It is very important for preventing the disaster by the landslide to estimate the landslide reaching distance. In this paper, it was tried to estimate the landslide reaching distance by analyzing past landslide cases. Before analyzing them, cases where the shape before and after the landslide movement was apparent were extracted from the literature and the report, etc., and they were objects of this study. As a result of patternizing the cases, mudstone and sandstone (including an alternation of sandstone and mudstone) amount to half the number of the population and Tertiary Period do to about 70 percent, therefore geological features of the landslide became clear. In multiple regression analysis, the extremely precise predicted expression was gained by assuming the prediction index to the slope gradient after the landslide movement : α, and doing the explanatory variable to between the slope gradient before it : θ and width of the landslide : W. Especially, the multiple correlation coefficient became R=0.923 on the mudstone in Tertiary Period. Moreover, significance and validity were confirmed by analysis of variance and residual analysis.
Groundwater flow due to rainfall was simulated using the Darcy's law both analytically and numerically to understand its effects on slope failure. Stationary solutions on the groundwater flow describing ground water table were obtained with implicit functions. In numerical calculations, effects of rainfall pattern, rainfall intensity and shape of slope layer on the development of groundwater flow were examined. The results were summarized as follows : 1) Under a constant rainfall condition, the water level of groundwater flow increased gradually with distance near the upper part of the slope, and developed with time in parallel to the slope at the middle and lower parts of the slope ; 2) An approximate solution applicable to a steep slope provided the boundary above, development and retreat of the flow numerically ; 3) When rainfall ceased, groundwater totally moved downward, keeping the water table ; 4) Heavy rainfall raised the water level of the flow at the top of the slope ; and continuous rainfall formed higher water level through the slope than intermittent rainfall in a condition of the same total amount of rainfall ; 5) Intermittent rainfall brought an increase in water level of the flow near the foot of the slope resulting in a small-scale slide at the foot that consequently triggered a large-scale landslide ; 6) When the slope was gentle toward its toe, the peak water level of the flow was found at the boundary and the peak water level continued to increase for a while after the rain ceased ; and 7) The stability of a unit slope was estimated as a function of time using an approximate solution.
The authors analyzed the landslide displacement and fluctuation of pore water pressure at a reactivated landslide which moves more than one meter a year in Niigata Prefecture from the results of monitoring every ten minutes. The distribution of landslide displacement caused by the earthquake matched that caused by rainfall. Pore water pressure showed variations at each monitored point. Considering a relationship between the distribution of landslide deformation and of the pore pressure, the variation in pore water pressure is considered to be generated by local compression or extension in the landslide mass. Although the amount of landslide movement was large before the earthquake, it fell immediately after the earthquake, and remained low until the beginning of the next snow-melt season.