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.

1. [1] K. Terzaghi, “Mechanism of Landslide,” S. Paige (Ed.), “Application of Geology to Engineering Practice: Berkey Volume,” pp. 83-123, Geological Society of America, 1950.
2. [2] G. Ter-Stepanian, “On the Long Term Stability of Slopes,” Norwegian Geotechnical Institute (NGI) Publication No.52, pp. 1-14, 1963.
3. [3] H. Eyring, “Viscosity, Plasticity, and Diffusion as Example of Absolute Reaction Rates,” J. Chem. Phys., Vol.4, pp. 283-291, 1936.
4. [4] E. Shimokawa, “Creep Deformation of Cohesive Soils and its Relationship to Landslide,” Mem. Fac. Agr. Kagoshima. Univ., Vol.16, pp. 129-156, 1980.
5. [5] J. Feda, “Interpretation of Creep of Soil by Rate Process Theory,” Géotechnique, Vol.39, No.4, pp. 667-677, 1989.
6. [6] S. Murayama and T. Shibata, “Rheological Properties of Clay,” Proc. of the 5th Int. Conf. on Soil Mechanics and Foundation Engineering, Part 1, pp. 269-273, 1961.
7. [7] R. W. Christensen and T. H. Wu, “Analysis of Clay Deformation as Rate Process,” J. Soil Mech. Found. Div., Vol.90, No.6, pp. 125-157, 1964.
8. [8] G. Ter-Stepanian, “Creep of a Clay During Shear and its Rheological Model,” Géotechnique, Vol.25, No.2, pp. 299-320, 1975.
9. [9] B. C. Yen, “Stability of Slopes Undergoing Creep Deformation,” J. Soil. Mech. Found. Div., Vol.95, No.4, pp. 1075-1096, 1969.
10. [10] J. N. Suhaydu and D. B. Prior, “Explanation of Submarine Landslide Morphology by Stability Analysis and Rheological Models,” Proc. of 10th Offshore Technol. Conf., pp. 1075-1082, 1978.
11. [11] R. M. Iverson, “A Constitutive Equation for Mass-Movement Behavior,” J. Geol., Vol.9, No.3, pp. 143-160, 1985.
12. [12] H. Sekiguchi and H. Ohta, “Induced Anisotropy and Time Dependency in Clays,” Proc. of the 9th Int. Conf. on Soil Mechanics and Foundation Engineering (ICSMFE), pp. 229-238, 1977.
13. [13] C. Zhou, J.-H. Yin, J.-G. Zhu, and C.-M. Cheng, “Elastic Anisotropic Viscoplastic Modeling of the Strain-Rate-Dependent Stress-Strain Behaviour of K₀-Consolidated Natural Marine Clays in Triaxial Shear Test,” Int. J. Geomech., Vol.5, No.3, pp. 218-232, 2006.
14. [14] C. S. Desai, N. C. Samtani, and L. Vulliet, “Constitutive Modeling and Analysis of Creeping Slopes,” J. Geotech. Eng., Vol.121, No.1, pp. 43-56, 1995.
15. [15] J. M. Pestana and A. J. Whittle, “Formulation of a Unified Constitutive Model for Clays and Sands,” Int. J. Numer. Anal. Meth. Geomech., Vol.23, No.12, pp. 1215-1243, 1999.
16. [16] S. J. Wheeler, A. Näätänen, M. Karstunen, and M. Lojander, “An Anisotropic Elastoplastic Model for Soft Clays,” Can. Geotech. J., Vol.40, No.2, pp. 403-418, 2003.
17. [17] Y. F. Dafalias, M. T. Manzari, and A. G. Papadimitriou, “SANICLAY: Simple Anisotropic Clay Plasticity Model,” Int. J. Numer. Anal. Meth. Geomech., Vol.30, No.12, pp. 1231-1257, 2006.
18. [18] M. Leoni, M. Karstunen, and P. A. Vermeer, “Anisotropic Creep Model for Soft Soils,” Géotechnique, Vol.58, No.3, pp. 215-226, 2008.
19. [19] M. Liingaard, A. Augustesen, and P. V. Lade, “Characterization of Models for Time-Dependent Behavior of Soils,” Int. J. Geomech., Vol.4, No.3, pp. 157-177, 2004.
