Genevois - The Vajont Landslides

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Genevois - The Vajont Landslides

  1. 1. International Conference Vajont, 1963-2013 Thoughts and Analyses After 50 Years since the Catastrophic Landslide October, 8-10 2013 Rinaldo Genevois, Department of Geosciences, University of Padova Pia Rosella Tecca, CNR-IRPI Padova, Italy
  2. 2. • THE VAJONT DAM HISTORY - THE DAM DESIGN : 1925 - JANUARY 1957 - THE DAM CONSTRUCTION: JANUARY 1957 – SEPTEMBER 1959 - THE CHRONICLE OF A CATASTROPHE • THE STATE OF KNOWLEDGE BEFORE THE FAILURE • STUDIES AND RESEARCHES AFTER THE FAILURE • FURTHER RESEARCH AND DEVELOPMENT • FINAL COMMENTS The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  3. 3. THE VAJONT DAM HISTORY THE DAM DESIGN: 1925 - JANUARY 1957 30th January 1929: SADE request the use the Vajont torrent waters; Eng. C. Semenza preliminary project for a 180 m high dam. Preliminary studies: 1) very early Prof. J. Hug hypothesis (1925): Casso bridge site 2) Prof. G. Dal Piaz suggestion (1928): Colomber bridge The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  4. 4. THE VAJONT DAM HISTORY MAIN REPORTS DURING CONSTRUCTION AND OPERATION TESTS • 25th March 1948 and 31st January 1957 integration - Prof. G. Dal Piaz: favorable to height increase (266m), but attention to dam shoulders. • 6th August 1957 - Dr. L. Mueller: poor rock mass characteristics on the left slope. • September 1957 - Prof. G. Dal Piaz: validity of all since then carried out studies. • 29th October 1958 - Prof. G. Dal Piaz: Forecast of only small rock falls on left Vajont slope (road construction). • 10th October 1959 - dr. L. Mueller: doubts of on the stability of the left slope . • 4th February 1960 - Prof. P. Caloi: left slope composed by sound rocks with a 10-12 m detritic cover. • June 1960 – dr. E. Semenza and dr. Giudici: presence on the left slope of an old wide and deep landslide. • 9th July 1960 - Prof. G. Dal Piaz: only small and slow phenomena possible. (continue) The Vajont Landslide: State-of-Art by Genevois & P.R. Tecca
  5. 5. THE VAJONT DAM HISTORY MAIN REPORTS DURING CONSTRUCTION AND OPERATION TESTS 15-16th November 1960 - Prof. Mueller sketch: Instability on deep slip surface following opening of M fissure. 26th November 1960 - Prof. Penta: M-fissure consequence of a deep landslide or a more superficial and slow one. 3rd February 1961 - dr. Mueller: Moving mass (volume of about 2x108 m3) split in independent parts. Landslide could be partially controlled by artificial landsliding. 10th February 1961 - Prof. Caloi: lower seismic wave velocities due to degradation of rock mass (deformation processes). 2nd March 1961 - Prof. Caloi: Deep bedrock sufficiently sound. 5th May 1961 - Dr. Pacher: Existence of many slip surfaces, then no unique deep shear surface. 31st October 1961- Prof. Penta: Existence of a shallow, possibly dormant landslide. 3 May 1962 - Eng. Beghelli (Belluno Public Works Bureau): No new landslides observed. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  6. 6. THE VAJONT DAM HISTORY THE CHRONICLE OF A CATASTROPHE 22.03-1959 Pontisei landslide 19.07.1959 1st CdC field survey 09.1959 Dam construction end 02.02.1960 1° filling (el. 595 m a.s.l.) 03.1960 Landslide at Pian della Pozza The Pontisei landslide (1959) The Vajont reservoir during 1962 The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  7. 7. THE VAJONT DAM HISTORY THE CHRONICLE OF A CATASTROPHE 06.1960 11.06.1960 W.L. at 660 m a.s.l. 10.1960 Semenza’s sketch of Colle Isolato as part of the paleo-landslide Semenza & Giudici paleo-landslide M-fissure open up The 1960 M-shaped fissure The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  8. 8. THE VAJONT DAM HISTORY THE CHRONICLE OF A CATASTROPHE 04.11.1960: A 7-8x105 m3 slide occurs on left slope of the reservoir The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  9. 9. THE VAJONT DAM HISTORY THE CHRONICLE OF A CATASTROPHE 15.11.1960 11.1960 31.01.1961 03.02.1961 21.02.1961 By-pass gallery plan Drawdown to 600 m a.s.l. Landslide modeling at Nove Mueller conjectures a 2x108 m3 landslide Risk of left slope failure by local daily Penta (CdC): no more displacements, no 10.04.1961 indication of deep landsliding, no indication of immediate danger 15.04.1961 Penta: no hazard with the w.l. at 600 m a.s.l. 20.04.1961 10.05.1961 08-09.1961 31.10.1961 10.04.1962 C. Semenza suspects landslides; Dal Piaz and Penta optimist By-pass gallery completed Piezometers installation Filling may continue Caloi: 1960 microseismic activity (w.l. 630 m) not observed for 650 m w.l.; velocities lower; no surface facts The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  10. 