Abstract
It is important to clarify the characteristics of groundwater, which may induce landslides, in order to plan the prevention of landslides, and also to clarify the three-dimensional characteristics of groundwater for large-scale landslides. In this study, for a large-scale landslide area located in a snowy region, we investigated the fluctuations of threedimensional groundwater during the snowmelt period by field observation and numerical analysis. The investigation focused on the thick pyroclastic flow deposit layer that had accumulated in the surface layer. It is considered that this layer plays an important role in transmitting groundwater pressure to the deep slip surface. The following results were obtained. The groundwater level of the landslide area increases as meltwater increases during the snowmelt period, and the magnitude of the rise in water level and groundwater flow direction of each point in the landslide are different. Groundwater tends to be stored in the caved-in part of the upper part of the landslide slope. These characteristics were estimated based on the topography and geological stratum shape. Furthermore, groundwater drainage for preventing landslides was examined using an analytical model. It was shown that draw-down by groundwater drainage affected the depth of drainage and flux of groundwater.
