Groundwater Flow Behavior at Landslide Area in Crystalline Schist Mountains

In this study, we performed the 1-m-depth ground temperature survey and continuous ground temperature monitoring at the Zentoku landslide area and the Nishi-ikawa landslide in the crystalline schist mountains to clarify the actual groundwater flow in landslide slopes. Our results showed that the ground temperatures at differing locations of the slope were different, probably due to the influence of veins of groundwater flow. We also found that the ranges of these differences in ground temperature were different for different landslides in the same mountain or different mountains. To clarify the reasons for these differences, we compared the data obtained from these two different landslide areas and also those data obtained from other case studies with groundwater dating. We inferred that during a rainfall, the groundwater from deep layer, which is relatively cold in summer and warm in winter than shallow groundwater, and wasn’t formed just after heavy rain, but infiltrated previously (approximately two to nine years ago), might have also been expelled out, resulting the greater difference in ground temperature. Namely, the groundwater flowing in the crystalline schist slope involves not only the newly infiltrated rainwater, but also those water concentrated flow from deep layer with some residence time. Therefore, it is suggested that this kind of groundwater flow should be considered in the instability analysis of landslides on the crystalline schist slopes.