Vajont Dam
Introduction
The Vajont Dam tragedy has been listed by UNESCO as one of the five worst man-made
disasters caused by “the failure of engineers and geologists”.
After many years of planning, the Vajont Dam was completed in 1961. It was one of the highest
dams in the world, being 262 metres high, 27 metres in width at the base and 3.4 metres in
width at the top.
Location and Background
The Vajont Dam is situated in the Dolomite Region of the Alps under Monte Toc, some 100km
north of Venice, Italy. The dam is in the Vajont Valley, which is a deep narrow gorge. The dam’s
purpose was to supply hydroelectric power to the cities of Turin, Milan and Modena as a result
of the growing demands for industrialisation.
SADE (Societa Adriatica di Elettricita) completed the construction of the dam in 1961, despite
delays due to opposition by the communities of Erto and Casso in the Valley. SADE insisted that
the mountain was sufficiently stable as they claimed that they had completed studies of the
geology of the gorge and of ancient landslides in the area.
Vajont Dam remains today one of the largest double-arched dams in the world. The dam’s
structure was not destroyed by the landslide.
Design
The Vajont Dam was designed by Dr. Carlo Semenza who later became director of SADE’s
hydraulic service division. He had completed the original feasibility study for the reservoir,
some 30 years previously. He was the chief engineer for the Vajont construction project.
The dam reaches a high of 262m and has a chord of 160m which allows it to hold a staggering
150 million cubic metres of water. It is a double arched dam with reinforced concrete wall of 27
metres thick at the base and 3.4 metres at the top.
Originally, the height of the dam was to be 200 metres and it was planned to be a reservoir for
58 million cubic metres of water. But it revised its planning application to make it 261 metres in
height and set to contain 150 cubic metres of water. This was a capacity almost three times the
size of the original plan!
The dam and the reservoir, which was created, were in a steep sided gorge. But the geology of
the gorge showed that the sides were made up of limestone and clay stone rocks interbedded
with slippery clay like horizons which were inclined towards the axis of the gorge.
Signs of Failure: Initial Concerns
The chief engineer raised concerns about the stability of the left bank of the damn, leading to
investigations that identified the possibility of an ancient landslide on the right bank. Some
concern was raised about the inclined synclinal nature of the strata.
 It was concluded that deep-seated landslides would be extremely unlikely
o Test borings did not identify areas of weakness
o Seismic analyses identified firm rock with a high modulus of elasticity
o Shear plane would have a chair-like structure – ‘braking effect’
 Smaller slides were likely, but not of great concern
Filling of the Reservoir: 1960
o In March, water in the reservoir was at 130 m above the river
 The first small landslide occurred.
 Continued monitoring of the movements in the banks
o By October, water in the reservoir was at 170 m above the river
 Rapid increase in the rate of displacement to approximately 3.5 cm/day
 2 km crack opened up - Large landslide mobilized?
o By November, water in the reservoir was at 180 m above the river
 Large landslide – 700,00 m3 in 10 minutes
 Decision made to reduce level in the reservoir to 135m
 Rate of displacement tapered to 1mm/day
 Left bank now known to be unstable
 Drainage tunnels were created in order to control the level in the reservoir. Concerns were
that these would lead to a blockage by the landslide mass. A bypass tunnel was constructed
on the right side, should the reservoir become bisected by the mass.
 The landslide could be initiated by filling the lake, whilst the level in the lake could now be
controlled by the drainage tunnels, in an attempt to prevent over-topping of the damn.
1961 to 1962
o Water in the reservoir was increased to 185m above the river, then to 235m
o Velocities of movement increased again to 1.2cm/day
o Second lowering began to bring water to 185m above river level
1963
o
o
o
o
o
o
Displacement rate was effectively zero
Reinforced belief that engineers could control the landslide
Reservoir level rapidly increased to 231m, then to 240m and then to 245m
Velocity increased to 0.3cm/day, then to 0.8cm/day and then to 3.5cm/day
Water level dropped to 235m to control rate of creep
Movement velocities continued to climb to 20cm/day by October 9th
Failure
At 22.38 GMT on October 9 1963, failure of the landslide occurred. The entire mass of
approximately 260 million cubic metres slid into the reservoir at up to 110km per hour. When
the mass hit the lake it displaced 50 million cubic metres of water.
A wave was pushed 260m up the opposite bank and destroyed the village of Casso. Another
wave went over the dam at a height of 250m and had a volume of about 30 million metres
cubed. Despite this, only the top metre of the dam was washed away while the remaining
structure remained largely intact.
The wave fell more than 500m onto the villages of Longarone, Pirago, Villanova, Rivalta and
Fae, totally destroying them. Air displacement from the wave also damaged other villages near
the landslide. More than 2000 lives were lost.
Conclusions
At first the disaster was attributed to an unexpected and unavoidable natural event, however it
quickly became clear that the designers and managers had ignored negative studies about the
stability of the mountain on the southern side of the basin.
When the landslide did occur, it is believed that the increase in the level of the reservoir caused
pore water pressures in the clay layers to increase, reducing the effective normal strength and
shear resistance. When the level of the reservoir was reduced, hydraulic pressures increased
the stresses as the water in the limestone tried to drain. The total thrust from this effect was
estimated at 2-4 million tonnes. This force caused brittle failure and a catastrophic loss of
strength.
There is still dispute over the contribution of some factors that may have triggered the
landslide. These include rainfall, dam levels changes and earthquakes. There are also differing
views about whether it was an old landslide or a completely new one.
After the disaster Longarone and other villages in the Piave valley were rebuilt in a modern
style. The parts of the dam that were damaged were rebuilt and since 2002 it is possible to visit
the dam.
Sade
 Owned by Mussolini’s former Minister for Finance
 Three studies undertaken when initial cracks in roadways were evident, claimed that the
entire side of Monte Toc was unstable – these were ignored, apparently so that the
company could claim maximum compensation pre nationalization of electricity
production. The company still managed the damn works even after handover to the
government.
 Journalists reporting landslides and cracks were sued for ‘undermining the social order’
 During the days where massive displacements (up to one metre) were recorded just
prior to the final collapse, no warnings were issued to the public.