Fluctuations of Pore-water Pressure in a Landslide of Heavy Snow Districts

Abstract

Pore-water pressure fluctuations were observed in a Tertiary landslide, which is located in a heavy snow district, using an automated data acquisition system. Meltwater and/or rainfall (MR (a/o)), snow-related factors including water equivalent of snow, and landslide movement were simultaneously obtained. The results of observation for two consecutive years showed the long-term effects of snow load on pore-water pressure, such as a gradual rise of pore-water pressure as snow accumulated. An investigation on the time-lag and response ratio of pore-water pressure against MR (a/o) for pre-snow, snow, and post-snow cover periods revealed that the time-lag of pore-water pressure against MR (a/o) was not uniform before snow started to accumulate, but showed a distribution curve with a clear peak during late snow cover periods. The response ratio of pore-water pressure against the MR (a/o) value integrated for one event was investigated, with followings results. The relationship between pore-water pressure and MR (a/o) was diverse in pre-snow cover periods but showed a high correlation in snow cover periods. The response ratio of pore-water pressure against MR (a/o) was high before the snow cover period, gradually decreased as snow accumulated, and was only one tenth of the value in the pre-snow cover period in late snow cover periods. The response ratio remained low even after snow had disappeared and required 500 to 600 hours to return to the level of the pre-snow cover period. Observations of pore-water pressure inside the landslide mass revealed that changes in pore-water pressure during snow cover periods are quite different from those in other periods. These results suggest that the hydrologic coefficients of landslide mass are strongly affected by snow cover.