In general, a seepage model of groundwater is calibrated through simulation of the monitored groundwater levels. However, it is often difficult to simulate groundwater levels of an area, including landslides, which tend to be scattered in both time and space due to irregular groundwater flow through inhomogeneous landslide mass. This paper describes one approach to appraise the distribution of groundwater that may be inducing the movements of the Jinnosuke-dani landslide, using 3D seepage model. First, emphasis was placed on close observation of the monitored data to find groundwater levels that exhibit similar trend in time. Furthermore, correlation between the groundwater levels, showing similarity, and the displacements suggests that it is possible to locate the groundwater tables that may be one of the triggering sources. Three dimensional seepage simulations were then performed to reproduce the measured groundwater levels. The simulated groundwater levels and estimated hydraulic properties are used in appraising the hypothetical mechanism of the movements suggested by the measurements.
The Kokugawa landslide occurred on 7th March 2012 in Joetsu, Niigata Prefecture. To clarify the characteristics of the landslide's movement, we analyzed the form of the landslide movement and displacement based on LiDAR data, aerial photography, field measurements, and trench profiles. The landslide slope was divided into initiation, transfer, and deposition zones. The massmovement type of the landslide was complex earthslide-earthflow, which initially occurred as a rotational slide in the initiation zone, transformed into an earthflow in the transfer zone, and then reached the deposition zone. The velocity of the mass reaching the deposition zone declined to 0.11m/h by 10th March. The movement lasted for 16days due to continuous movement with a velocity of less than 1m/h. The linear movement of the mass along the slope direction at the transfer zone continued after the mass had reached the deposition zone. Deflection and slowdown of the mass did not occur, except at the landslide toe, probably due to the generation of excess pore pressure by undrained loading on the impermeable alluvial clay layer, which caused a significant decrease in shear resistance in the clay layer. Snow in the deposition zone was pushed by the landslide and formed a swelled snow wall around the mass. This suggests that the snow wall confined the lateral spread of the mass in the deposition zone, but the mass did not entrain much snow at its base and interior.