Vajont Dam Failure
Introduction
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Location and Background
Design
Signs of Failure
Failure
Result
Conclusion
Location and Background
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100km north of Venice Italy under Mont Toc
Built between 1956-1961
Provide hydro-electric power to Milan, Turin
and Modena
Constructed by SADE and ENEL
Public assured geology of gorge studied
sufficiently
Design
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262m high
World’s highest thin double-arched dam
Chord measuring 160m and the volume of
impounded water being 115 million m3
27m thick at base and 3.4m thick at top
Signs of Failure
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Concerns for stability of valley walls during
construction
Many reports compiled during 1958-59
Possible prehistoric slide on Mont Toc side of
bank
Concluded smaller slides in looser surface layers
were likely, but deep-seated landslides were
extremely unlikely
Signs of Failure
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During test phase
Raised water levels to 660m
 As water level rose, creep zone moved
 50million m3 piece of rock slid down into reservoir
 Meter-wide crack had opened up, high up the
mountain
 As a result, seismographs were installed on top of
the dam to assess movements
 Second filling and draw-down of reservoir
performed without problems
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Failure
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During third filling draw-down, with water levels
at 710m above sea level, rates of creep increased
and phase velocities increased to 20cm/day
Removal of the water would be catastrophic as
would leaving the body of water in the reservoir
Failure
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On October 9th 1963 failure of the landslide
occurred
260 million cubic metres of earth and rock slid
500m north at up to 30 m /sec
The landslide pushed a wave up the opposite
bank 260m above the original reservoir level and
then over-topped the dam by up to 245m
Result
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30 million cubic metres
of water fell on five
villages killing almost
2000 people
The dam's structure was
largely undamaged - only
the top meter of
masonry was washed
away
Causes of Failure
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Reactivation of old landslide or new landslide
Increase in pore water pressure due to the increasing
level of the reservoir reduced the effective normal
strength and 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
Contribution of rainfall, dam level changes and
earthquakes as triggers for the landslide