this type of soils. To reliable design of pile foundations in the liquefaction susceptible soils, it is necessary to have a accurate evaluation of the lateral pressure which will be exerted on the pile if the subsurface layers undergo liquefaction and lateral spreading in the course of earthquake. In this study, a coupled Soil-Pile-Structure Interaction (SPSI) analysis method has been used to investigate the behavior of piles in liquefiable soils. Interaction of soil-pile was simulated by using nonlinear p-y springs. The liquefaction effects were taken into account by introducing a degradation multiplier to the lateral resistance of soil. The degraded lateral resistance of liquefied soil was considered equal to 5% of its initial value for loose sand and 10% for medium sand. Fully coupled dynamic analysis of a soil column in the free-filed condition was performed in the OpenSEES (Open System for Earthquake Engineering Simulation) software. For simulation of the interaction of solid-fluid phases based on the theory of saturated porous medium, u-p formulation has been used. Liquefied soil behavior was modeled using pressure dependent multi yield material model. From the coupled analysis, the time histories of excess pore pressure ratio at the different levels are obtained. The value of excess pore pressure ratio (0.0 to 1.0) is used to interpolate the transient lateral resistance of soil from its initial value in the static condition (excess pore pressure ratio equal to 0.0) to its final degraded value in the fully liquefied condition (excess pore pressure ratio equal to 1.0). In order to verify the numerical model, results of numerical modeling have been compared with two centrifuge experiments' measurements. Both of experiments include two soil layers and the pile is extended into the lower layer. In the first experiment, the loose sand layer is above the medium dense layer and in the second experiment the medium dense sand layer is above the dense layer. After verification of the numerical model, parametric analysis was performed to study the effect of various parameters on the dynamic response of piles and applied lateral pressure from the spreading liquefied soil to pile. Investigated parameters are liquefaction layer thickness, the input excitation frequency, fixity of the pile cap, pile stiffness, maximum input acceleration and the relative density of liquefiable soil. The results of the parametric analysis show that the maximum bending moment in the case of fixed head occurs at the top of pile and in the case of free head at the depth of 1 to 3 meters. The maximum bending moment of pile is also greater in the case of fixed head pile; however, its lateral deformation is lower. Increasing of frequency of input motion and soil relative density or decreasing of liquefied soil thickness lead to decreasing of maximum bending moment and deformation of pile. Regarding the lateral pressure exerted on the pile, the results of analysis indicate that the lateral pressure is relatively constant at the depth of liquefied layer and is equal to 7 to 10 percent of the total vertical pressure at the base of liquefied layer.