Research On A New Generation Technique For Ground Seismic Response Analysis

There are abundant macroscopic earthquake damage investigations and strong motion observation data show that the local site conditions can significantly affect the seismic ground response, especially the soil deposits have tremendous amplification effect on the surface ground motions. One of the main approaches for earthquake preparedness and disaster reduction is seismic fortification, for both general and major engineerings. The rationality and reliability of design ground motion parameters will directly affect the safety and economy of construction projects. As one of core techniques in seismic regionalization and seismic safety evaluation of major engineering, seismic response analysis of soil sediments is currently the most important method on the research of site condition effect to the earthquake response, whose purpose is to provide reasonable and reliable design ground motion parameters for seismic fortification of engineering structure.The equivalent linear program LSSRLI-1 is mainly used for seismic safety evaluation of engineering sites in our country at present, which represents the international advanced level in the 1980 s. It has been almost 30 years since the program was proposed, and which has made outstanding contributions for seismic zoning and seismic safety evaluation career in China. But in the engineering practice, LSSRLI-1 gradually shows some deficiencies, either in some soft field or when the input vibration intensity is strong, the calculated results is sometimes unsatisfactory. SHAKE2000 and DEEPSOIL respectively represent the international advanced level of the equivalent linear method in the frequency domain and the absolute ninlinear method in the time domain. Comparative analysis results indicate advantage than LSSRLI-1 on the whole, however the underestimation of ground motion amplification on thick soft sites is also quite obvious.This thesis is based on the urgent needs of our country engineering construction, and aims at the major remained deficiencies of international earthquake response analysis methods, including the severe underestimation of seismic ground response at deep soft sites and the lack of high frequencies components when using the conventional iterative method at hard sites, put forward the new generation calculation method of soil layer seismic response and compiled the corresponding program SOILQUAKE. The main salient points and achievements obtained can be outlined:(1) On the basis of the classical wave motion theory, the linear viscoelastic exact solution was derived, which is used for site earthquake response analysis. According to the comparison results of 126 soil groups within four different site classes, it can be clearly seen the most critical problem of our current seismic safety evaluation program is that the shear strain is always overestimated at soft sites, which predicts a ground motion of lesser intensity. By improving the shear strain calculation method, the equivalent linear response analysis program is rebuilded, which was named SHAKE-China and proved to be having the same precision with SHAKE2000.(2) New cyclic triaxial(CT) divice with high precision was used to carry out a series of expert experimental studies, the main objective of which is to evaluate the influence of loading frequency on the shear modulus and damping ratio of typical sand and clay samples. On the basis of the test results, the influence models of vibration frequency on the reference shear strain γr and the maximum damping ratio Dmax were given, as well as the correction coefficient curves. The necessity of considering the frequency correlation of shear modulus and damp was evaluated from the perspective of ground motion. The results show that the loading frequency has little effect on dynamic shear modulus and damping ratio for cohesionless soil samples, the influence can be ignored. For clay soil, the shear modulus and normalized modulus ratio increase but damping ratio decreases with the increase of loading frequency, and the effects are extreme significant. This study therefore demonstrates that the significant deviation would happen to the ground response analysis results on the sites which contains clay layer if without considering the frequency correlation of shear modulus and damping ratio.(3) The accurate theoretical solution on the relationship between shear strain and vibration velocity was proposed and emphasized on the situation of horizontally layered sites undergoing earthquake loading. It can be easily and clearly seen that the relationship between shear strain and vibration velocity was strongly affected by the wave frequency, therefore the now available frequency-considered methods imply an obvious qualitative error, because all these existing so called improved methods are generally based on the hypothesis that the ratio between dynamic shear strain and vibration velocity is a constant proportion.(4) The ultimate aim of this dissertation is to put forward the new generation seismic response analysis method through the approach of taking into account a direct calculation way of frequency-dependent shear modulus and damp ratio. The new technique was compiled and named SOILQUAKE(with Chinese meaning of the first one to overcome a key problem), which means this program made a great progress in solving the stubborn shortage of avaiable site response analysis programs, specifically speaking, the extreme underestimation of ground response at deep soft sites and the lack of high frequencies components when using the conventional equivalent linear method at hard sites.(5) The new program SOILQUAKE was verified using actual downhole array strong motion records. Analysis results show that SOILQUAKE outputs are very close to SHAKE-2000 and DEEPSOIL and behave small deviations to the real acceleration histories at hard and medium hard sites, but SOILQUAKE result is more reasonable than SHAKE2000 within the range of high frequencies and about the same with DEEPSOIL. For general soft ground and thick soft sites, SHAKE2000 and DEEPSOIL substantially underestimated the local site effect, the results of these two programs are of little reliability, while SOILQUAKE result is the closest to the actual records on the whole with its small relative deviation on peak acceleration values less than ±20%, within the acceptable range. The performance of SOILQUAKE on general soft ground is of significant advantages, while on thick soft sites, its behavior is even more excellent.No matter in theory or from the point of comparison results, the new calculation program SOILQUAKE proposed in this thesis made successful improvements to the traditional methods, the reliability and accuracy reveal comprehensive breakthrough than today’s international advanced equivalent linear and completely nonlinear methods, i.e. SHAKE2000 and DEEPSOIL. SOILQUAKE has generally overcome the common defect of existing methods that is the excessive underestimation of earthquake ground motion at soft sites, so it could be called the new generation of seismic response analysis method.