20. [20] M. Calvello, L. Cascini, and G. Sorbino, “A Numerical Procedure for Predicting Rainfall-Induced Movements of Active Landslides Along Pre-Existing Slip Surfaces,” Int. J. Numer. Anal. Meth. Geomech., Vol.32, No.4, pp. 327-351, 2008.
21. [21] N. Huvaj and A. Maghsoudloo, “Finite Element Modeling of Displacement Behavior of a Slow-Moving Landslide,” Proc. of Geo-Congress 2013, pp. 670-679, 2013.
22. [22] J. A. Fernández-Merodo, J. C. García-Davalillo, G. Herrera, P. Mira, and M. Pastor, “2D Viscoplastic Finite Element Modelling of Slow Landslides: The Portalet Case Study (Spain),” Landslides, Vol.11, No.1, pp. 29-42, 2014.
23. [23] D. R. Bhat, N. P. Bhandary, R. Yatabe, and R. C. Tiwari, “Residual-State Creep Test in Modified Torsional Ring Shear Machine: Methods and Implications,” Int. J. of GEOMATE, Vol.1, No.1, pp. 39-43, 2011.
24. [24] D. R. Bhat, N. P. Bhandary, R. Yatabe, and R. C. Tiwari, “A New Concept of Residual-State Creep Test to Understand the Creeping Behavior of Clayey Soils,” Proc. of GeoCongress 2012, pp. 683-692, 2012.
25. [25] D. R. Bhat, N. P. Bhandary, and R. Yatabe, “Residual-State Creep Behavior of Typical Clayey Soils,” Nat. Hazards, Vol.69, No.3, pp. 2161-2178, 2013.
26. [26] D. R. Bhat, R. Yatabe, and N. P. Bhandary, “Creeping Displacement Behavior of Clayey Soil in a New Creep Test Apparatus,” Proc. of Geo-Shanghai 2014, pp. 275-285, 2014.
27. [27] D. R. Bhat and R. Yatabe, “A Regression Model for Residual State Creep Failure,” Proc. of 18th Int. Conf. on Soil Mechanics and Geotechnical Engineering, pp. 707-711, 2016.
28. [28] E. Conte, A. Donato, and A. Troncone, “A Finite Element Approach for the Analysis of Active Slow-Moving Landslides,” Landslides, Vol.11, No.4, pp. 723-731, 2014.
29. [29] Y. Ishii, K. Ota, S. Kuraoka, and R. Tsunaki, “Evaluation of Slope Stability by Finite Element Method Using Observed Displacement of Landslide,” Landslides, Vol.9, No.3, pp. 335-348, 2012.
30. [30] E. Eberhardt, L. Bonzanigo, and S. Loew, “Long-Term Investigation of a Deep-Seated Creeping Landslide in Crystalline Rock. Part II. Mitigation Measures and Numerical Modelling of Deep Drainage at Campo Vallemaggia,” Can. Geotech. J., Vol.44, No.10, pp. 1181-1199, 2007.
31. [31] L. Picarelli, G. Urciuoli, and C. Russo, “Effect of Groundwater Regime on the Behaviour of Clayey Slopes,” Can. Geotech. J., Vol.41, No.3, pp. 467-484, 2004.
32. [32] H. G. Brandes and D. D. Nakayama, “Creep, Strength and Other Characteristics of Hawaiian Volcanic Soils,” Géotechnique, Vol.60, No.4, pp. 235-245, 2010.
33. [33] J. D. Nelson and E. G. Thompson, “A Theory of Creep Failure in Overconsolidated Clay,” J. Geotech. Eng. Div., Vol.103, No.11, pp. 1281-1294, 1977.
34. [34] F. D. Patton, “Groundwater Pressure and Stability Analyses of Landslides,” Proc. of the 4th Int. Symp. on Landslides, Vol.3, pp. 43-60, 1984.
35. [35] L. K. Walker, “Undrained Creep in a Sensitive Clay,” Géotechnique, Vol.19, No.4, pp. 515-529, 1969.
