[H-DS07] Landslides and related phenomena

[HDS07-10] Unraveling the role of halloysite on the initiation and movement of coseismic landslides of pyroclastic fall deposits: a case study

*Gonghui Wang¹, Baichuan Liu², Issei Doi¹, Toshitaka Kamai¹, Takahiro Ohkura³ (1.Disaster Prevention Research Institute, Kyoto University, 2.(former) Graduate School of Science, Kyoto University, 3.Graduate School of Science, Kyoto University)

Keywords:coseismic landslide, tephra slope, Halloysite, shear behavior, water retention

Coseismic landslides of pyroclastic fall deposits are normally characterized by their high mobility and long runout, and usually present significant hazard to human safety and properties. This kind of landslides could be exampled by those triggered by the 2016 Kumamoto earthquake on Minami Aso area, Kumamoto, and by those triggered by the 2018 Hokkaido Eastern Iburi Earthquake in the hills near the epicenter. Field study on those landslides revealed that most of the landslides occurring on tephra slopes are shallow but destructive, the displaced landslide materials mainly consist of tephra, and their sliding surfaces are mostly developed within the weathered tephra layer, where the water content is very high and a clay mineral of halloysite resulting from the weathering of volcanic glass is normally presented. However, the role of halloysite on the initiation and movement of these landslides has not been clarified yet. Therefore, in this study, taken the landslide triggered by the 2016 Kumamoto earthquake on Takanodai area, Minami Aso as the example, we conducted a series of field monitoring and laboratory tests (including pF tests and undrained ring shear tests) to examine the effects of introduction of halloysite on the water retention, shear strength of tephra materials. Our test results show that introduction of halloysite in the tephra can elevate the water retention capacity such that the tephra can be in high water content state in natural condition. Introduction of halloysite does not affect the peak shear strength of tephra materials, but can elevate the liquefaction potential, such that the shear failure could be easily triggered and the mobility of displaced landslide material be elevated.