Paper:

# Rigorous Analysis of Stress-Dependent Landslide Movements with Groundwater Fluctuations Applicable to Disaster Prevention in Monsoon Asia

## Deepak Raj Bhat^{*1,†}, Soichiro Osawa^{*2}, Akihiko Wakai^{*2}, Katsuo Sasahara^{*3}, Netra P. Bhandary^{*4}, Fei Cai^{*2}, Hirotaka Ochiai^{*5}, and Norihiro Tanaka^{*1}

^{*1}Okuyama Boring Co., Ltd.

3-5-9 Higashi-nihonbashi, Chuo-ku, Tokyo 103-0004, Japan

^{†}Corresponding author

^{*2}Gunma University, Gunma, Japan

^{*3}Kochi University, Kochi, Japan

^{*4}Ehime University, Ehime, Japan

^{*5}Japan Forest Technology Association, Tokyo, Japan

In this study, novel finite element approaches are proposed for numerical analysis of stress-dependent landslide movement with groundwater fluctuation by rainfall. Two new constitutive parameters that are capable of directly controlling the relationship between the apparent factor of safety and sliding velocity are incorporated into a specific material formulation used in finite element analysis for the first time. For the numerical simulation of the measured time history of the sliding displacement caused by the groundwater fluctuations, such required analytical parameters can also approximately be determined by back analysis. The proposed models are applied to a landslide field experiment on a natural slope caused by rainfall in real time in Futtsu City, Chiba Prefecture of Japan to check its applicability. The predicted and measured time histories along the horizontal direction on the upper, middle, and lower slope are compared. In addition, the deformation pattern, shear strain pattern, and possible failure mechanisms of the natural slope of such a field experiment landslide are discussed in detail based on the analysis results of the finite element method (FEM)-based numerical simulation. Moreover, the creeping landslides and possible landslide sites for further application of the proposed models are briefly discussed in the cases of Nepal and Japan as examples in Asia. It is believed that the proposed newly developed numerical models will help in understanding the secondary creep behavior of landslides triggered by extreme rainfall, and at the same time, long-term management of such landslides will be much easier in monsoon Asia. Finally, it is expected that this study will be extended for simulation of the tertiary creep behavior of landslides induced by rainfall in the near future.

*J. Disaster Res.*, Vol.16, No.4, pp. 658-673, 2021.

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