Study On Dynamic Properties Of Cement Mixed Pile Foundations And The Soil-Structure Interactions During Earthquakes

Soft soils are widely distributed in the prosperous coastal areas in China. A great deal of structures will be constructed with the economical development and urbanization. Cement Deep Mixing Method (CDMM) is usually adopted to improve the engineering properties of soft soil foundations in engineering practice. Each severe earthquake brings great damages and losses to people in China, which lies in a high-frequency earthquake region. Therefore, study on dynamic properties of cement mixed pile foundations and the soil structure interaction during earthquakes is of great theoretical and practical importance in seismic design.This dissertation studies the static and dynamic properties of cemented composite foundations systematically through tests and parameters of Duncan-Chang model and Hardin-Drnevich model are generalized. The seismic acceleration is conversed and used as input acceleration through nonlinear method. With the self-developed finite element program, the soil-structure interactions and the seismic settlement of structures under earthquakes are studied, which led to several conclusions useful to seismic design in engineering practice.The preparation method of composite cemented clay samples is modified based on former researches. The static triaxial test is performed on composite cemented clay samples and parameters of Duncan-Chang model are obtained considering the effects of replacement ratio and confining pressure. The effects such as replacement ratio, strain amplitude and confining pressure on the dynamic modulus and damping ratio of composite cemented clay samples are analyzed through undrained dynamic triaxial test. Fitting equations estimating the dynamic modulus and damping ratio through regression analysis are deducted with the effects of replacement ratio, strain amplitude and confining pressure.The ground seismic acceleration is conversed as the input seismic acceleration from bedrocks, which is useful in the next step. The formula of dynamic FEM is deduced based on dynamic Biot's consolidation equation considering the coupling of seepage and dynamic analysis with time-history analysis method. Parameters are determined from tests and a FEM program is developed. Dynamic responses of free field and superstructure considering different replacement ratio are analyzed. Inside the area improved by cement mixed piles, the peak acceleration decreases and the peak shear stress increases with the increment of replacement ratio. In a point closer to the ground, there are higher peak acceleration and lower peak shear stress at the same replacement ratio. Outside the improved area, the peak acceleration increasesand the peak shear stress decreases with the increment of replacement ratio. Compared with free field, the peak acceleration decreases and the peak shear stress increases under the overload transmitted from superstructures either inside or outside improved area. At the same replacement ratio, the peak acceleration and the peak shear stress both increase with the increment of layers in superstructures. The peak acceleration and the peak shear stress in superstructures increase with the increment of replacement ratio.Parameters of softening model are obtained through seismic settlement tests of composite cemented clay samples and relative curves are plotted considering different replacement ratio. After FEM analysis, conclusions can be made: the seismic settlement of structures under which the soils are improved by cement mixing piles is smaller than that of untreated soils, which indicates the CDMM is effective in reducing the seismic settlement of superstructures.In general, some practical and useful fitting equations are deduced after researches on static and dynamic tests of composite cemented clays and corresponding regression analysis, which can be referred at the shortage of test data. The dynamic interaction between composite cement pile foundations and structures as well as the seismic settlement of superstructures are analyzed with the conversed seismic acceleration pr...