36. [36] Z.-Y. Yin, C. S. Chang, M. Karstunen, and P.-Y. Hicher, “An Anisotropic Elastic–Viscoplastic Model for Soft Clays,” Int. J. Solids and Struct., Vol.47, No.5, pp. 665-677, 2010.
37. [37] W. Z. Savage and A. F. Chleborad, “A Model for Creeping Flow in Landslides,” Bull. Assoc. Eng. Geol., Vol.19, No.4, pp. 333-338, 1982.
38. [38] K. Forbes and J. Broadhead, “Forests and Landslides: The Role of Trees and Forests in the Prevention of Landslides and Rehabilitation of Landslide-Affected Areas in Asia,” p. 3, Rap Publication, 2011.
39. [39] R. K. Dahal, “Rainfall-Induced Landslides in Nepal,” Int. J. Eros. Control Eng., Vol.5, No.1, pp. 1-8, 2012.
40. [40] S. Hasegawa, R. K. Dahal, M. Yamanaka, N. P. Bhandary, R. Yatabe, and H. Inagaki, “Causes of Large-Scale Landslides in the Lesser Himalaya of Central Nepal,” Environ. Geol., Vol.57, No.6, pp. 1423-1434, 2009.
41. [41] D. N. Petley, G. J. Hearn, A. Hart, N. J. Rosser, S. A. Dunning, K. Oven, and W. A. Mitchell, “Trends in Landslide Occurrence in Nepal,” Nat. Hazards, Vol.43, pp. 23-44, 2007.
42. [42] D. R. Bhat and A. Wakai, “Investigation of Creeping Landslides Along the Major Highway of Nepal Triggered by the 2015 Gorkha, Nepal Earthquake,” Proc. of the Research Committee on Strong Linear Phenomena of Surface Ground and its Effects at the Great Earthquake Symp., Ground Society Association of GeoKanto, pp. 77-80, 2018.
43. [43] Japan Landslide Society and National Conference of Landslide Control, “Landslides in Japan,” 6th Revision, 2002.
44. [44] A. Wakai, D. R. Bhat, K. Kotani, and S. Osawa, “Numerical Simulation of a Creeping Landslide Case in Japan,” B. Tiwari, K. Sassa, P. T. Bobrowsky, K. Takara (Eds.), “Understanding and Reducing Landslide Disaster Risk: Volume 4 – Testing, Modeling and Risk Assessment,” pp. 273-280, Springer, 2021.
45. [45] D. R. Bhat, R. Yatabe, and N. P. Bhandary “Study of Preexisting Shear Surfaces of Reactivated Landslides from a Strength Recovery Perspective,” J. Asian Earth Sci., Vol.77, pp. 243-253, 2013.
46. [46] D. R. Bhat, A. Wakai, and K. Kotani, “A Finite Element Approach to Understand the Creeping Behaviour of Large-Scale Landslides,” Proc. of the 19th Int. Summer Symp., pp. 9-10, 2017.
47. [47] D. R. Bhat and A. Wakai, “Numerical Simulation of a Creeping Landslide Induced by a Snow Melt Water,” Technical J., Vol.1, No.1, pp. 71-78, 2019.
48. [48] H. Saito, O. Korup, T. Uchida, S. Hayashi, and T. Oguchi, “Rainfall Conditions, Typhoon Frequency, and Contemporary Landslide Erosion in Japan,” Geology, Vol.42, No.11, pp. 999-1002, 2014.
49. [49] Y. Hong, H. Hiura, K. Shino, K. Sassa, A. Suemine, H. Fukuoka, and G. Wang, “The Influence of Intense Rainfall on the Activity of Large-Scale Crystalline Schist Landslides in Shikoku Island, Japan,” Landslides, Vol.2, No.2, pp. 97-105, 2005.
50. [50] Y. Sasaki, A. Fujii, and K. Asai, “Soil Creep Process and its Role in Debris Slide Generation – Field Measurements on the North Side of Tsukuba Mountain in Japan,” Eng. Geol., Vol.56, Nos.1-2, pp. 163-183, 2000.
