The Influence Of Soil Stratification And Ground Characteristics On The Microtremor Tests

Microtremor test has been widely used for soil characteristics of sites. It provides useful information for earthquake-resistant evaluation of building and seismic design of foundation. The common test is used to provide predominant period and frequency analysis for site classification and earthquake-hazard investigation. Because microtremor is a kind of micro random vibration, and it is influenced by site condition, test results may not be able to reflect real characteristics of site soils. To obtain true data for engineering earthquake-resistant, researchers must make a good comparison for natural frequency result from field tests and theoretical calculations.The general characteristics of site microtremor spectrum and distributed regularity are studied based on a large number of site tests and analyses in Fuzhou district. The data indicate that the thickness of cover layer and soil stratification will influence microtremor spectrum and distributed regularity directly. The foundation soil layer could be regarded as a system, waves transmitted from different seismic origin travel in this system, then, the response will be different as the soil foundation structure changed. In light of this theory, when the primary ground is covered with fresh filled soil or the foundation is improved significantly, the soil structure will be changed at the same time, thus, the predominant frequency of the same site will be also changed. This phenomenon has been verified by several site investigations.A simple ground soil stratification moldel has been founded in this paper, finite element method and simplified V-H method, including average method, sumed method, revised formula method , are used to calculate site period, and the two solution (finite element method and simplified V-H meth) are comparied used in natural ground period anlysis.Contrast and analysis have been made based on different observe points in the same site, and the influence of three factors (including top layer with loose sand, foundation improvement and near building) for microtremor test are also discussed, some worthy conclusions are obtained which could be used to consult practice projects.Meanwhile, a good understanding of the site spectral structure characteristic could help us to acquire overburden soil physical property. The conclusion will contribute to the application of microtremor theory in ground evaluation. It also offers the reference basis for earthquake-resistant design and site selection in Fuzhou.Finally, a brief discussion is made in terms of how to apply microtremor theory to practice project in better manner.