4A6 Structures Project
The Failure of Vajont Dam, Italy
Group 14- 4A6 Structures Project
Vajont Dam Failure
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Introduction
Completed in 1961, The Vajont Dam was one of the highest dams in the world. It stood
262 metres high, and 27 metres wide at the base. On October 9, 1963, it experienced a colossal
disaster. After heavy rainfalls, approximately 260 million cubic metres of forest, earth and rock
fell into the reservoir in landslides creating a massive wave which washed over the dam and into
the valley below, tragically killing up to 2,000 people.
Location and Background
Situated about 100 kilometres north of Venice, Italy, the Vajont Dam was located in the
Dolomite Region of the Alps. It was constructed under the Monte Toc Mountain to provide
hydroelectric power to the cities of Milan, Turin and Modena. It was the world’s highest thin arch
dam and held a huge 115 million m3 of water.
The Dam was built by SADE (Societa Adriatica di Elettricita), who assured the public
that the geology of the area was fully understood. The mountains in this area are large, with nearvertical cliffs formed in the Jurassic Dogger formation and underlying Triassic formations. The
valleys tend to be associated with outcrops of weaker formations, which contain more clays and
are bedded thinly. The generalised geological structure is of a syncline cut by the valley.
Design
At a height of 265.5m, the Vajont Dam was the largest thin-arched dam in the world
when it was completed in 1960. As was the norm for a dam of this size it was designed and built
as a doubly curved arched dam made from reinforced concrete. Approximately 360,000m³ of
concrete was used in its construction. The dam was 3.4m thick at its crest widening out to 27m at
its base. The chord of the dam was 160m, and the volume of impounded water was 115 million
m³.
Failure
The first concerns about the stability of the valley walls were raised during construction
of the dam, with a number of reports being compiled during 1958 and 59. These reports identified
a possible prehistoric slide on the right bank. In view of the possibility of previous land slides and
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Vajont Dam Failure
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the synclinal form of the strata there was considerable discussion on the stability of the valley
walls. The conclusion they reached was affected by the fact that:
 3 test borings had failed to identify areas of weakness;
 It was assumed that any shear plane would have a ‘chairlike’ form and exert a
‘braking effect’;
 Seismic analyses had suggested that the banks consisted of very firm in-situ rock
with a high Young’s modulus.
The company building the dam SADE (Società Adriatica di Elettricità ) informed the
public that the geology of the valley had been studied and the area was considered to be
sufficiently stable. They concluded that, although smaller slides in the looser surface layers were
likely (with low velocities and volumes), deep-seated landslides were extremely unlikely.
The First small detachment occurred in march 1960 during the first filling of the dam. The water
at this stage was 130m above the level of the river. They continued filling the reservoir while
monitoring the banks. In October, when the level reached 170m a joint 2km in length opened up,
indicating that an area about 1700m long and1000m wide moving with an observed speed of
3.5cm per day.
On the 4th of November, when the level of the reservoir reached 180m, 700,000 cubic
metres of material slid into the lake in ten minutes. Because of this they gently dropped the level
down to 135m and the movement reduced to close to 1mm per day. The designers realised that
the large mass of the left bank was inherently unstable. However as there was no realistic way of
arresting the slide artificially and stop the slide, or safely cause the mass to slide down all at
once, it was decided to use varying levels and drainage tunnels to try and gain control of the
sliding mass. They believed that by carefully elevating the level movement of the large landslide
mass could be started and then controlled by altering the level of the reservoir. It was calculated
that, should a sudden movement occur, providing it did not lead to a filling of the reservoir in
under 10 minutes, it would not cause over-topping of the dam.
The dam was drawn down and filled up twice without any major incidents occurring. In
late September 1963 the level was dropped for the 3rd time. This was being done to bring the
creep rate back under control as had been done before. However velocities of movement
continued to increase slowly. By the 9th of October speeds of up to 20 cm per day were recorded.
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Vajont Dam Failure
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At approximately 10:35 pm catastrophic failure of the slope occurred. The entire mass of
approximately 260 million cubic metres of earth and rock slid 500m north at up to 30 m sec-1
(110 km per hour). When the mass fell into the lake it had a level of 235m and contained about
115 million cubic metres of water. The landslide pushed a wave up the opposite bank 260m
above the original reservoir level and destroyed the village of Casso and then over-topped the
dam by up to 245m. An estimated 30 million cubic metres of water fell on five villages
decimating them. About 2000 lives were lost. Despite this the dam was left structurally intact,
only a metre of masonry was washed away at the top.
Conclusion
There has been a lot of work done to find the exact cause of the failure, the most likely
causes are,
 An increase in pore water pressure due to the increasing level of the reservoir
reducing the effective normal strength and the shear resistance.
 The chair-like form of the shear surface resisted movement. Then when the level was
dropped hydraulic pressures were induced as water in the jointed limestone tried to
drain. The force from this caused a brittle failure and a catastrophic loss of strength.
 Despite noticing shifts in the rock during the filling of the dam, the reservoir was
filled under controlled monitoring in an effort for SADE to sell the dam and its
generators to the newly formed state-owned electric company.
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Vajont Dam Failure
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