10. THE VAJONT DAM HISTORY THE CHRONICLE OF A CATASTROPHE 15.05.1962 17.11.1962 02.12.1962 Acceleration with 660-685 m filling. New fissures, different surface velocities. Landslide of limited mass is considered. Request of possible evacuation. 20.03.1963 22.07.1963 W.L. at 700 m a.s.l. Drawdown to 647.5 m a.s.l. Velocity increases with the drawdown 700 to 650 m drawdown Filling to 715 m a.s.l. requested Alarm given by Erto Major 02.09.1963 Continuous increase of displacements 04.09.1963 W.L. at 710 m a.s.l. 15.09.1963 New fissures open and M-fissure widens 27.09.1963 Drawdown begins 02.10.1963 New fissures and further displacements 05.10.1963 Settlements in Pian della Pozza 07.10.1963 New 10 m long fissure. Evacuation of Mt. Toc inhabitants 10.01.1963 The ‘Pian della Pozza’ hollow The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  11. 11. THE VAJONT DAM HISTORY THE CHRONICLE OF A CATASTROPHE 09.10.1963 Morning: discharge channel clogged; velocities increase 9.10.1963 13:00 l.t.: new 5 m long fissure 09.10.1963 20:00 l.t.: left slope road impassable 9th October 1963, 22:39 local time The catastrophic landslide Volume: Width: Max. depth: Max. velocity: Total displaced water: Water wave heigth (above dam crest) 2.7x108 m3 about 2000 m 250 – 280 m 25-30 m/s about 50x106 m3 200-250 m The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  12. 12. THE VAJONT DAM HISTORY THE CATASTROPHE The exposed failure surface and the slid mass on the foreground The landslide mass filling up the reservoir The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  13. 13. THE VAJONT DAM HISTORY THE CATASTROPHE The resulting water wave flows to both the Est and the West The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  14. 14. THE VAJONT DAM HISTORY THE CATASTROPHE The water wave washed out the slope up to Erto and overtopped the dam with an height of more than 200 m. But the dam and its shoulders remain intact! The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  15. 15. THE CATASTROPHE After overtopping the dam and rushing into the narrow Vajont gorge, the wave swept onto the Piave valley razing everything and killing 1910 people. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  16. 16. LONGARONE After Before The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  17. 17. THE STATE OF KNOWLEDGE BEFORE THE FAILURE Aeschilus: πάθει μάθος [There is] learning in suffering The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  18. 18. THE STUDIES BEFORE THE FAILURE Geological and geomorphological setting studied by Prof. G. Dal Piaz (1928 – 1962). Vajont valley is very narrow from the Piave river up to the Casso village. Towards East the valley widens, remaining however very narrow in its lower part. The valley is set in dolomitic limestones, no tectonic accidents are evident and rock masses are uniform and sound. At the Colomber Bridge section they are uniform and compact; permeability problems will be easily passed. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  19. 19. THE STUDIES BEFORE THE FAILURE Semenza and Giudici (1960) intuition Ancient landslide on the left side in the area of “Pian del Toc” and “Pian della Pozza with failure surface bounded: to North by the cataclasites outcropping at 600 m a.s.l.; to South by the "Pian della Pozza” depression of and to East by a vertical fault. Conclusions: the landslide mass could be re-mobilized. Semenza-Giudici hypothesis considered substantial by L.Mueller after Mfissure opening (October 1960): two sectors separated by Massalezza .Total volume about 2x108 m3 ; shear surface at 250 m (West) and 200 m (East) depths. Displacement vectors in western sector: N20 and inclinations from 27 -34 to zero. Antithetic fractures in the flat part of the "chair" . The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  20. 20. International Conference Vajont, 1963-2013 THE STUDIES BEFORE THE FAILURE Field investigation (boreholes, seismic surveys and displacements measurements) were carried out. Boreholes didn’t get the sliding surface. Main Muller ‘s persuasions: - pulsating glacier-like movement; - causes: low shear strength, orientation of discontinuities, high water pressure and cyclic stresses ; - displacement rate influenced by water level in the reservoir ; - fast movements occur with drawdown operations; - landslide cannot be stopped, but only controlled . - greater landslides will determine water waves higher (40 m), causing the instability even of the dam abutments. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  21. 