INFLUENCE OF LOCAL SITE CONDITIONS ON STRONG GROUND MOTION

Abstract

It is a well established fact that local site conditions have a very important role in the response of structures towards strong ground motion. The soil and rock at a site have specific characteristics that can significantly alter the input motion in terms of its amplitude, frequency contents and duration. Several evidences are found in the literature about the influence of local site conditions on earthquake vibration. The dissertation work tries to understand some of the basic aspects of the effects of local site conditions on strong ground motion through the perspective of one dimensional ground response analysis. The local site conditions investigated are type of soil, subsurface layering and bed rock depth. Bhuj earthquake, 2001 is considered as the input motion. Acceleration time history at the surface, response spectra at 2, 5, 10 and 20% damping, amplification ratio and Fourier spectra are obtained using SHAKE2000 [equivalent linear analysis]. An attempt is made to correlate these results to the effect of bed rock depth in seismic site characterization for sites with shallow shear wave velocity model. Average Shear Wave Velocity up to 30 meters [ V (30) ] is a common parameter for classifying sites. But often problems are encountered in obtaining V(30) for sites where soil column doesn't extend upto 30 metres. In those cases, extrapolation is done. However, none of these methods considers the effect of bed rock depth on site classification. A method is proposed in this dissertation work to incorporate the effect of bed rock depth by taking into account the actual bed rock shear wave velocity. Site classification is done according to NEHRP site classification scheme and Eurocode-8. A comparative study of - the two conventional extrapolation techniques: extrapolation assuming constant velocity (Boore, 2004); extrapolation using the correlation between V. (30) & V (d) (Boore, 2004); and the proposed method is also included in this thesis. Better site class is obtained adopting the proposed method.