Surface soil effects on vertical ground motion

摘要

Strong ground motions recorded at Port Island, Kobe, in the 1995 Kobe earthquake showed that, while the horizontal accelerations were de-amplified at the surface, the vertical acceelration was greatly amplified. It is expected that the strong amplification of vertical motion was attributed to the partial saturation of surface fill layers. In this paper, a study is conducted on such surface soil effects in vertical ground motions. The characteristics of surface soil properties such as P-wave velocity, Poisson's ratio and degree of saturation are discussed based ont he two-phase theory. An analytical study is then made for a simplified problem to investigate the partial saturation effect on vertical amplification function. Finally, an nonlinear, fully coupled finite element analysis is carried out to study the response of the Port Island site to the recorded earthquake excitations. The partial saturation condition of surface fills is included in the numerical analysis.