Modeling the interaction of geomaterials with pore fluids is one of the key issues in numerical geomechanics. In 2004, many disasters caused by heavy rainfall and flooding occurred in various areas throughout Japan. These rain-related disasters were probably accompanied by geodisasters, e.g., flows of debris and slope failures in mountain areas and river dike breaks in plain areas. Although the disasters are of course very unfortunate, it is anticipated that data from them will contribute to geomechnics in terms of future geo-disaster prevention. Because most of these geo-disasters are related to the behavior of pore fluids in geomaterials, it is important to construct and use an appropriate model of the interaction of geomaterials with pore fluids.
In the present paper, a newly developed evaluation method for the safety of river dike embankments during flooding is introduced first. This method consists of a deformation analysis that can simultaneously consider stability and unsaturated seepage flow. It is quite different, therefore, from the conventional evaluation method in which stability and seepage flow are considered separately. A soil skeleton-pore water coupled elasto-plastic finite element analysis is applied to the problem by incorporating the unsaturated seepage characteristics, and by assuming the pore air pressure in the unsaturated soil region to be atmospheric pressure. The deformation and the stability of a river dike embankment during a flood are investigated for various cases of initial saturation and permeability. The results of the analysis show that the existing evaluation criterion for the seepage failure of river dike embankments is not always on the safe side.
Secondly, the problems to be solved are discussed. A multi-phase coupled analysis, which considers the interaction of geomaterials not only with pore water, but also with pore air, is shown. The present analysis has been applied to simulate the experimental results of triaxial tests on unsaturated silt under unexhausted and undrained conditions. This method is expected to be a useful numerical tool for predicting deformation and stability of unsaturated river dike embankments during floods in practical fields. However, more complex problems exist which are difficult to solve with the present numerical analysis method, which is based on continuum mechanics. Some examples of these unsolved problems are also introduced, namely, river dike breaks due to flooding and the progressive seepage failure of sandy deposits that include discrete air bubbles.