Mechanism of a long-runout landslide triggered by the August 1998 heavy rainfall in Fukushima Prefecture,Japan |
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| Institution: | 1. School of Civil Engineering, Guangzhou University, Guangzhou 510006, China;2. Department of Environment and Civil Engineering, Toyama Prefectural University, Imizu 9390398, Japan;3. Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Ube 7558611, Japan;1. Three Gorges Research Center for Geo-hazard, China University of Geosciences, Wuhan, 430074, China;2. Department of Earth and Planetary Sciences, Washington University, One Brookings Drive, Saint Louis, United States;1. Department of Civil Engineering, National Cheng Kung University, 1 University Road, Tainan 70101, Taiwan;2. National Cheng Kung University, 1 University Road, Tainan 70101, Taiwan;3. Former Graduate Research Assistant, National Cheng Kung University, 1 University Road, Tainan 70101, Taiwan;1. College of Geological Engineering and Surveying of Chang''an University, Key Laboratory of Western China Mineral Resources and Geological Engineering, Xi''an 710054, China;2. State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Collaborative Innovation Center of Geological Prevention, Chengdu 610041, China;3. Research Centre on Landslides, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan;4. Department of Earth Sciences, Abbottabad University of Science and Technology, Abbottabad, Pakistan;1. Key Laboratory of Mountain Hazards and Surface Process, Institute of Mountain Hazards and Environment, Chinese Academy of Science, Chengdu, Sichuan 610041, China;2. Saitama University, Department of Civil & Environmental Engineering, Saitama 338-8570, Japan;3. Chuo Kaihatsu Corporation, Shinjuku-ku, Tokyo 169-8612, Japan |
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| Abstract: | Heavy rainfall from 26 to 31 August 1998 triggered many landslides in Nishigo Village of southern Fukushima Prefecture, Japan. The Hiegaesi landslide, a long-runout landslide with travel angle of 11°, which occurred in loamy volcanic-ash/pumice layer and was deposited in a nearby rice paddy, was investigated. In an observation pit dug in the middle part of the landslide deposit, the sliding zone just above the deflected rice plants was observed, and it was confirmed that grain crushing occurred in the sliding zone. The triggering and sliding mechanisms of this landslide then were investigated by ring-shear tests in laboratory. For the triggering mechanism, one saturated naturally drained test (test A: torque-controlled test) and one saturated undrained test (test B: speed-controlled test) were conducted on the samples taken from the source area of the landslide. Even in the naturally drained test opening the upper drain valve of the shear box, a temporary liquefaction occurred. In the undrained test, excess pore-pressure was generated along with shearing, and “sliding-surface liquefaction” phenomenon was observed. The effective stress and shear resistance finally decreased to near zero. These results can explain the observed phenomenon of small friction resistance like a flow of liquid when the sliding mass slid out of the source area. For the sliding mechanism of the landslide in the rice paddy, saturated undrained test (test C: speed-controlled test) was performed on soil sample above the deflected rice plants. The apparent friction angle obtained in this test was 8°. In addition, the residual friction angle measured after test B and test C was the same value of 41°. Combining with the observation on the shear zone in the ring-shear box after test C, it is concluded that, during the sliding in rice paddy, the undrained shear strength of the soil layer itself mainly influenced the high mobility of the landslide, probably because the friction between rice plants and soils is greater than the undrained shear strength inside the soil mass. |
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