Seismic Response Analysis And Simplified Method Of Bridge Pile Foundation In Liquefiable Ground

The seismic hazards indicate that soil liquefaction is one of the main reasons of bridge pile foundation damage during earthquakes. However, the content of Chinese seismic design code just presents some qualitative and general items for bridge pile foundation in liquefiable ground, and these items lack of systematicness and maneuverability for practical projects. So, there are some shortcomings on the study of bridge pile foundation in liquefiable ground. The study of seismic response of bridge pile foundation is the key to solve this problem. Dynamic p-y curve method is one of the effective ways to study seismic pile-soil interaction, and it provides a technical approach to develop a seismic design method based on displacement for bridge pile foundation in liquefiable ground. But, as for adopting this method to analyze seismic pile-soil interaction, the most important problem is to reasonably determine the dynamic p-y curves of sand during liquefaction. In fact, although the field do not completely liquefy, the dynamic property of sand greatly changes due to high pore pressure. Therefore, it is necessary to study seismic response and simplified analysis method of bridge pile foundation during field liquefaction. Aiming at seismic response of bridge pile foundation in liquefiable ground, three-dimensional finite element method (3D FEM) is established to study seismic pile-soil interaction based on shaking table test and numerical analysis. The dynamic p-y curves of different relative densities sand are obtained by this method. By dividing pore pressure ratio time histories into several segments, dynamic p-y curves of sand and backbone curves are established for the same pore pressure ratio at the different time interval, and then the backbone curves of sand are obtained. Based on the above results, the modified formula of saturated sand p-y curves is raised considering the effect of pore pressure, and developing the simplified analysis method for bridge pile foundation in liquefiable ground. The main content and understandings obtained are as follows:(1) Adopting reinforced concrete single pile-pier, a series of shaking table tests are conducted successfully for seismic response of bridge pile foundation in liquefiable ground, corresponding to the typical liquefiable ground, whose surface and bottom of the field make up clay layer and the interlayer is saturated sand layer. Dynamic property of the field and seismic response of bridge pile foundation are researched during field liquefaction. One approach of p-y curves establishment for seismic pile-soil interaction in liquefiable ground is given, and fundamental property of dynamic p-y curves for sand is analyzed.(2) Based on OpenSees finite element numerical simulation platform, aiming at the finished shaking table tests, and adopting finite element implementation of u-p formulation governing equation, a 3D finite element model and calculation method is developed to study seismic response of bridge pile foundation under controlling the test conditions by introducing plastic multi-yield surface constitutive model which can take soil dynamic properties into consideration. The pile was simulated by beam-column element according to the theory of Euler-Bernoulli beam. The pile linked with soil by rigid connector element, which considers volume effect in the model. According to the comparative analysis between numerical simulation and shaking table tests included in the seismic response of bridge pile foundation and dynamic characteristics of field etc., the correctness of finite element model and calculation method was verified, which can effectively simulate seismic pile-soil interaction in liquefiable ground.(3) Based on finite element method, a series of numerical simulation of bridge pile foundation in liquefiable ground are conducted for different relative densities sand under different amplitudes of El Centro waves, and p-y curves of seismic pile-soil interaction are obtained under different amplitudes and different relative densities in liquefiable ground. A series of p-y curves corresponding to different pore pressure ratios are obtained by dividing pore pressure ratio time histories into several segments, and a modified method is proposed for establishing backbone curves of dynamic p-y curves by adopting the establishing approach for backbone curves of stress-strain curve, which takes pore pressure ratio as a control factor. Based on p-y curves of saturated sand recommended by API, a modified equation of p-y curves for different relative densities of saturated sand is established, which takes pore pressure ratio as a control factor and reflects the variation of the soil properties caused by pore pressure. And then, concrete expression and method of determining parameter are given.(4) Based on the response analysis methods of free liquefiable ground, aiming at free liquefiable ground, numerical simulation models of shaking table tests under controlling the test conditions is established. Numerical simulation is verified by comparing the results of shaking table tests with one of numerical simulation models. Meanwhile, the seismic response of overlying clay layer, middle sand layer and lower soft clay layer are simulated by numerical simulation, and their property is analysed.(5) The p-y element model of seismic pile-soil interaction in liquefiable ground is established by combining spring element and damping element reasonably, and the calculation parameters and formulation are provided. In order to consider the effect of sand liquefaction on the spring of seismic pile-soil interaction, the p-y element model combines the modified equation of saturated sand p-y curve considering the effect of pore pressure. Based on the nonlinear Winkler foundation model, a simplified analytical method aiming at shaking table test is developed by using the established p-y element, which considers the effect of lumped soil mass, inertial force from superstructure, mass inertial force from soil around pile and radiation damping on dynamic response of the system. The displacement and pore pressure ratio time histories obtained from free liquefiable ground are taken as the input of the simplified numerical analyses model of seismic response of bridge pile foundation in liquefiable ground. The simplified analytical method is verified by comparison between results of shaking table tests and calculations, and the step of simplified analytical method is given. Then, the effects of relative density, initial modulus ratio between pile and soil, superstructure mass and radiation damping, on seismic pile-soil interaction are analyzed by adopting the proposed modified model.