Mechanism of creep movement caused by landslide activity and underground erosion in crystalline schist, Shikoku Island, southwestern Japan

The mechanism of creep movement of the Zentoku landslide in crystalline schist has not been studied in detail because of the steepness of the slope, very slow movement, low population density and complex topographic and geologic characteristics. Sassa et al. (1980: Proc. INTERPRAEVENT 1, 85–106) and Sassa (1984: Proc. 4th International Symp. on Landslides, Toronto, vol. 2, pp. 179–184; 1985. Geotechnical classification of landslides, Proc. 4th International Conference and Field Workshop on Landslides, Tokyo, pp. 31–40; 1989: Landslide News, Japan Landslide Society, No. 3, pp. 21–24) monitored landslide movement and groundwater level at the Zentoku landslide on Shikoku Island, southwestern Japan, and suggested that the mechanism may be caused by underground erosion. To study the influence of underground erosion at this site, continual monitoring of suspended sediment and water discharge from a groundwater outlet (i.e. a spring) was implemented. The locations of groundwater flow paths were determined, as were the amounts of discharged sediment. Slope deformation was monitored by means of a borehole inclinometer. The conclusions were as follows: (1) the flow paths were found to be on or above the shear zones in which underground erosion has occurred; (2) in addition to being a result of precipitation and groundwater discharge, sediment discharge is affected by landslide activity; and (3) the mechanism of creep movement is an interrelated chain process that combines underground erosion caused by landslide activity with landslide activity caused by underground erosion. Thus, landslide activity increases erosion susceptibility and transportation of soils within the mass, and underground erosion causes instability of the landslide mass, in turn.

This mechanism can explain the observed phenomenon that the Zentoku landslide not only moves actively during heavy rain, but also continues to creep throughout the year.