21. THE STUDIES BEFORE THE FAILURE At the beginning of 1961 the Hydraulic Modeling Center - University of Padova charged to model the effects of a landslide on the Vajont reservoir. Considering the maximum elevation discharges over the dam for a 3 or 1.5 minutes landslide range from 12.000 m3/s to 30.000 m3/s and wave heights over the dam from 11.5 m to 22.0 m. Considering two different sectors discharge results of 20.000 m3/s and a wave height over the dam of 16.0 m.(Prof. A. Ghetti 1962 report). Until approximately the Spring 1959 the concern was mainly, if not only in the stability and characteristics of rocks of the dam abutments, while the stability of the whole reservoir slopes was not considered with exception of the Erto area. Studies on the whole reservoir area began when the construction of the dam body was already accomplished (1959), driven also by the 1960 landslide occurred with a water level in the reservoir at 650 m a.s.l. (1st filling operation initiated on February 1960). The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  22. 22. THE STUDIES BEFORE THE FAILURE Geological knowledge considering also the trend of monitored data in conjunction with both the fillingdrawdown operations and specific events. First Filling and Draw-Down Beginning on February 1960. - On March 1960 small detachments when the w.l. reached 610 m a.s.l. - On October 1960, w.l. at 650 m a.s.l., fast increase of displacements rate (3.5 cm/day). - A long fissure opens up. - On 4th November a 700,000 m3 landslide. - Slow drawdown to 615 m a.s.l. : displacements rate reduces to less than 1 mm/day. - Designers conviction: i) landslide controlled varying w.l.; ii) over-topping of dam avoided, if landslide sliding time less ten minutes. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  23. 23. THE STUDIES BEFORE THE FAILURE Second Filling and Draw-Down Initiate on October 1961. - The w.l. reaches 665 m a.s.l. in early February 1962 and 715 m a.s.l. in November 1962. - At the end of filling (March 1963) displacements velocities increase to 1.2 cm/day. - The four months draw-down to 665 m a.s.l. leads to zero the displacements rate. - designers convinced that the control of the landslide was possible. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  24. 24. THE STUDIES BEFORE THE FAILURE Third Filling and Draw-Down W.L. increased to 711 m a.s.l. in April-May 1963. - Velocities never excede 0.3 cm/day. - Velocities reach 0.4 cm/day for w.l. at 717 m a.s.l. (June1963) and 0.5 cm/day for w.l. at 720m a.s.l. (mid-July 1963). - w.l. maintained, but velocities increase to 0.8 cm/day (mid-August) and to 3.5 cm/day (early September) for a w.l. at 725 m a.s.l.. - w.l. slowly lowered to 715 m a.s.l. by 9th October 1963 , but velocities continuously increase up to a registered 20 cm/day. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  25. 25. THE STUDIES BEFORE THE FAILURE Some more data Other data have been gathered in those years: - Piezometers rather closely follow the w.l. in reservoir with the exception of Piezometer n. 2. - Increasing w.l. an increase in sliding velocity is obtained, but relationship is not linear rather tending towards an asymptotic limit, indicating the failure. The second reservoir filling leads to a different asymptotic value: movements along seem to be not governed by effective stresses acting on the shear surface. This may have induced to the third filling and to the following lowering of the w.l. in the attempt to reduce the velocity of the slide. - During 1960-1963 a number of extensional and compressional events happened due to a regional N–S compressional stress field. Seismic activity registered on the Vajont valley on April-May 1962 and September 1963 is attributed to natural phenomena. At 22:38 GMT on 9th October 1963 the catastrophic landslide occurred. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  26. 26. THE VAJONT VALLEY BEFORE AFTER The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  27. 27. STUDIES AND RESEARCHES AFTER THE FAILURE The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  28. 28. STUDIES AND RESEARCHES AFTER THE FAILURE REPORTS JUST AFTER THE LANDSLIDE The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  29. 29. STUDIES AND RESEARCHES AFTER THE FAILURE The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  30. 30. STUDIES AND RESEARCHES AFTER THE FAILURE Study on the water wave induced by the Vajont landslide) by Prof. M. Viparelli and Prof. G. Merla, charged by the Government Board of Inquiry (1964) Maximum height of water wave: 200 m. Volume of the water overtopping the dam: 25 x106 m3; discharge at the dam site: 50-100.000 m3/s. Height of water wave at the entrance in the Piave river valley: 100 m. The wave reaches at 8:00 o’clock of the day after a site 84 km distant, as high as 2.33 m. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  31. 