maria chini viki ioannou themistoklis teratsias. o vajont valley: part of the ne italian alps region...
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VAJONT DAM FAILUREVAJONT DAM FAILURE
MARIA CHINIVIKI IOANNOUTHEMISTOKLIS TERATSIAS
INTRODUCTIONINTRODUCTIONo Vajont valley: part of the NE
Italian Alps region
o The dam is located between the
mountains Toc and Salta
o Narrow, deep side slopes and
glacial morphology
o Height: 262 m above the valley
floor
o Width: 190 m at the top and
27 m at the bottom
o Construction: 1956-1960
o Volume: 150 million m³ of
water
GEOLOGYGEOLOGY
o Vajont Valley dominating rocks :
• Limestones
• Dolomites
o Two locations under consideration
o Location with more rigid limestones and bigger dam capacity
More or less fractured
GEOLOGYGEOLOGY
Colle Isolato (1959):
Provides evidence for the existence of the prehistoric landslide
o Overlying alluvial deposits from old river
o Different from in-situ rock mass
CHRONICLE - RECORDSCHRONICLE - RECORDSo June 1960: a report pointed out that
potential reservoir filling would
cause sliding
o July 1960: a report indicated that
sliding could occur only in the surface
o December 1959: seismic velocity
of rock 5,000-6,000 m/s
o Reduced to 1,000-2,000 m/s in one year
CHRONICLE - RECORDSCHRONICLE - RECORDSo 4th November 1960: a mass of debris
estimated to 700,000 m3 slipped
o May 1960 - August 1963: seismographs
placed on the dam
o May 1963: Velocity of slip 1 mm/day
o August 1963: Velocity of slip
5 mm/day
DAY OF CATASTROPHEDAY OF CATASTROPHE
o By September the velocity of movements = 20mm/day
=> Operations to lower reservoir level began
=> Velocity continued to increase rapidly
9th of October 1963, at 22.39:
Sudden slide of the main rock mass
DAY OF CATASTROPHEDAY OF CATASTROPHEo Landslide detached from the southern slope of Mt. Toc
o Volume of the landslide = 270 million m3
o Collapsed in 45s
o 25 million m3 of water was displaced
o Wave crested 140 m above the dam’s crest and that still had a height of 70 m downstream
DAY OF CATASTROPHEDAY OF CATASTROPHEo Energy of spilled water = 1.5 x Hiroshima’s atomic bomb
o The wave hit the town of Longarone and other villages killing more than 2000 people
o The dam remained almost intact even if the load at the abutment was 8 times greater than design load
CAUSES OF FAILURECAUSES OF FAILURE
Generally agreed that failure
happened along bands of clay
(5-15 cm thick) inside the
limestone mass:
Clay layers
CAUSES OF FAILURECAUSES OF FAILURELandslide initiation factors:
I.Prehistoric landslide located in the northern part of the slope :
=> Reactivated by the water level rise
II.Clay layers below slide
=> Mobilized by the increase of water pore pressures
High velocity of displacement:
I.Existence of the old landslide indicates a low friction angle of rock mass => low resistance in movement
II.Frictional heat created a cushion during sliding and converted slow rate sliding into rapid failure
OMISSIONSOMISSIONSI. Presence of cataclastites in mylonitic zone: fault plane
II. Seismic activity: no connection with the formation of Vajont gorge
III. Wrong interpretation: increasing water level not favourable
IV. Omission: to analyse the slope or take a sample for laboratory tests
V. The superficial slides: verified but they were deeper than expected
VI. Rock degradation: indication that the rock was not as strong as before
OMISSIONSOMISSIONS
Carlo Semenza was aware of the problems:
“...things are probably bigger than us and there are no adequate practical measures...”
“...I am in front of a thing which due to its dimensions seems toescape from our hands...”
POST-FAILURE CONSIDERATIONSPOST-FAILURE CONSIDERATIONS
o Modification in the design standards of ongoing projects
o Geodetic station => sustain water level in safe limits
o Microseismic monitoring => localize and forewarn for an oncoming
slope failure
o Acoustic emissions => forecast future movements
o Engineers and geologists observe and inspect the slopes of
projected basins
CONCLUDING REMARKSCONCLUDING REMARKS
o Construction giant dams need to take all
factors under consideration
o Continue checking, supervising and
maintaining the construction, after its
completion
There are always unpredictable and ‘‘time independent hazards’’
THANK YOU FOR YOUR THANK YOU FOR YOUR ATTENTION!ATTENTION!