The Study On Method Of Seismic Response Analysis Of Nonlinear Soil Layers

The study on method of seismic response analysis of nonlinear soil layers is an important content in research domain of geotechnical earthquake engineering now. The technique of soil layers seismic response analysis is also one of core techniques in seismic safety evaluation of engineering sites currently. Early in the beginning of 21th century, site conditions having certain effect on ground motion had been found from seismic damage of The Great 1906 San Francisco Earthquake. In order to estimate reasonably the effection of site conditions on ground motion, researchers predigest sites into mechnics models, suggest all kinds of soil dynamic constitutve models, and estimate sites'response of ground motion by analytical or numerical methods. Currently, the seismic response analysis method of soil layers applied in engineering widely is equivalent linear method all the same. The soil nonlinear problem is translated into linear problem in equivalent linear method when equivalent linear method is used to estimate site's sesmic response. For having no appropriate dynamic constitutve model, some unreasonable situations have been found from the engineering application of equivalent linear method. Further more, intensity anomaly is a phenomenon fallen across frequently when earthquakes accur. Espelcially, seismic disaster of soft site is more serious. The study on earthquake disaster of soft site is also a hotspot in the research domain of earthquake engineering. In view of these facts, in this paper some research works are made of method of seismic response analysis of nonlinear soil layers and intensity anomaly phenomenon of special site in Wenchuan earthquake. The main works is as blow:1. Some problems of current method of seismic response analysis of soil layers are pointed outCurrent method of response analysis of soil layers is equivalent linear method which is widely applied in ingineering. The basic principle of equivalent linear method is introduced in brief, its merits and shortcomings are summarized, the latest progress of equivalent linear method is also introduced. Engineering examples show some main problems of the method applied in engineering, the problems are as following: the method cannot compute when the acceleration time historys have a certain discrete points, the result is abnormal when site is soft, and it is unreseanable when the input groung motion is strong, the program may occur perpetuating cycle. The way to improve the method is also put forward. 2. Soil dynamic constitutive model with exponential form is constructedBased on boundedness and the form of exponential faction, and also based on general rules for constructing soil dynamic constitutive model, function image of y=ex in interval (-∞,0] is overturned and translated and also magnified or reduced so as to gain the skeleton curve of soil dynamic constitutive model. In the term of unloading and opposite side loading, let the stress-strain curve have the same form of skeleton curve, the curve uses the last unloading point as its starting point, and uses soil's ultimate stress level as its asymptote. Then the hysteretic curve is constructed. Thus soil dynamic constitutive model which has exponential form is gained, which is suitable for unsymmetrical cyclic loadings. The model is also called UE model. UE model has such traits as flowing: First, UE model is suitable for unsymmetrical cyclic loadings. Secnond, UE model need a less memory. Third, UE model's hysteretic curves under unloading-reloading follow Masing's rule automatically. Fourth, UE model has less parameteres, which have clear physical meaning and could be determined by conventional tests.3. UE model is tested by the results of resonance column testFormula of equivalent shear modulus ratio is deduced. UE model is tested by the the results of resonance column tests of the soils, which are obtained from seismic microzontioon work in Yanliang, Xi'an. The results indicate UE model could fit the testing results well. At the same time, the shear moduls ratio values recommended by Yuan Xiaoming are also fitted by UE model, the fitting results also indicate UE model is dependable.4. The method of seismic response analysis of nonlinear soil layers based on UE model is testedOne dimension method of seismic response analysis of nonlinear soil layers is put forward based on UE model. The corresponding computer program NDSoilUE-1D is also compiled. The program is tested by strong ground motion records of Xiangtang three dimension array in Tangshan, the acceleration response at ground surface and analysis resultsof spectral ratio confirme that the new method can reflect the trend more reasonable than equivalent linear method, which soils'nonlinear trait changes from weak to stong when the input ground motion varies from weak to strong.5. Preliminary studies are completed about the effect of special site in Wenchuan earthquakeIn order to study the reason why intensity anomaly phenomenon occurs at Hanyuan county in Wenchuan earthquake, an investigation is taken on seismic damage and the landform and physiognomy of this town. Considering the site data, five sites'microtremor datas are tested according the degree of seismic damage. The testing result is that the predominant periods of the sites with severe damage are larger than the ones of the site with slight damage. The results indicate that the reason why Hanyuan town has severe earthquake damage is that the site is soft. The conclusion based on sand liquefaction point of view points out that the slop of Beihoushan Mountain is a potential threat to Hanyuan town.