31. STUDIES AND RESEARCHES AFTER THE FAILURE The Commission of the Board of Inquiry ENEL (1964) composed by lawyer M. Frattini and Professors F. Arredi, A. Boni, C. Fassò and F. Scarsella, charged to investigate geological and mechanical factors and hydraulic effects of the landslide. - Landslide volume: 250 Mm3, contemporary in the eastern and western parts in 10’30” running up on the opposite valley side up to 125 m with small rotation. - Exceptional volume and velocity consequence of rocks nature, the “chair” structure and, effective stresses decrease due to reservoir infilling. - Rather good correlation between horizontal displacements and reservoir levels, but not with rainfall and seismic activity. - Displacement could be interpreted as simply preparatory to a series of small landslides and not to an instability involving the whole slope. - Triggering factors: low friction resistance of clayey interbeds and seismic activity. - Induced water wave: total volume 48 Mm3 ; maximum elevation 200 m on reservoir level; volume overpassing the dam 30 Mm3. - Results of Ghetti model hydraulically reliable, differences due to involved volumes and movement rate. - The exceptional phenomenon is the reactivation of the ancient landslide. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  32. 32. STUDIES AND RESEARCHES AFTER THE FAILURE Müller (1964) first scientific paper on 1963 Vajont landslide and related events. After description of the studies carried out and of the phenomena observed, considering velocities before the failure and that calculated during collapse, he concludes: transition from initial long creeping stage to true rock slide was caused by, “the slight excess of driving forces, due to the joint water thrust or to the decrease in resisting forces, resulting from the buoyancy and softening of clayey substances during higher water level [...] with a progressive rupture mechanism at the base of the moved mass”. He attributes to the landslide a velocity of 25-30 m/s, consequence of a “spontaneous decrease in the interior resistance”, favoring the hypothesis of a new first-time landslide contrasting his initially agreement with Giudici & Semenza (1960) on the existence of a prehistoric landslide. In conclusion, he strongly believes in the substantial unpredictability of many aspects of the landslide. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  33. 33. STUDIES AND RESEARCHES AFTER THE FAILURE Kiersh’s hypothesis and sections (1964, 1965) assumed valid in many subsequent studies on the Vajont landslide: existence of a prehistoric landslide; presence of highly fractured rocks due to the effects of the last glacial period; collapse triggered by a rise of the groundwater level with increased hydrostatic uplift and swelling pressures. Selli et al. (1964) comprehensive work on phenomena that accompanied the event (in Italian). Authors hypothesis: mass moved with a generally pseudo-plastic behavior; main causes ascribed to particular geological structure, slope morphology and variations in reservoir water level. Maximum velocity is calculated in 17 m/sec. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  34. 34. International Conference Vajont, 1963-2013 STUDIES AND RESEARCHES AFTER THE FAILURE GEOLOGICAL ASPECTS After 1960 Giudici & Semenza geological studies, many other were carried out, but basic lithostratigraphy is still considered valid except some chronological considerations. Carloni and Mazzanti (1964): first detailed geological and geomorphological. Rossi and Semenza (1981): geological maps of the Vajont landslide area, before and after the catastrophe, published for the first time. Stratigraphy of the Vajont valley by Carloni & Mazzanti (1964), to the left and by Besio & Semenza1963), to the right. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  35. 35. STUDIES AND RESEARCHES AFTER THE FAILURE Carloni and Mazzanti (1964): first detailed geological and geomorphological. Rossi and Semenza (1981): Colle Isolato, located on right side, interpreted as remnant of an ancient landslide; geological maps of the Vajont landslide area, before and after the catastrophe, published for the first time. M. Ghirotti (1993, 1994) completed geological data with a geomechanical survey of rock masses. E. Semenza (2000) palinspastic evolution from postglacial to 1963 failure Paronuzzi (2009) new engineering-geological model: presence at the base of a 30-70 m thick shear zone. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  36. 