The Vajont Dam Failure
Introduction:
The Vajont Dam disaster is an excellent example of engineers not fully understanding and
accepting the importance of attention to detail and warning signs. Mt. Toc and the southern
slope of the Vajont valley were more prone to instability due to the fact that the mountain’s
outer sides consisted of an ancient landslide deposit and not of bed rock as initially inferred.
After several times of raising and lowering the reservoir level, the southern margin of Mt. Toc
eventually destabilised and after almost three years of intermittent creeping, at 22.39, on
October 9, 1963 a huge landslide occurred, resulting in 260 million m3 of forest, earth and
rock to fall into the reservoir at speeds of up to 110 km/hr.
Location and Background:
The Vajont Dam is located under Mount Toc in the south-eastern part of the Dolomite
Region of the Italian Alps, 100km north of Venice, Italy. The dam was built as part of the
post-war development of Italy so as to provide hydro-electric power to the growing cities of
Milan, Turin and Modena. Initially SADE (Società Adriatica di Elettricità) managed the Vajont
project and they assured the public that the geology of the valley had been sufficiently
studied, including analysis of ancient landslides and that it was sufficiently stable for
construction. The project was later taken over by ENEL (Ente Nazionale per l'energia
Elettrica). The general geological structure of the valley is of a syncline cut by the valley as
shown below. It is based in middle Jurassic limestone, overlain with successive layers of
upper Jurassic limestone with clay and Cretaceous limestones.
Design
The Vajont Dam was completed in 1960 and was one of the highest dams built in the world.
It measured 262m high, 27m thick at the base and 3.4m thick at the top. The Vajont dam
was the world’s highest thin double-arched dam, the chord measuring 160m to contain a
volume of 150 million m3 of water.
The Timeline to Disaster
1957: SADE sought a permit to increase the height of the Vajont dam from 200m to
261.60m. The reservoir was now to contain 150 million m3 of water as opposed to the 58
million m3 of water it was originally designed for.
March, 1959: a nearby hydroelectric reservoir at Lake Pontesei, developed problems.
Yellow splotches were observed bubbling to the surface of the Pontesei Reservoir, always in
the same place, indicating that the sides of the mountain were absorbing water and a
landslide was approaching.
Also in 1959: cracks developed on the Mount Toc side of the valley, as a result of the
construction of a ring road around the reservoir. The builders were instructed to fill in the
cracks and continued on working but the cracks reappeared. The chief engineer, Dr.
Semenza then requested a survey of the area. From this survey, it was concluded that the
Mount Toc side of the valley was a prehistoric landslide and was in the process of moving
down Mount Toc. Further studies indicated that the ancient landslide was stable but would
eventually creep down the valley. SADE, however, decided that a deep-seated landslide
was highly unlikely and informed the public that the dam was sufficiently stable because:
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three test borings had failed to identify any areas of weakness
it was assumed that any shear plane would have a 'chairlike' form that would exert a
'braking effect'
analysis using seismic reflection profiling had suggested that the banks consisted of
very firm in-situ rock with a high elastic modulus (thus is very stiff and makes it less
likely to fail under a compressive/tensile load)
January 2, 1960: SADE completed the Vajont Dam and proceeded to the testing phase.
This involved impoundment of various amounts of water to about halfway up the dam. At the
same time, Italy decided to nationalise its hydroelectric industry and as a result, SADE
decided to raise the water level to 660m (as opposed to the allowed 600m) above sea-level.
However, they realised that the more the water level rose, the more the foot of the creep
zone moved, resulting in yellow/brown bubbles being sent to the surface of the reservoir
(similar to Pontesei). An indication that a landslide was imminent!
March 1960: The increasing water level resulted in a small rupture.
October 1960: A further increase in the water level to 170m lead to an increase in the rate
of movement to approximately 3.5cm/day.
November 1960: 700,000 m3 of rock slid down into the reservoir in the space of 10 minutes,
resulting in a giant wave. Also a metre-wide crack appeared, high up the mountain.
Seismographs were then installed on top of the dam to assess displacements.
The level of the reservoir was then gently dropped back to 135m above river level and the
movement reduced to close to 1mm/day. In the meantime, a bypass tunnel was constructed
under the reservoir, so that it could still produce hydroelectric power, if the landslide was to
occur.
December 1961: A second test was organised by the government, which involved the
impoundment of water to an even higher level. As the water level increased, the
seismographs at the top of the dam began to set off two out of every three days.
As a result, on October, 1962, all the water was drained out of the reservoir and the
mountain settled down again. The engineers now understood that control of the landslide
was possible by altering the water level of the reservoir.
March 14, 1963: the Vajont project was passed to ENEL (National Energy Agency).
Because a required third impoundment had not taken place, SADE received six days
temporary custody of the project to bring the water level to 715m above sea level, despite
the fact that they were informed in the early phase of the design process to never exceed
700m above sea level else the dam might fail. This water level was maintained through
early September, 1963, and the rates of displacement increased to as much as 3.5cm/day.
ENEL, took over complete ownership of the project and by the end of August, 1963, the
water level reached 710m above sea level.
September 2, 1963: An earthquake (Grade 7 on the Mercalli Scale) occurred at Mount Toc.
This was followed by another tremor on September 15, 1963. The new chief engineer,
Biadene, insisted that the situation was under control.
September 17, 1963: the engineers turned off the water entering the dam for a week to help
stabilise the rate of creep. Although the water level was stationary, Mount Toc continued to
slide. Removal of the water would be catastrophic as would leaving the body of water in the
reservoir. By now, the velocity of displacement was up to 20cm/day, but instead of informing
the public, the chief engineer said nothing. The people in Erto were told to “remain alert” by
the police.
At 22.39 on October 9, 1963, the landslide occurred. It slid approximately 500m northwards
with speeds up to 30m/second. The mass of material completely blocked the valley to a
depth of up to 400m and it travelled up to 140m on the opposite side. The landslide lasted
for approximately 45 seconds. At the time, the reservoir contained 115 million m 3 of water.
A tsunami wave then moved upstream (up the opposite side of the valley) and destroyed the
village of Casso, before overtopping the dam by up to 245m. The foul-smelling mass of
water, approximately 30 million m3, preceded by a roaring wind, destroyed the villages of
Longarone, Pirago, Villanova, Rivalta and Fae and killed 2,500 people.
However the dam was not destroyed in the landslide and is still standing today.
Conclusions
There are various different suggestions as to why this disaster occurred:
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reactivation of an old landslide
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a completely new landslide
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increasing the level of the reservoir drove up pore pressures in the clay layers,
reducing the effective normal strength and hence the shear resistance
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contribution of rainfall, dam level changes and earthquakes as triggers of the
landslide
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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.
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The profit motive and capitalism have been blamed for cutting corners in the building
of the dam
After researching this disaster it was clear to see that all the warning signs were there, years
in advance. The engineers chose to ignore these signs and not to warn the nearby villages.
The chief engineer needed to swallow his pride and admit defeat. Our group have learned
an important lesson from this, that in the engineering world it is far better to be safe than
sorry!