Study On Lateral Dynamic Response Of Pile Foundation In Liquefiable Soil

The pile foundation disasters occur often, especially in liquefiable soil layer through all previous earthquakes. Study on lateral capacity of pile in liquefiable soil layer has generally become important subject in aseismic design of soil and foundation engineering in the world.There are obvious deficiencies in aseismic design method of pile in liquefiable site for methods used at present. One of the main causes is that the existing theory and experience for aseismic design method of pile foundations depend on static condition. Except of that, the shortage or misunderstanding to mechanism of liquefied-related pile-soil interaction is the other troubles.State-art-of researches on the subject are conducted and, moreover, several important problems are pointed out based on shaking table tests as well as post earthquake investigation and numerical simulation. The main contents and achievements in the thesis are:1. The design of pile-soil interaction shaking table test is completed to reach the purpose for comparison of multi-cases tests. A lot of data and acknowledges are obtained.In test design, four destinies saturated sand, non-liquefiable sand and silt;two kinds of pile and pile-head models static test under sinusoidal cyclic loading and seismic wave input are included;The time dependent processes of accumulating pore water pressure and the strain of pile at different soil layer depth as well as the interaction force of soil and pile and displacement, acceleration of the pile head.2. Relationship among acceleration, pore water pressure, deformation and modulus of soil layer, the inputting acceleration, bending moment of pile and interaction force of soil and pile, response of pile-head is analyzed through many tests. Some new phenomena and acknowledgement are obtained.The results in the paper show the movement of soil layer surface and dynamics response of pile foundation in the liquefied soil has specialties compared with non-liquefiable soil. The relative displacement between soil and pile in liquefied soil is much greater than non-liquefied soil. The raising rate of relative displacement is greater than decreasing rate of soil modulus. The soil layer acceleration and pile-head acceleration, bendingmoment of pile and input acceleration wave in the non-liquefied layer coincide with each other in the form and bending moment of pile near bottom is less than in soil surface. Surface acceleration in liquefied soil is analogous with pile-head initially. Then the responses raise and reach to maximum as pore water ratio reaches to 0.8. Then, the response decreases obviously under the sinusoidal wave and for seismic input, high frequent decreases in and low frequent amplify. However, they are both agreeable with the base input in the form. The bending moment of pile in liquefied soil layer is greater than that near soil surface as the pore water pressure reaches to 0.8. After that, the bending moment does not decrease but is amplified.3. The mechanism of liquefaction-induced dynamics pile-soil interaction is revealed by post earthquake analysis, two scales shaking table tests and numerical simulation.The results in the paper indicate inertia force of pile-head dominates the pile destruction in non-liquefied soil and the soil is pushed by the pile. In this case, soil and pile assume high frequent responses, which are all, be controlled by soil layer acceleration. In liquefied soil layer, however, the relationship between soil and pile is soil-pushing pile since the pile is pushed by the soil and the rate of displacement rising is greater than decreasing of soil modulus. There are obvious correspondence relations in the pore water pressure, soil deformation and soil-pile interaction force. The destruction of pile foundation is controlled by liquefaction-related soil deformation not by the inertia of pile-head. The relative displacement and force of soil-pile and bending moment of pile behave at low-frequency response since they are all be controlled by liquefaction-related soil displacements.4. The reasonability of the main computational method of seismic lateral capacity of pile is discussed and the disadvantages of the pseudo-static and coefficient-discounted method are also pointed.The results in the paper indicate both the coefficient-discounted method and using lateral displacement of pile instead of soil-pile relative displacement are not agreeable with the real situation. Fundamental idea of pseudo-static method is not agreeable with mechanism in actual. The pile response can not be obtained simply by discounting lateral stiffness of non-liquefiable soil layer. Calculated results by using pseudo-static method disagree with real situation especially in small displacement range. Calculating results by using pseudo-static method are quiteconservative even can not be accepted in engineering practice.5. Modified model of dynamic p-y curves of pile foundation in Hquefiable soil layer are proposed and its reliability are also verified.The basic form of main trunk circuit of pile p-y curve is gained and the difference and connection with the p-y curve in API Code is got through the study on the influencing factors, modification method and mode of the new model construction of the p-y curve in Hquefiable soil strata. Using double-parameters modified method construct a new computational model. The new formula overcome shortage of rapid rise of soil-pile interaction force within the deformation is small. The reliability of the new formula is verified through shaking table test in this paper.