36. STUDIES AND RESEARCHES AFTER THE FAILURE Hendron & Patton work (1985, 1986): first attempt to analyze the landslide from geological, geomorphological and geotechnical points of view. - Landslide is considered a reactivation of a post-glacial one slid over one or more clay levels, acting as a continuous impermeable layer and as a level with residual friction angle as low as 5°. Two aquifers In the northern slope of Mt.Toc : the upper one (highly fractured and permeable landslide mass) influenced by reservoir’s level; the lower one (Calcare del Vaiont Fm.) fed by both rainfall in the hydrogeological basin and reservoir’s water. The hydrogeological scheme may give rise to high water pressures in the hypothesis that clay strata separating the two aquifers were continuous. They conclude: ‘‘The piezometric data of the Vajont slope are too little and questionable; it is not sufficient for drawing up a reliable hydrogeological model, which is necessary in these cases to make reasonable assumptions about the pore water pressures for slope stability analysis’’. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  37. 37. STUDIES AND RESEARCHES AFTER THE FAILURE RESEARCH ON SPECIFIC ASPECTS Many papers published differently interpreting and strongly debating particular aspects of the landslide event, ranging from geotechnical properties to physical and rheological behavior and to stability analyses, in the attempt to understand the role of factors involved in the landslide triggering and development. First-time or reactivated landslide The first primordial question is : was the landslide a first-time or the re-activation of an old one? A question initially faced up by Hendron and Patton (1983, 1985), but still now object of studies and discussions. Generally accepted the idea that landslide occurred, at least in part, on the slip surface of an old landslide, but different hypotheses exist. For instance, slip surface coincides with a normal fault plane juxtaposing Cretaceous limestones and highly fractured rock mass (Mantovani & Vita-Finzi, 2003). The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  38. 38. STUDIES AND RESEARCHES AFTER THE FAILURE HYDROGEOLOGY The lack of reliable water pressure data is a very great drawback . Schematic hydrogeological section drawn by Semenza & Dal Cin. Besio (1986) indicates four aquifers (quaternary deposits; Scaglia Rossa, Calcare di Soccher and Rosso Ammonitico, impervious bedrock Fonzaso Formation; the Calcari del Vajont, impervious bedrock Igne Formations; the Calcari di Soverzene and the Dolomia Principale, the main aquifer. Springs are numerous, but only a few with significant discharges (1-10 l/s). The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  39. 39. STUDIES AND RESEARCHES AFTER THE FAILURE Existence and continuity of clay beds in the calcareous sequence A rather discussed aspect. Broili (1967), as Müller (1964), concludes for the absence of clay beds, but just some films of pelitic materials, a few mm thick, seldom observed that could not have played any significant role in slope failure. Müller (1968), re-analyzing data reaches different conclusions stressing, instead, the relevance of the “chair-like” shape of the slip surface. Nowadays , is generally accepted that failure occurred along a few cm thick clayey beds intercalated to the limestones strata, rather continuous over large areas, acting as impervious layer. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  40. 40. STUDIES AND RESEARCHES AFTER THE FAILURE High velocity A significant loss of strength is required, but which mechanisms controls the rate of movement and sudden acceleration? Various interpretations depending on first-time landslide or reactivation of an old prehistoric one (Mencl,1966). Several mechanisms responsible for the frictional evolution have been investigated using different models and formulating different assumptions for the evolution of friction coefficient with both time and deformation. Müller (1968) reduction of frictional resistance related to creep phenomena and progressive failure, as at limit equilibrium required friction resistance is too small with respect to the strength properties of material involved. Relevant loss of strength in terms of pore water frictional heating and thermal pressurization. Thermoplastic softening behavior of clays studied by many authors (Hicher, 1974; Despax, 1976; Modaressi & Laloui, 1997): friction angle at critical state of some clays may be a decreasing function of the temperature, depending on clay mineralogy. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  41. 