Genevois - The Vajont Landslides

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

• 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

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


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

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 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

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


04.11.1960: A 7-8x105 m3 slide occurs on left slope of the reservoir

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


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

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


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


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 CATASTROPHE

The exposed failure surface and the
slid mass on the foreground
The landslide mass filling up the reservoir

THE CATASTROPHE

The resulting water wave flows to both the Est and the West


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 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.

LONGARONE

After

Before


THE STATE OF KNOWLEDGE
BEFORE THE FAILURE

Aeschilus: πάθει μάθος
[There is] learning in suffering

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.


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" .



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.


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).

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.



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.


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.


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 VALLEY
BEFORE

AFTER


STUDIES AND RESEARCHES
AFTER THE FAILURE


STUDIES AND RESEARCHES AFTER THE FAILURE

REPORTS JUST AFTER THE LANDSLIDE



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 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.

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.



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.



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.



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.


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’’.

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).


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).

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.


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.

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.

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.


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.


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.


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.


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

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


47

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


48


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


49

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


50


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


51

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


52

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


53

Genevois - The Vajont Landslides

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