Abstract
Heavy rainfall over a wide area of northern Japan from August 26 to 31, 1998, triggered many landslides which caused serious damage to human lives as well as houses and civil engineering structures, particularly in Fukushima Prefecture. We report preliminary finding of a hydro-geological study of two landslide sites, Nishigo and Taishin, Fukushima. When the stability of slopes during a rainstorm is discussed, both the thickness and hydraulic conductivity of the surface soil must be taken into account, in addition to the rainfall intensity and cumulative rainfall. These hydro-geological factors are important, in particular for the prediction of the time of slope failure. In certain cases, the hydro-geological structure of the surface soils can be used as a diagnostic factor for slope stability. For example, an impermeable zone can obstruct the outflow of seepage water from a slope, so that a kind of underground dam is accidentally constructed in situ. In such a case, a site investigation, in which much attention is paid to the hydro-geological structure, is crucial for the discussion of slope stability.