41. STUDIES AND RESEARCHES AFTER THE FAILURE Ciabatti (1964) proposes a pore water pressure rise due to frictional heating and, considering a variable friction coefficient, estimates a maximum velocity of 17 m/s . The pressurization mechanism, explaining total loss of strength, based on the original "vaporization" concept of Habib (1967, 1975), later discussed by other authors (Voight & Faust, 1982; Vardoulakis, 2000 and 2002; Veveakis & Vardoulakis, 2007; Goren & Aharonov, 2007). Frictional heat and the consequent vaporization of pore water may lead, if the surface of failure is deep enough, to a strong increase in pore water pressure causing the loss of frictional resistance. Critical displacement necessary to create vapor in the slide zone and relation between critical displacement and rate of shear displacement can be calculated (Habib, 1975). Voight & Faust (1982), starting from Ciabatti (1964) model, propose a thermal mechanism for the low kinetic friction mobilized and calculate acceleration, velocity (maximum: 26 m/s) and elapsed time as functions of displacements. Nonveiller (1978, 1987) propose the same mechanism but estimates a maximum velocity of 15 m/s. Semenza & Melidoro (1992) try to explain both high velocity and run-out considering the final accelerated phase, and conclude that this mechanism can really induce high velocities, but is effective only after rather long The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca times.
  42. 42. STUDIES AND RESEARCHES AFTER THE FAILURE Vardoulakis (2002): frictional heating can trigger an explosive pressurization phase after a long-term phase of accelerating creep. Final total loss of strength explained by thermal pressurization, triggered by the temperature rise within clay layers. The pore pressure increase can convert the slow sliding into a catastrophic failure. Author calculates in this way a velocity of 20 m/s just 8 s after its activation, that is a slide displacement of 74 m. Goren & Aharonov (2007): using a similar thermo-poro-elastic mechanism obtain the development of high pore water pressure and reduced friction resistance, and then large sliding velocities and run-out. Furthermore, pore pressure diffusion rates is mainly controlled by the depth-dependent permeability, and then greater landslides are able to maintain high pore pressure for longer times, resulting in lower values of the dynamic friction angle. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  43. 43. STUDIES AND RESEARCHES AFTER THE FAILURE Tika & Hutchinson (1999): ring shear tests on two clay samples from the slip surface obtaining relevant loss of strength increasing the shear rate. Residual friction angle at slow rate (9.7 -10.6 ) compares rather well with previously reported values (8 -11 ), but at rates greater than 100 mm/min, after an initial increase, the dynamic residual value falls to 4.4 . There is then no need to invoke other strength loss mechanisms. Ferri et al. (2010, 2011): Laboratory tests with higher shearing rates show even smaller values of the friction coefficient. Friction angle initially increases from 24.2 25.6 at low velocity to 34.2 at 0.04 m/s and falls to 5.1 for velocity up to about 1.3 m/s due to temperature increase up to about 260 C. Increase in water content reduces the shear strength enhancing the velocity weakening mechanism as already observed by Tika & Hutchinson (1999) and in agreement with the Helmstetter et al. (2004) and Sornette et al. (2004) models. In saturated conditions, thermal and thermo-chemical pressurization are not required to explain the high slip rates achieved during the final collapse of the landslide, at least for shear rate lower than 1.3 m/s, that is the maximum velocity investigated. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  44. 44. STUDIES AND RESEARCHES AFTER THE FAILURE Sornette et al (2003): slider-block model, derived from Voight (1988) model gives, good predictions of the critical time-to-failure up to 20 days before the collapse. Helmstetter et al. (2004): apply the state and velocity-dependent friction law established and used to model earthquake friction. Observed displacements can be reproduced with the slider block friction model, suggesting that Vajont landslide belongs to the velocity-weakening unstable regime. High velocity has been attributed also to other deformation mechanism such slow rock cracking process. Kilburn & Voight (1998): stresses concentration at micro-cracks tips make the cracks to grow at an accelerating rate and to coalesce in a unique shear plane or band. Burland, 1990; Petley, 1995: Laboratory studies show that brittle behavior can occur also in undisturbed and water-saturated clays stressed to loads corresponding to depths more or less coinciding with the depths of the deforming clay layers in Mt Toc. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  45. 