Theoretic Analysis Of Dynamic Site Response And Experimental Study Of Cyclic Soil Behavior

Theoretic analysis of dynamic response of free sites and dynamic triaxial test on dynamic modulus and damping ratio of soils are carried out, as a part of the project aided by nature science national fund, analysis of soil-foundation-structure dynamic interaction (NO.50278033).The base of seismic design is seismic response analysis of sites and structures. In the soil-foundation-structure dynamic interaction analysis it is of first importance to analyze dynamic response of free sites, which is affected by soil behavior. And because of it's complexity much research has only been carried out to investigate the seismic response of sites with a uniform shear modulus or a shear modulus linear or power variation with depth. Based on the research and the practical datum, a site model is presented that the shear modulus is distributed exponentially increasing with depth. Using a one-dimensional shear beam model, analytical solutions are developed for the displacement amplitudes of the sites with a single layer, subjected to the excitation of steady-state and vertically propagating shear seismic waves. Furthermore, seismic responses are analyzed by the mode superposition method. The conclusions are as follows: the mean value of shear wave velocity of this site model can be used to analyze seismic response when the parameter a doesn't exceed 0.5. The predominant period can be still estimated to be four times of the total shear wave travel time through the layer. The modal frequencies can be approximately estimated by the mean value of shear wave velocity. These conclusions are drawn for simplified calculation and analysis of seismic design in engineering.In dynamic analysis it is important to make clear dynamic behavior of site soils, of which the parameters are necessary for theoretical analysis and numerical analysis. Then one of the problems in the project is how to determine the parameters and their regularity using the test equipments. Therefore dynamic triaxial test is conducted to determine the dynamic modulus and damping ratio of silty clay of the site, where a model test about soil-foundation-structure dynamic interaction will be done. Because the relationship curves of dynamic stress and dynamic strain from the test are approximately hyperbolic, the silty clay regarded as a visco-elastic body is studied by means of an equivalent linear model. After two affecting factors of consolidation ratio and lateral pressure being studied, the empirical formulas of maximum shear modulus, dynamic modulus and damping ratio are suggested for being applied intheoretic analysis and practical engineering and also supplying the essential parameters for analysis of the seismic response of sites and soil-structure dynamic interaction.