Stability Analysis of Slopes with Terraced Topography in Sapa, Northern Vietnam: Semi-Infinite Slope Assumption with Specific Lengths for Slope Failure
Akihiko Wakai*1,, Akino Watanabe*1, Nguyen Van Thang*1, Takashi Kimura*2, Go Sato*3, Kazunori Hayashi*4, Nanaha Kitamura*1, Takatsugu Ozaki*1, Hoang Viet Hung*5, Nguyen Duc Manh*6, and Tran The Viet*5
1-5-1 Tenjincho, Kiryu, Gunma 376-8515, Japan
*2Ehime University, Ehime, Japan
*3Teikyo Heisei University, Tokyo, Japan
*4Okuyama Boring Co., Ltd., Miyagi, Japan
*5Thuyloi University, Hanoi, Vietnam
*6University of Transport and Communications, Hanoi, Vietnam
Numerous annual slope failures are induced by heavy rainfall during the monsoons, especially in developing countries in Asia. The authors have developed a simple method to predict rising groundwater levels in natural slopes at a relatively shallow depth based on parametric studies conducted using the finite element method. An assumption of a semi-infinite homogeneous slope was adopted in the analysis. Addtionally, the authors numerically modelled the vertical infiltration and the lateral seepage flow. Using this method implies that the finite element analysis is not mandatory in the evaluations of practical slopes. Such a simplified approach helps avoid time-consuming tasks in rigorous computations. However, a semi-infinite assumption used in the developed method may provide us with unsuitable solutions, particularly in cases where the slopes include heavily terraced topography with local small cliffs, because theoretically, the first slope failure tends to occur in steep slopes, like the edge of a rice terrace, even though they are very small cliffs. Nevertheless, these local solutions do not affect the conclusions for disaster risk reduction. Moreover, such unsuitable alternatives must be eliminated during analysis. To address this matter, the current study proposes a novel concept of specific lengths. This procedure provides a representative length within the specified length range. The averaged slope gradient is defined by focusing on the secant lines between each topographical grid, while those defined outside the specified range – for example, local cliff angles – are ignored in the slope stability calculation. Consequently, the proposed concept was confirmed to be efficient and can be applied to evaluate the terraced rice fields in Sapa, northern Vietnam. In the past, this area had experienced rainfall-induced slope failures; hence, the proposed method may be able to simulate these occurences. The proposed concept’s effectiveness when applied to terraced fields should continue to be verified through case studies conducted in areas with extensive smallterraced topography.
-  T. Okimura, N. Torii, Y. Osaki, M. Nambu, and K. Haraguchi, “Construction of real-time type hazard system to predict landslides caused by heavy rainfalls,” J. of the Japan Society of Erosion Control Engineering, Vol.63, No.6, pp. 4-12, 2011 (in Japanese).
-  A. Kinoshita, T. Kanno, A. Okamoto, M. Hitokoto, M. Onodera, M. Sakuraba, and M. Sugiyama, “Construction of real-time hazard map system in Rokko mountain area,” J. of the Japan Society of Erosion Control Engineering, Vol.66, No.1, pp. 15-22, 2013 (in Japanese).
-  Z. Shi, F. Wei, and V. Chandrasekar, “Radar-based quantitative precipitation estimation for the identification of debris flow occurrence over earthquake-affected regions in Sichuan, China,” Natural Hazards and Earth System Sciences, Vol.18, No.3, pp. 765-780, 2018.
-  T. Ozaki, A. Watanabe, A. Wakai, and F. Cai, “A simple prediction model for shallow groundwater level rising in natural slopes based on finite element analysis,” Proc. of the 4th Int. Conf. on Geotechnics for Sustainable Infrastructure Development, pp. 993-1000, 2019.
-  D. Tien Bui, T. A. Tuan, N.-D. Hoang, N. Q. Thanh, D. B. Nguyen, N. Van Liem, and B. Pradhan, “Spatial prediction of rainfall-induced landslides for Lao Cai area (Vietnam) using a hybrid intelligent approach of least squares support vector machines inference model and artificial bee colony optimization,” Landslides, Vol.14, No.2, pp. 447-458, 2017.
-  T. V. Tran, V. H. Hoang, H. D. Pham, and G. Sato, “Use of Scoops3D and GIS for the Assessment of Slope Stability in Three-Dimensional: A Case Study in Sapa, Vietnam,” Proc. of the Int. Conf. on Innovations for Sustainable and Responsible Mining (ISRM 2020), Vol.2, pp. 210-229, 2020.
-  D. M. Nguyen and Q. H. Tran, “Features of large-scale landslide at Hau Thao Area, Sa Pa Town, Lao Cai Province,” Proc. of the 4th Int. Conf. on Geotechnics for Sustainable Infrastructure Development, pp. 917-922, 2019.
-  P. H. Giao and B. X. Hanh, “Analysis of post-landslide electric imaging data at a site in Sapa, Vietnam,” Proc. of the EAGE-GSM 2nd Asia Pacific Meeting on Near Surface Geoscience and Engineering, pp. 1-5, 2019.
-  D. G. Tarboton, “A new method for the determination of flow directions and upslope areas in grid digital elevation models,” Water Resources Research, Vol.33, No.2, pp. 309-319, 1997.
-  M. Watari and S. Kobashi, “Prediction and Countermeasures for Landslides and Slope Failures,” Sankaido Publishing, pp. 115-116, 1987 (in Japanese).
This article is published under a Creative Commons Attribution-NoDerivatives 4.0 Internationa License.