Atsushi Mikami

Simple formulations of ground impedance functions for rigid surface foundations

A differential equation is formulated for the dynamic response of ground medium by using a simplified ground model. Applying Galerkins procedure for weighted residual, this equation leads to a governing equation only at the ground surface. The equation indicates that the ground surface behavior can be computed even further by a simplified model. By solving the governing equation for the boundary conditions along the surface, expressions in simple closed forms are developed for the dynamic response analysis of a massless rigid foundation that rests on the ground surface. Despite their significant simplicity, the developed expressions compute the values very close to those computed by far more complex rigorous solutions. They are found to be capable of capturing the important characteristics of the dynamic ground behavior well.

Deformation build up within a granular assemblage during an intense earthquake

The Great Hanshin-Awaji Earthquake of 1995 has left traces of its intense seismic motion in granular soils at various places in Kobe, and these signs are found to be somehow correlated with the features of the disaster at those places. Strong ground shaking was responsible for a number of slope failures. Considerable displacements seem to have been built up even within nearly flat sandy soil deposits, a fact proven by the data regarding dislocated rings of manholes. Thus the importance of studying behaviours of granular soil experiencing this large strain emerges from the findings obtained through the investigation. Irreversible deformation of a granular material is mainly due to a change in its fabric. This process is accompanied by noticeable dilation, through which a certain amount of energy is consumed. It is therefore noted that ground velocity as well as acceleration is another important key which provides a stability index. On the basis of findings through experiments using a new visualisation tech...