1. The Hungtsaiping landslides- from a rock slide to a colluvial slide
K.T. Chang
Department of Soil and Water Conservation, National Chung Hsing University, Taichung, Taiwan
M.L. Lin, C.M. Lo
Department of Civil Engineering, National Taiwan University, Taipei, Taiwan
J.J. Dong
Institute of Applied Geology, National Central University, Taoyuan, Taiwan
September 21, 2009

2. Dong et al. (2007)
Introduction
Jhongliao, Nantou N

3. Landslides triggered by the 1999 Chi-Chi earthquake
CGS(2002)
tension cracks,
subsidence,
scarps,
squeezes
subsidence

4. Features of the landslide triggered by the 1999 Chi-Chi earthquake
average displacements around 20 meters in a large region (100 hectares).
Different sliding directions in different areas (WNW at the upper slope and then toward NNW to the Yonglu stream).

5. Geologic investigation
Topographic analysis
Field Reconnaissance
Borehole drilling
Geophysical exploration
The strength of rock mass
→numerical simulation

6. Locations for boreholes and geophysical exploration
scanlines
photo location p1 p5 p4

7. Topographic change
Event: 1916 Nantou series earthquakes (ML 6.8, 6.2, 6.5, & 5.5)

8. Field reconnaissance p1
colluvium
bedrock
colluvium
bedrock

9. p2
fault & shear zone
fold p3

10. Drilled cores bedrock depth : 6~82 m sandy granular support colluvium
clayey matrix support colluvium
interface of colluvium & bedrock

11. Earth resistivity explorationB B’

12. Geologic map

13. Geologic profiles

14. Evaluation of rock mass mechanical behavior
砂岩破壞包絡線 50
100
150
200
250
-10 10 20 30 40
實驗值
迴歸值 σ 3
(MPa) 1
sandstone failure envelope
12.760 i m =
experiment
estimate
Field Reconnaissance
Core Intact Rock Experiments
GSI, D
Hoek-Brown Failure Criterion
σci , mi, Ei
σcm , Em, c,
mi=12.76
頁岩破壞包絡線 20 40 60 80
100
120
140
-10 10 30
實驗值
迴歸值 σ 3
(MPa) 1
shale failure envelope
12.760 i m =
experiment
estimate
mi=6.129

15. GSI Range of Rock Mass at Hungtsaiping
Red : Sandstone
Blue : Shale

16. Rock mass parameters estimated by the Hoek–Brown failure criterion

17. History of Slides & Mechanisms
At least 2 events N

18. Simulation using DEM (PFC)

19. Profiles for numerical modeling

20. Calibration of microparameters
The slide surface in shale rock mass
Initial friction coefficient= 0.5 (ψ=26.6°)
(refer to the friction angle of the shale mass)
The slide surface on slope
Friction coefficient= 0.6 (ψ=31°)
The microparameters of the sandstone mass is calibrated through uniaxial compressive strength and deformation modulus of rock mass
Parameters of disks
Parameters of parallel bonds
normal stiffness
(GN/m)
shear stiffness
friction coefficient
(GPa/m)
normal strength
(MPa)
shear strength
normal damping ratio
shear damping ratio
3.9
1.64
0.65
0.975
0.410
7.84
0.216
0.016

21. Simulations for rainfall and earthquake
Pore water pressure on the slide surface is taken into account by reduction of the friction coefficient on the slide surface. The maximum reduction multiplier is around 0.6.
Infinite dry slope
Infinite slope with groundwater on surface
The acceleration records at the strong motion station TCU072 are used for both the ancient and the 1999 earthquakes
Horizontal acceleration
Vertical acceleration 21

22. The ancient profile subjected to heavy rainfall
The initial friction coefficient of the slide surface= 0.50 (ψ=26.6°)
The reduced friction coefficient of the slide surface= 0.30 (ψ=16.7°)

23. Before the 1999 earthquake (modification of the profile)
After the 1999 earthquake
The friction coefficient of the slide surface is kept as 0.6 (ψ=31°)

24. The ancient profile subjected to earthquake
The initial friction coefficient of the slide surface= 0.50 (ψ=26.6°)
The reduced friction coefficient of the slide surface= 0.32 (ψ=17.7°)

25. Before the 1999 earthquake (modification of the profile)
After the 1999 earthquake
The friction coefficient of the slide surface is kept as 0.6 (ψ=31°)

26. Conclusion
The ancient landslide was a large rock slide with large displacement and caused the thick-layered colluvium, which is composed of mainly disintegrated sandstone material at upper portion and disintegrated shale material at lower portion. The slide surface was within a shale formation.
The ancient landslide was along the dip direction of the bedding on the upper slope and then along the lower slope surface.
The slide during the Chi-Chi earthquake was mainly the movement of colluvium on the bedrock.
The ancient rock slide is speculated as a result of a strong earthquake.

27. Thanks for your attention.

28. Laboratory Experiments