45. STUDIES AND RESEARCHES AFTER THE FAILURE Kilburn & Petley (2003): The slow cracking mechanism can accelerate under constant applied stress and is quickly enhanced by circulating water, chemically attacking molecular bonds at crack tips. Initially it is dominated by the formation of new cracks increasing exponentially with time (Main and Meredith, 1991), and, later, by the exponential growth of cracks length (Main et al., 1993) until a unique failure plane is formed (McGuire and Kilburn, 1997; Kilburn and Voight, 1998). The Authors propose a new model for the development of progressive failure in brittle landslides based on Saito (1965, 1969), Voight (1988, 1989) and Fukozono (1990) works: the linear inverse-rate velocity trend with time is representative of the existence of movements dominated by crack growth, a process that indicates the approach to catastrophic collapse. The Vajont Landslide: State-of-Art by R. Genevois & P.R. Tecca
  46. 46. STUDIES AND RESEARCHES AFTER THE FAILURE Burland, 1990; Petley, 1995 : Clays can behave as brittle material under high loads and observations on slope movements (Voight, 1988) are consistent with the failure behavior of clays at high pressure: failure mechanism evolves from ductile process (creep phase), to a brittle process (collapse phase) (Petley & Allison, 1997, and Petley, 1999). Alonso & Pinyol (2010): reduction to minimum values of available strength along “dormant” sliding surfaces in high plasticity clays, but even full degradation of cohesion cannot lead to velocities higher than 3 m/s. The process must be accompanied by other mechanisms. Cecinato et al. (2010) model accounts for frictional heating pressurization and shows: thermal friction softening may be considered secondary, with respect to static and dynamic friction softening. Thermal pressurization will cause, besides, thicker slides to accelerate faster. October, 8-10 2013 The Vajont Landslide: State-of-The-Art by R. Genevois & P.R. Tecca 46
  47. 47. STUDIES AND RESEARCHES AFTER THE FAILURE Stability analises A large number of stability analyses have been performed using limit equilibrium methods and concentrating on the evaluation of friction angle necessary for stability. Obtained values are rarely >12 (Jäeger, 1965; Nonveiller, 1967; Mencl, 1966; Skempton, 1966; Kenney, 1967). Kenney (1967): filling of the reservoir reduces stability by only 5-10%. Lo et alii (1971): consider two wedges separated by a vertical discontinuity ; the friction angle at limit equilibrium is 13 for a groundwater level corresponding to the water level in the reservoir. Hendron & Patton (1985) back-calculated friction angle required for stability with two-dimensional limit equilibrium, obtaining 17 -28 , rather high with respect to 5 - 16 obtained by laboratory tests: other factors were not considered. The necessary resisting force to ensure equilibrium is provided by the side friction on the Eastern edge of the slide (three-dimensional stability analyses). October, 8-10 2013 The Vajont Landslide: State-of-The-Art by R. Genevois & P.R. Tecca 47
  48. 48. STUDIES AND RESEARCHES AFTER THE FAILURE Chowdhury (1978) considered the progressive failure : in the landslide mass exists an upper portion, fundamentally unstable, that gradually creeps down and thrusts on a lower more stable portion: forces are so progressively increased up to cause the sudden failure of the lower portion. A model consistent with the existence of a nonuniform weakening zone separating upper from lower part in sliding mass (Jäeger,1972). Sitar & Maclaughlin (1997) introduced the DDA (Discontinuous Deformation Analysis) technique. Subdividing the landslide mass in a number of blocks from 1 to 105, the friction angle required for stability increases from 7 to 16 , depending on the inter-block friction and the position of the vertical discontinuity. Obtain a trend rather similar to that of Chowdhury (1978) modeling the progressive failure. Later, Sitar et al. (2005) indicate that disintegration of the landslide during failure results in acceleration of the slide mass; the process of landslide mass disintegration has an effect similar to the pore pressure rise consequent to the frictional heating. Considering both of these processes, peak velocities (25–30 m/s) are comparable to those estimated by Hendron and Patton (1985): both mechanisms might have had a fundamental role. October, 8-10 2013 The Vajont Landslide: State-of-The-Art by R. Genevois & P.R. Tecca 48
  49. 49. STUDIES AND RESEARCHES AFTER THE FAILURE Erismann & Abele (2001) stated that, considering the scientific knowledge at that time, the Vajont catastrophe could have been foreseen as regards the transition from slow to fast motion. Alonso & Pinyol (2010) stability analyses of a simple two-wedge models, show that rock shear strength mobilized between the two wedges is practically in accordance with the expected strength of present Cretaceous marls and limestones. October, 8-10 2013 The Vajont Landslide: State-of-The-Art by R. Genevois & P.R. Tecca 49
