LANDSLIDES TRIGGERED BY HEAVY RAINFALL IN AMAMI-OSHIMA ISLAND, KAGOSHIMA, JAPAN

Many landslides were triggered by the three heavy rainfalls in the Amami-Oshima Island, Kagoshima Prefecture, Japan in October, 2010, September, 2011 and November, 2011. Especially, the 2010 heavy rainfall on the Amami city from October 18th to 21st was the highest recorded torrential downpour in a century. During recent years, climatic aberrations that result in this type of heavy rainfall have been common and could expect to increase the risk of landslide disasters in the Amami-Oshima Island in the future. In this study, we examined the topographical features and geotechnical properties of landslides of Amami-Oshima Island in order to consider a rational procedure for preventive and mitigation measures for landslide disasters.

The study includes landslides that are located in the area of the base of the Shimanto geological group, which had been formed during the Mesozoic and Cretaceous periods. The topographical features of cross sections of landslides, namely, length ”L”, thickness ”D” and slope “α”, do not show any relationship between each other.

In the c′-tanφ′diagram drawn for pre-occurrence stage, which was computed by a 2D stability analysis formula, the values of intercept, c,′ranged between 34.9-108.7kN/m², of tanφ′ranged between 0.65-0.76. For the post-occurrence stage, intercept, c,′and tanφ′shifted to 23.9kN/m² and between the range of 0.46-0.53, respectively. The value of c,′has varied widely, while tanφ′fallen within a narrow range. Influence of L, D and α on the c′-tanφ′diagram were discussed in relation to the alteration of c′-tanφ′line. Obtained results are as outlined below. The influence of L is very minimal. D has an effect on c′-intercept, but has little impact on tanφ′-intercept, whereas α has a significant impact on c′-interceptandtanφ′-intercept.

Soil samples were collected from the scarp at the four sites of Ura, Ashikebu, Akinagawa and Sokaru. The soil sample of Ashikebu landslideis characterized by a high clay content and a high plasticity index. As for the mineralogical properties, all samples were dominated by quartz, and did not contain smectite, which is often found in landslide soils. The fully-softened strength and residual strength of the Ashikebu sample were 38.2°(φ_sf) and 17.7°(φ_r) at an assumed c of 0 kPa under the normal stress of 200 kPa.