Landslide information
THE VAJONT (Vaiont) LANDSLIDE
Dr David Petley
Figure
0: The Vajont landslide looking from upstream (image thanks to Professor E.
Bromhead, University of Kingston)
INTRODUCTION
The Vajont reservoir disaster is a classic example of the consequences of the
failure of engineers and geologists to understand the nature of the problem that
they were trying to deal with. During the filling of the reservoir a block of
approximately 270 million m3 detached from one wall and
slid into the lake at velocities of up to 30 m sec-1
(approx. 110 km h-1). As a result a wave over topped the
dam by 250 m and swept onto the valley below, with the loss of about 2500 lives.
Remarkably the dam remained unbroken by the flood.
LOCATION AND BACKGROUND
Vajont is located in the south-eastern part of the Dolomite
Region of the Italian Alps, about 100 km north of Venice (Fig. 1). It was built
as a part of the on-going, post-war development of Italy in order to provide HEP
for the rapidly-expanding northern cities of Milan, Turin and Modena. Whilst a
proposal to site a dam at this location was made in the 1920's, excavation of
the site began in 1956 and the dam was completed in 1960. The completed doubly
curved arch dam was, at 265.5 metres above the valley floor, the worlds highest
thin arch dam. The chord of the dam was 160 m, and the volume of impounded water
was 115 million m3.
Figure 1: Schematic map of
the location of the Vajont dam and reservoir
The dam was built across the
Vajont Valley, a deep, narrow gorge. The geological setting of the valley was
fully understood. In this area, the mountains tend to be characterised by
massive, near-vertical cliffs formed in the Jurassic Dogger formation and
underlying Triassic formations. The local valleys tend to be associated with
outcrops of the weaker formations, particularly the Upper and Lower Cretaceous
and Tertiary units, which contain more clays and are more thinly bedded. Thus
the generalised geological structure is of a syncline cut by the valley (Fig.
2). The syncline is based in middle Jurassic limestone, overlain with successive
layers of upper Jurassic limestone with clay and Cretaceous
limestones.
Figure 2: Schematic
cross-section through the Vajont valley
ORDER OF EVENTS
1. Prior to the Completion of the Dam
It appears that during the construction of the dam the chief engineer was
concerned about the stability of the left bank of the dam, and a number of
reports were compiled on this during 1958 and 1959, which identified a possible
prehistoric slide on the right bank. Whilst there was considerable discussion of
the stability of the valley walls in view of the inclined synclinal form of the
strata and the possibility of old slides in this area, it was concluded that
deep-seated landslides were extremely unlikely as (see Muller 1964 for a review
of this):
- areas of weakness were not identified in the three test borings;
- it was assumed that any shear plane would have a 'chairlike' form that
would exert a 'braking effect';
- seismic analyses had suggest that the banks consisted of very firm in-situ
rock with a high modulus of elasticity.
Smaller slides in the looser surface layers were considered to be likely,
although volumes and velocities of movement were expected to be low.
2. During the First Filling of the Reservoir
Filling was initiated in February 1960 (Fig. 3), before final
completion of the dam (which occurred in September 1960). By March 1960 the
level of the reservoir had reached 130 m above the level of the river, when the
first small detachment occurred. Continued filling of the reservoir occurred
whilst monitoring of the movements in the banks was undertaken. In October 1960,
when the depth of the reservoir had reached 170 metres, a rapid increase in the
rate of displacement to approximately 3.5 cm day-1 was
observed. At the same time a huge joint of 2 km length opened up, defining an
area about 1700 m long and 1000 m wide, suggesting that a very large landslide
had been mobilised. On 4 November, with the depth of the reservoir at 180 m, a
large failure occurred when 700 000 m 3 of material slid
into the lake in about ten minutes. As a result the level of the reservoir was
gently dropped back to 135 m. At this point movement reduced to close to 1 mm
day-1.
It was realised by the designers of the dam
that the large mass of the left bank was inherently unstable. However Muller
(1964) states that:
'It appeared hopeless to arrest the slide
artificially, because all means that would have had to be applied were beyond
human bounds. It was also impossible to either seal the surface of the area, to
shift the weight or to cement the rock by means of injections. On the other hand
the possibility of accelerating the sliding movement in order to let the entire
mass to slide down all at once had to be excluded. The danger arising for the
formation upstream of the slide by an uncontrollable level of the storage lake
would have been too great.'
Figure 3: Chronology of
events leading up to the catastrophic failure
Thus it was decided that an attempt could be made to gain
control of the sliding mass by varying the level of water in the reservoir
whilst controlling the joint water thrust within the rock mass by means of
drainage tunnels. It was realised that this could lead to the blockage of that
section of the reservoir by the landslide mass. However the volume of water in
the unblocked (upstream) section would still be sufficient to allow the
generation of electricity. Hence a bypass tunnel was constructed on the opposite
(right) bank such that if the reservoir was divided into two sections the level
of the lake could still be controlled.
It was assumed that by elevating
the level of the reservoir in a careful manner movement of the large landslide
mass could be initiated. The rate of movement could be controlled by altering
the level of the lake. It was realised that a final sudden movement might occur,
and it was calculated that, so long as the movement did not exceed a rate that
would lead to filling of the reservoir by the landslide in ten minutes or less,
over-topping of the dam would be avoided.
