THE VAJONT DAM
Slope Stability Failure
INTRODUCTION
• The Vajont Dam is located 100 km north of
Venice under Mount Toc in the Italian Alps.
• Completed in 1961, it was the biggest dam in the
world measuring 354.6 metres high, 27 metres
thick at the base and 3.4 at the tip.
• 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.
INTRODUCTION
• 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 160m, and the volume of impounded water
was 115 million m³.
LOCATION AND BACKGROUND
BACKGROUND
• The dam was built across the Vajont
Valley, a deep, narrow gorge.
• Testing of the Mount Toc, which took place
after the project had already been
approved, found that the mountain had a
tendency to free huge amounts of rock,
causing unexpected landslides.
• The project went ahead…
Schematic Cross-Section through the Vajont Valley
PRIOR TO COMPLETION
• 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.
FILLING OF RESERVOIR
• Filling was initiated in February 1960.
• The artificial lake was continuously filled
and flared in order to test the stability of
Mount Toc, the drainage basin and the
subsoil. This caused the detachment of
rock which caused minor landslides.
• It was realized by the designers of the
dam that the large mass of the left bank
was inherently unstable.
CATASTROPHIC FAILURE
•
•
•
•
At 22:38 GMT on October 9 1963 a 250 million metres cubed rock mass slid
approximately 500m northwards at up to 30 m/sec.
The mass completely blocked the gorge to a depth of up to 400m, and it
traveled up to 140m 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 245m.
CATASTROPHIC FAILURE
• The water, estimated to have had a
volume of about 30 million m³, then fell
more than 500m 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, 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 whilst others
claim that it was a first-time movement.
• 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
FAILURE OF DAM
• 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.
• Remarkably the dam
remained unbroken by
the flood.
Credits
Chief Whip - Paul Rogers
Dr. of Love - Alan Mathews
Music - Laura Hartmann
Editor - Paul Conlon
In Association with Group 14 Productions