9oct hungr vaiont-2013

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9oct hungr vaiont-2013

  1. 1. STABILITY AND MOTION ANALYSIS OF THE VAIONT SLIDE Oldrich Hungr University of British Columbia, Canada (ohungr@eos.ubc.ca)
  2. 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. 3. Data: Selli and Trevisan (1964)
  4. 4. E. Semenza (1960)
  5. 5. Semenza sections plus sliding surface reconstruction
  6. 6. Failure mechanism (Hendron and Patton, 1984) Side shear Internal shear
  7. 7. BEDDING C=0, ϕ=12º ROCK MASS C=1500, ϕ=34º 1700 10a 5 2 Strength distribution
  8. 8. 2D LE analysis Bishop’s Simplified Morgenstern-Price
  9. 9. 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
  10. 10. 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
  11. 11. 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.
  12. 12. 3D Analysis, Method of Columns (Hungr et al., 1989) Ground Sliding Simplified column assembly
  13. 13. Sliding surface Volume: 294 million m3 (excluding water)
  14. 14. BEDDING C=0, ϕ=12º ROCK MASS C=1500, ϕ=34º 10º Distribution of strength. Optimize sliding direction.
  15. 15. 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
  16. 16. 2-stage failure? (Superchi, 2011)
  17. 17. 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?)
  18. 18. 2D dynamic analysis “Flexible block concept” (Romero and Molina, 1974)
  19. 19. Flexible block ϕ=23º, ru=0.4 C=0, ϕ=12º 10-sec. intervals
  20. 20. 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
  21. 21. Resultant forces DRIVING RESISTING ROTATION
  22. 22. Sliding mass displacement: ϕ=23º on bedding, plus cohesive constraint, piezometric surface Original Real (after slide) Analysis
  23. 23. 2-stage failure
  24. 24. 10 m/s 20 m/s
  25. 25. CASSO Water displacement: V MAX = 10 m/s
  26. 26. CASSO Water displacement: V MAX = 20 m/s
  27. 27. 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
  28. 28. 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. THANK YOU

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