UNCERTAINTY IN SITE CHARACTERIZATION USING SURFACE WAVE TECHNIQUE

Abstract

Application of surface wave method is being increasingly used in geotechnical site characterization of near surface materials. The dispersion property, which makes the Rayleigh wave velocity frequency dependent is utilized in surface wave methods. Surface wave methods determine the small strain shear modulus of shallow subsurface materials, and this shear modulus is the key input in the evaluation of the soil response under dynamic loading. So, the accuracy of testing is very much important, otherwise it may lead to serious consequences on the seismic hazard studies. There are different uncertainties associated with surface wave methods. These uncertainties can be broadly classified into three categories: Model based uncertainty, Data measurement uncertainty and Inversion uncertainty. In this study, these uncertainties are studied with the help of numerical simulations and experimental studies to examine the error associated with each of them in order to increase the reliability of the application of surface wave methods. Surface wave methods, which may suffer from near-field effects result in underestimation or overestimation of Rayleigh wave phase velocity due to the contamination of body waves near to the source. A detailed numerical and experimental study have been conducted to simulate the near-field effects. The study shows that the impedance contrast between the half-space and overlying soil layer is having a considerable effect on the underestimation of phase velocity. With the increase of impedance contrast the level of underestimation also increases. The near-field effects not only depend on impedance contrast, number of receivers and type of soil profiles also play an important role. Lesser number of receivers and inversely dispersive soil profiles are found to have a much significant effect. Finally, the underestimation of Rayleigh wave phase velocity for different types of soil profiles with different impedance scenarios is quantified in terms of two normalized parameters. Data measurement uncertainty is another major source of uncertainty, which arises while conducting the surface wave tests. Data measurement uncertainty basically arises from the noise present in the surroundings in the form of continuous or transient signals. Noise results in a scatter in the measured dispersion curve and makes the results uncertain. Now, while to extract the shear wave velocity profiles, this scatter in the dispersion curve may provide different velocity profiles which are falling in the range of measured data variation. Data uncertainty largely depends on the noise present in the surrounding environment. ii Different sites may show variable data error if a repetition test is performed keeping the configuration same. All the experimental results from large number of repetition tests at 9 different sites are compiled to generate a data variation bound after statistical analysis. The equations of the upper bound curve and mean curve are proposed, which can be used to account the effect of data uncertainty, where the repetition tests are not conducted. The consequence of data uncertainty on 1D seismic ground response analysis is also studied. The study shows the considerable effect of data uncertainty on seismic ground response analysis. In surface wave tests, experimental dispersion curve is constructed from the field data using different processing techniques. This experimental dispersion curve is then used to extract the shear wave velocity profile using inverse problem solution. The solution of the inverse problem is non-unique and results in several equivalent velocity profiles, with a good fit with the experimental dispersion curve. In this study, an attempt has been made to quantify the consequences of inversion uncertainty in the surface wave methods on 1D seismic site response. The study shows that equivalent profiles resulting from non-uniqueness of surface wave inversion are not equivalent in terms of their seismic site response. It is also found that effect of non-uniqueness of surface wave inversion on seismic ground response is considerably influenced by characteristics of the input ground motion.