51. [51] S. Ogita, W. Sagara, D. Higaki, and Research Committee on Elucidating Mechanisms of Large-Scale Landslides, “Shapes and Mechanisms of Large-Scale Landslides in Japan: Forecasting Analysis from an Inventory (WCoE 2014–2017),” K. Sassa, M. Mikoš, and Y. Yin (Eds.), “Advancing Culture of Living with Landslides: Vol.1 – ISDR-ICL Sendai Partnerships 2015–2025,” pp. 315-324, Springer, 2017.
52. [52] H. Yagi and T. Inokuchi, “Aerial Watching of Landslides in Japan -26- Jin-Nosuke-Dani Landslide, Mt. Hakusan, Central Japan,” J. Jpn. Landslide Soc., Vol.49, No.6, pp. 348-349, 2012.
53. [53] H. Yagi and T. Inokuchi, “Aerial Watching of Landslides in Japan -50- Epilogue of the Series “Aerial Watching of Landslides in Japan,’”’ J. Jpn. Landslide Soc., Vol.54, No.6, pp. 289-293, 2017.
54. [54] L. Vulliet and K. Hutter, “Viscous-Type Sliding Laws for Landslides,” Can. Geotech. J., Vol.25, No.3, pp. 467-477, 1988.
55. [55] N. Sugawara, “Variation of Surface Displacement Rate with Factor of Safety for Creep-Type Landslides (As a Consequence from Reviewing Literatures),” Oyo Technical Report, No.23, pp. 1-18, 2003 (in Japanese with English Abstract).
56. [56] D. R. Bhat, A. Wakai, and K. Kotani, “A Comparative Study of Two Newly Developed Numerical Models to Understand the Creeping Behaviour of Landslides,” Proc. of the 20th Int. Summer Symp., pp. 101-102, 2018.
57. [57] The Japanese Geotechnical Society, “FEM Series 3 for Ground Engineers: Using the Elasto-Plastic Finite Element Method,” 2003 (in Japanese).
58. [58] T. Tanaka, “Deformation and Stability Analysis by Finite Element Method,” The Japanese Society of Soil Mechanics and Foundation Engineering, “Principle of Soil Mechanics,” 1st Revision, pp. 109-154, 1992 (in Japanese).
59. [59] O. C. Zienkiewicz, C. Humpheson, and R. W. Lewis, “Associated and Non-Associated Visco-Plasticity and Plasticity in Soil Mechanics,” Géotechnique, Vol.25, No.4, pp. 671-689, 1975.
60. [60] I. Sakuramoto, T. Tsuchida, K. Kuramoto, and S. Kawano, “A Study on Nonlinear Viscoelastic Viscoplastic Finite Element Method Using Iterative Method,” Trans. Jpn. Soc. Mech. Eng. A, Vol.66, No.651, pp. 1984-1989, 2000 (in Japanese with English Abstract).
61. [61] The Japanese Geotechnical Society, “Japanese Geotechnical Society Q & A,” Vol.2, pp. 276-278, 2009 (in Japanese).
62. [62] K. Ugai and D. Leshchinsky, “Three-Dimensional Limit Equilibrium and Finite Element Analysis: A Comparison of Results,” Soils Found., Vol.35, No.4, pp. 1-7, 1995.
63. [63] F. Cai, K. Ugai, A. Wakai, and Q. Li, “Effects of Horizontal Drains on Slope Stability Under Rainfall by Three-Dimensional Finite Element Analysis,” Comput. Geotech., Vol.23, No.4, pp. 255-275, 1998.
64. [64] Y. M. Cheng, T. Lansivaara, and W. B. Wei, “Two-Dimensional Slope Stability Analysis by Limit Equilibrium and Strength Reduction Methods,” Comput. Geotech., Vol.34, No.3, pp. 137-150, 2007.
65. [65] A. Murakami, A. Wakai, and K. Fujisawa, “Numerical Methods,” Soils Found., Vol.50, No.6, pp. 877-892, 2010.
66. [66] A. W. Skempton, “Long-Term Stability of Clay Slopes,” Géotechnique, Vol.14, No.2, pp. 77-102, 1964.
67. [67] A. W. Skempton, “Residual Strength of Clays in Landslides, Folded Strata and the Laboratory,” Géotechnique, Vol.35, No.1, pp. 3-18, 1985.