1. STABILITY AND MOTION ANALYSIS
OF THE VAIONT SLIDE
Oldrich Hungr
University of British Columbia, Canada
(ohungr@eos.ubc.ca)

2. Old technology: Method of Slices
General method:
1) Work out the equilibrium of each slice
2) Calculate the equilibrium of the slice assembly
3) Results depend on assumptions regarding the
interslice force X (mobilization of internal strength)

3. Data: Selli and Trevisan (1964)

4. E. Semenza (1960)

5. Semenza sections plus sliding surface reconstruction

7. Failure mechanism
(Hendron and Patton, 1984)
Side
shear
Internal shear

8. BEDDING
C=0, ϕ=12º
ROCK MASS
C=1500, ϕ=34º
1700
10a
5
2
Strength distribution

9. 2D LE analysis
Bishop’s Simplified
Morgenstern-Price

10. MP Factors of Safety
F=0.44
F=0.66
BEDDING
C=0, ϕ=12º
F=0.43
C=0, ϕ=12º
C=0, ϕ=12º
F=2.28
ROCK MASS
C=1500, ϕ=34º

11. SARMA Method
(Hoek et al., 1997)
Assumption:
internal strength
fully mobilized
F=0.48 to 0.66
C=0, ϕ=12º
(rock mass
strength)
F=0.70 to 0.86
c=1500 kPa
–if available
ϕ=34º
C=0, ϕ=12º

12. Bishop's
Simplified
MorgensternPrice
Sarma
c=0
Sarma
c=1500 kPa
#2, x=540
0.43
0.44
0.48
0.66
#5, x=860
0.60
0.66
0.70
0.86
#10a, x=1310
0.42
0.43
No conv.
No conv.
x=1700 (rock)
2.29
2.28
circle
circle
Section
Conclusions – 1) highly asymmetric case,
2) internal strength increases F by 0 to 50%,
depending on cohesion and shape.

13. 3D Analysis, Method of Columns (Hungr et al., 1989)
Ground
Sliding
Simplified column
assembly

14. Sliding
surface
Volume:
294 million m3
(excluding water)

15. BEDDING
C=0, ϕ=12º
ROCK MASS
C=1500, ϕ=34º
10º
Distribution of strength. Optimize sliding direction.

16. Single stage, Volume = 323 million m3 (including water)
Condition
Bishop's
Simplified
Bishop's
Rotated
10ᵒ
MorgensternPrice
No constraint,
uniform strength
0.42
0.47
0.46
Constraint, ϕ=34ᵒ,
c=1500 kPa
1.07
0.92
1.19
Constraint, ϕ=34ᵒ,
c=0
0.60
0.62
0.67
Conclusions – 1) direction optimization is
necessary, 2) Constraint (with cohesion) is needed

17. 2-stage
failure?
(Superchi, 2011)

18. 2-stage failure, Volume = 282 million m3 (including water)
Condition
Bishop's
Simplified
Bishop's
Rotated
10ᵒ
MorgensternPrice
No constraint,
uniform strength
0.46
0.51
0.51 (+9%)
Constraint, ϕ=34ᵒ,
c=1500 kPa
0.82
0.59
0.74 (-38%)
Constraint, ϕ=34ᵒ,
c=0
0.58
0.59
0.61 (-9%)
Conclusion – Partial failure is possible (likely?)

19. 2D dynamic analysis
“Flexible block concept” (Romero and Molina, 1974)

20. Flexible block
ϕ=23º, ru=0.4
C=0, ϕ=12º
10-sec.
intervals

21. 3D flexible block
model (Aaron, 2013)
Assumptions:
1) Differential movement
of columns is
permitted in the
vertical direction but
not in the horizontal
plane.
2) No shear stress on
column faces

22. Resultant
forces
DRIVING
RESISTING
ROTATION

23. Sliding mass displacement:
ϕ=23º on bedding, plus cohesive
constraint, piezometric surface
Original
Real (after slide)
Analysis

24. 2-stage failure

26. 10 m/s
20 m/s

27. CASSO
Water displacement: V MAX = 10 m/s

28. CASSO
Water displacement: V MAX = 20 m/s

29. Conclusions:
1) Landslide was highly asymmetric
2) Internal strength contributed to the stability of typical
sections, to a limited extent
3) The structural constraint on the right flank was
important
4) Cohesion of the intact rock mass played an
important role, but is difficult to quantify
5) The speed of the landslide is not difficult to explain
and was due largely to the loss of cohesion in the
interior and in the side constraint
6) The slide may have been slower than assumed
7) A two-stage failure mechanism is very plausible

30. REFERENCES
Hendron, A.J. and Patton, F.D. (1985) - The Vaiont Slide, a Geotechnical Analysis Based on New Geologic Observations
of the Failure Surface. Technical Report GL- 85-5, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS.
I, II.
Hoek, E. (1997) - General 2-dimensional slope stability analysis. Analytical and Computational Methods in Engineering
Rock Mechanics, 95-128, E.T. Brown, Ed., Allen and Unwin, London.
Hungr, O., Salgado, F.M. and Byrne, P.M. (1989) - Evaluation of a three-dimensional method of slope stability analysis.
Canadian Geotechnical Journal, 27: 679-686.
Romero, S.U. and Molina, R. (1974) - Kinematic aspects of the Vaiont Slide. Proceedings, 3rd. Congress
ISRM, Denver, Colorado, 2:865-870.
Selli, R., Trevisan, L., Carloni C.G., Mazzanti, R. and Ciabatti, M. (1964) - La Frana delVajont. Giornale di Geologia. serie
20, XXXII (I), 1–154.
Superchi, L. (2011) - The Vajont rockslide: new techniques and traditional methods to re-evaluate the catastrophic event.
Ph.D. Thesis, University of Padova, 188p.
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