Abstract
In recent years, many slope failures occurred in the zone of Shimanto formation, which produce the sediment amounted to be in the range of 104-105m3. By the comparative investigation of the failure mechanism with those occurred in the mountains of Granitic rocks which will produce one order less volume of sediment (<104m3) and assuming that the ratio of depth against longitudinal length of moving soil mass in both geology to be almost the same, authors found that the important morphological factor is the steep and longer slope length, whereas the inclination has similar tendency of the distribution. In Granitic rock mountains, slides are generally thin and the underlying base rock will never collapse, on the other hand, for the case of Shimanto formation, slip surface will develop deep beneath the surface until the depth of loosened fissure rich base rock. By the analysis of measured records and the video of a slope failure occurred in Aki county, Kochi prefecture in 1998, precursive movements show the creep characteristics and the function of root network system which is believed to work for defending the slope failure could not recognized in case of the reforested Sugi (Cryptomeria japonica). The growing effects of a single tree onto the soil were simulated in use of the direct shear test apparatus.
