DISCUSSION

6. DISCUSSION Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

7. Yuefeng Zhoua, W. M. Yana, L. G. Thama, Fuchu Daib and Ling Xub aDepartment of Civil Engineering, The University of Hong Kong bInstitute of Geology and Geophysics, Chinese Academy of Sciences Stability Analysis of a Loess Slope with Water Infiltration Affected by Cracks

8. Introduction Field investigation Numerical modeling Conclusion Outline 8 Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

9. Introduction • In China, loess covers proximately 6.6% of the total area. • Loess Plateau is the greatest bulk accumulation of loess on earth, the area of which is 31700 km2 (containing 50% of loess in China).Loess plateau is one of the most severe terrain in China for geohazards. • In recent years great range of loess plateau is subject to serious and accelerated soil-water erosion. Loess landslide, as a typical geological disaster in China, occurs frequently in loess plateau and has been paid special attentions in recent years.

10. Introduction • Location: Heifangtai Loess Plateau 60 km southwest of Lanzhou, China • Description of the Study Area: • Loess thickness is 40 to 50 m. • The bedrock is mainly mudstone and siltstone. • Extensive cracks developed on the edge of loess plateau so that failure can be easily initiated. • Agricultural production is the major economic source for local residents. The Site 10 Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

11. Introduction • Climate: The climate at Heifangtai plateau is temperate semiarid. The average annual precipitation amount is 287 mm. Under such a dry condition, a typical character shown at Heifangtai plateau is that extremely steep slope.

12. Introduction From 1968 to 2010, more than 60 major landslides occured at Heifangtai Loess Plateau (13.7km2). Yellow River historic landslide groundwater seepage 12 Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

13. Introduction • The reason for large amount of landslides at Heifangtai Plateau is the rise of groundwater table due to long-term and continuous irrigation of agricultural lands. • Based on early stage investigation, a typical slope was chosen at the edge of plateau to conduct this field test to simulate flooding irrigation to study stability of slope with water infiltration.

14. Field test Principal section Range of installation 14 Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

16. Field test Details of flooding irrigation

17. Field investigation

18. Field investigation

19. Field investigation The slope angle was measured by compass 19 Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

20. Numerical modeling Seepage (FE method) and stabilility analysis (LE method) 20

21. Numerical modeling Soil properties (strength) Four layers of undisturbed samples were taken at a slope nearby at the depth of 5m, 10m, 15m and 30m respectively. Then the samples were airproofed by preservation membranes and wax. ICU and CS tests were performed on undisturbed loess samples. 21 Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

22. Numerical modeling Soil Properties (permeability) Kv is several times to several dozens times higher than Kh. Permeability adopted here: Kv= 4.8 × 10-6 m/s; Kh=7 × 10-7m/s

23. Numerical modeling • Fitting curves of SWCC for whole range by equation (Fredlund et al. 1994) • Prediction of unsaturated permeability curves(Fredlund et al. 1994) 23 Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

24. Numerical modeling Suction for (a) with cracks and (b) without cracks, and volumetric water content for (c) with cracks and (d) without cracks after irrigation on 17 Oct., 2009(3rd day)

25. Numerical modeling Suction for (a) with cracks and (b) without cracks, and volumetric water content for (c) with cracks and (d) without cracks after irrigation on 20 Oct., 2009 (6th day)

26. Numerical modeling Suction for (a) with cracks and (b) without cracks, and volumetric water content for (c) with cracks and (d) without cracks after irrigation on 24 Oct., 2009 (10th day)

27. Numerical modeling Suction for (a) with cracks and (b) without cracks, and volumetric water content for (c) with cracks and (d) without cracks after irrigation on 26 Oct., 2009 (12th day)

28. Numerical modeling

29. Conclusions • With the consideration of cracks, the wetting zone is much deeper than without consideration. • Short term irrigation could only trigger local failure rather than global failure of slope. • The existence of cracks could be regarded as an effective way of discharging water for the stability of slope in short time. • Strength parameters from CS tests are more reliable than that from ICU tests Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

30. Thank you! Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

31. Numerical modeling Comparison between simulated and monitored data

32. Numerical modeling Comparison between simulated and monitored data

33. Numerical modeling Due to substantial cracks, joints and cavities existing in the site, the accurate matching between field testing and numerical modeling results are not expected in this study. The overall comparison between monitored and simulated data is acceptable.