3. First Draw-Down of the Reservoir
Creep had been initiated by the initial filling of the reservoir. As the
level was subsequently drawn down, rates of movement decreased from a maximum of
about 8 cm day-1 to 3 mm day-1 at a
level of 185 m and less than 1 mm day-1 at 135 m. By this
time the main landslide mass had moved an average of about 1 m.
4. Second Filling of the Reservoir
From the beginning of October 1961 through to early February 1962 the water
level was raised to 185 m, followed by a phase of slow impoundment such that in
November 1962 the level had reached 235 m. During the early part of this phase
velocities did not substantially increase, but by the end of the phase
velocities had increased to 1.2 cm day-1.
5. Second Draw-Down of the Reservoir
In November 1962 a second lowering of the level was slowly undertaken, with
the water depth decreasing to 185 m after four months. Initially displacements
remained high but in December they began to reduce and, by early April when the
water height had reached 185 m, the rate was effectively zero. The experiences
gained from the second phase of filling and the subsequent draw-down confirmed
to the engineers that control of the landslide was possible by altering the
level of the reservoir. In consequence a third filling of the reservoir was
undertaken.
6. Third Filling of the Reservoir
Between April and May 1963 the reservoir level was rapidly increased to 231
m. Slight increases in velocity were noted, but rates never exceeded 0.3 cm
day-1. During June the level was increased to 237 m and the
rate of displacement increased to 0.4 cm day -1. In mid
July the level reached 240 m and some of the control points indicated small
increases in displacement to 0.5 cm day-1. The level was
maintained through to mid-August, but during this time velocities increased to
0.8 cm day-1. In the latter part of August the level was
increased once more such that by early September the depth of water was 245 m.
In some parts of the slide velocities increased to as much as 3.5 cm day-1.
7. Third Drawing Down of the Reservoir
In late September the water level was slowly dropped to bring the rates of
creep back under control. By October 9 a depth of 235 m was reached. However
velocities of movement continued to slowly increase, and by October 9 rates of
up to 20 cm day-1 were recorded.
8. Catastrophic Failure
At 22:38 GMT on October 9 1963 catastrophic failure of the landslide
occurred. The entire mass slid approximately 500 m northwards at up to 30 m
sec-1. The mass completely blocked the gorge to a depth of
up to 400m , and it travelled up to 140 m up the opposite bank. Movement of the
landslide mass ceased after a maximum of 45 sec. At the time the reservoir
contained 115 million m3 of water. A wave of water was
pushed up the opposite bank and destroyed the village of Casso, 260 m above lake
level before over-topping the dam by up to 245 m. The water, estimated to have
had a volume of about 30 million m3, then fell more than
500 m onto the villages of Longarone, Pirago, Villanova, Rivalta and Fae,
totally decimating them. A total 2500 lives were lost. However the dam was not
destroyed and is still standing today. The by-pass tunnel is used for the
generation of HEP.
CAUSES OF FAILURE
Since the catastrophic failure, a huge range of work has been undertaken on
the causes of the failure. Initially the was a large amount of speculation about
the location of the sliding surface, but more recent studies have confirmed that
it was located in thin (5 - 15 cm) clay layers in the limestone. It is claimed
by some that as such it represents a reactivation of an old landslide (Hendron
and Patten, 1985; Pasuto and Soldati, 1991), whilst others claim that it was a
first-time movement (Skempton, 1966; Petley, 1996). It is likely that increasing
the level of the reservoir drove up pore pressures in the clay layers, reducing
the effective normal strength and hence the shear resistance. Resistance to
movement was created by the chair-like form of the shear surface. Dropping the
level of the reservoir induced hydraulic pressures that increased the stresses
as water in the jointed limestone tried to drain. It has been estimated that the
total thrust from this effect was 2 - 4 million tonnes (!?) (Muller, 1964).
Failure occurred in a brittle manner, inducing catastrophic loss of strength.
The speed of movement is probably the result of frictional heating of the pore
water in the clay layers (Voight and Faust, 1982, 1992).
REFERENCES
Hendron, A.J., and Patten, F.D, 1985, The Vaiont Slide.
US Corps of
Engineers Technical Report GL-85-8.
Jaegar, C., 1980,
Rock
mechanics and Engineering. Cambridge University Press, 523
pages.
Kiersch, G.A., 1964. 'Vaiont reservoir disaster'.
Civil
Engng,
34, 32-39.
Müller, L., 1964, The rock slide in the
Vaiont valley.
Felsmechanik und Ingenieur-geologie,
2, 148-212.
Pasuto, M. and Soldati, A. 1990. 'Some cases of deep-seated
gravitational deformations in the area of Cortina d'Ampezzo (Dolomites)',
The
Proceedings of the European Short Course on Applied Geomorphology, 2,
91-104.
Petley, D.N. 1996. 'The mechanics and landforms of deep-seated
landslides'. Brooks, S., and Anderson, M (eds).
Advances in Hillslope
Processes, John Wiley, Chichester.
Skempton, A.W. 1966.
'Bedding-plane slip, residual strength and the Vaiont landslide',
Geotechnique, 16, 82-84.
Voight, B. and Faust, C., 1982,
Frictional heat and strength loss in some rapid landslides
Geotechnique,
32, 43-54.
Voight, B. and Faust, C. 1992. 'Frictional heat and
strength loss in some rapid landslides: error correction and affirmation of
mechanism for the Vaiont landslide',
Geotechnique,
42,
641-643.
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