  50. 50. STUDIES AND RESEARCHES AFTER THE FAILURE Landslide generated water waves Use of mathematical theories, physical models and numerical simulations in order to describe: (i) triggering of the landslide (quality and quantity of data); (ii) propagation (material rheology); iii) landslide–water interaction, (permeability of collapsing material); (iv) propagation of wave (usually modeled with depth integrated models). Panizzo et al (2005): empirical formulations for impulse waves; application to Pontisei and 1960 and 1963 Vajont landslides estimates quite well the values of both the maximum generated wave height and run-up. Roubtsova &Kahawita (2006): three-dimensional simulation of the Vajont landslide induced water wave using SPH and technique to treat free surface problems. Ward & Day (2011): three dimensional simulation of Vaiont landslide (semicoherent slump) and flood disaster applying the «tsunami ball» method . Findings : a landslide of that volume can splash water up to about 200 m, push more than 30 million m3 of water over the dam and flood the valley below. October, 8-10 2013 The Vajont Landslide: State-of-The-Art by R. Genevois & P.R. Tecca 50
  51. 51. STUDIES AND RESEARCHES AFTER THE FAILURE General comments Technical literature quite copious, mainly due to inconsistencies in the interpretations of the event but, great part of major questions have not yet been adequately explained (e.g., relevance of clayey layers; nature of landslide; high velocity). However, the scientific literature shows: i) the critical relevance of geological features; ii) we are at the beginning in the development of concepts and tools in modeling and predicting these type of events. iii) issues emerging from the Vajont disaster induced to a more integration between different geoscience disciplines and also engineering for a better understanding of landslide events, triggers, and processes. October, 8-10 2013 The Vajont Landslide: State-of-The-Art by R. Genevois & P.R. Tecca 51
  52. 52. FURTHER RESEARCH AND DEVELOPMENT ἓν οἶδα ὅτι οὐδὲν οἶδα - Socrates (I know one thing, that I know nothing) Researches on Vajont landslide and, in general, on large gravitational phenomena, are carried on everywhere in the world. In the proceedings of this conference you will find some of them relating in particular to the Vajont landslide (e.g.: Bistacchi et al., Fabbri et al.,Paparo et al., Petronio et al., Wolter et al., Zaniboni et al.) Even today, to deal with very large landslide is a formidable task and areas that need to be developed are mainly: 1. 3-D modelling, for a better representation of spatial and kinematic effects, especially as regards the complexity of sliding surfaces. 2. brittle behavior of large landslides including assessment of shear strength of weak fissured clay rocks and relationship between displacement velocity, mechanics, and shear strength on failure surfaces. 3. pre-failure deformation trend in large landslides for a more reliable model of landslide evolution. 4. better understanding of fatigue and progressive failure processes, studying the development of deep-seated rockslide. October, 8-10 2013 The Vajont Landslide: State-of-The-Art by R. Genevois & P.R. Tecca 52
  53. 53. CONCLUDING REMARKS A mess of almost impenetrable and contrasting events, technical aspects and human behavior imprinted the long inexorable approach to the 9th October disaster. All but E. Semenza have some technical responsibilities in the events that preceded the catastrophic landslide, even considering the knowledge at that time. C. Semenza istinctively knew that something terrible was going on since April 1961, when wrote : “[…] things are probably bigger than us and there are no adequate practical measures […] I am in front of a thing which due to its dimensions seems to escape from our hands […] The question nowadays is just : an improved knowledge of the field situation and a deeper knowledge of the rock masses behavior could have provided reliable criteria to avoid or at least stop the landslide? This is a scientific, and not yet completely solved problem. However, as important pressures were done to complete the dam in planned time, enormous and unforgivable responsibilities relapse on the way works were run. What it was said more than 2000 years ago should be considered still effective? Nihil tam munitum quod non expugnari pecunia possit. "Nothing is so fortified that it can't be conquered with money." (Cicero) October, 8-10 2013 The Vajont Landslide: State-of-The-Art by R. Genevois & P.R. Tecca 53

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