The Vaiont Landslide

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Landslide

y

avid N

etley Ph D

of landslide scarp looking toward South w i t h landslide mass in foreground

e remarkable Vajont reservoir disaster of Northern Italy Venice (Figure 1 ). Th e dam was built as a part of the po st-

is a salient example of an attempt to apply engineering war deve lopm ent of Italy to provide hydroelectric power for

measures to a poorly understood bu t highly hazardous

the rapidly-expanding northern cities of Milan an d Turin.

tem. The disaster occurred when approximately 270 million While siting a da m at this loc ation was first propo sed in

of material o n one of the banks of a newly-constructed th e 1920 Js,construction did not begin until 1956, When

voir failed catastrophically durin g th e filling. The move-

completed in 1960, the doubly-curved arch dam was, at

265 .5 above h e alley floor, the world s highest thin arch

be as high as 30 m/sec. The landslide displaced a wave that dam . The chord of the dam was 1 60 m, an d the volum e of

topped the d am by u p to 250 m and swept on to the valley

impoun ded water was 115 million m3.

ng the deaths of mo re tha n 2,500 peo ple. Remark-

The dam was built across the Vajont Valley, a deep, nar-

y, the da m remained unbroken by the flood.

row gorge. In this area, the mountains are characterized by

This article presents background information regarding

massive, near-vertical cliffs form ed in t he Jura ssic Dogger

formation and underlying Triassic formations. The valleys

e failure, causes of the failure, and s om e of the

are generally formed by synclinal geological structures that

por tan t lessons learned from this tragedy.

expose weaker Cretaceous and Tertiary units, which contain

more clays and are more thinly bedded (Figure 2). Hence,

the valley walls are formed from mid dle Jurassic limestone,

Vajont is located in the southeastern part of the Dolo-

overlain with successive layers of uppe r Jurassic limesto ne

with clay an d Cretaceous limestones.



imit of

196

landslide

ua

0 0 5 1.0

f rom Ja eg ar 1980

Figure 1 . Map showing location of

Vajont Dam

and Reservoir

ORDER

OF

EVENTS

efore

Dam

Completion

I t appears tha t dur ing cons t ruc -

t i o n o f t h e d a m in 1 9 5 8 a n d 1 9 5 9 ,

engineers were concerned abo ut the

s tab i l i ty o f t he banks o f th e reser -

voir. As a result , a possible prehis-

toric slide was identified o n the right

bank. While there was considerable

d i s c u s s i o n o f t h e s t a b i l i t y o f t h e

valley walls in view of the inclined

syncl inal form of the s t ra ta and the

possibili ty of old slides in this area,

deep-seated landsl ides were deem ed

extremely unlikely because:

areas of weakness were not identi-

fied in th e test borings (a ltho ugh it

appears that only three boreholes

had been drilled in th e reservoir

walls);

a p otential shear plane was

assumed to have a chair-like form

that wou ld resist movement;

seismic analyses suggested that the

banks consisted of very firm, in-

situ rock with a high m odu lus of

elasticity.

Smaller slides in the looser surface

layers were con sidered to be likely, but

the volumes and velocities of move-

me nt w ere expected to b e low.

First Reservoir Filling and

Drawdown

Filling was initiated in February

196 0 (Figure 3 before final comple-

tion of the dam in September of that

year.

A

month af ter f i l l ing s tar ted,

when the reservoir level had reached

13 0 m above th e river level, a small

s l o p e m o v e m e n t o c c u r r e d o n t h e

south bank. As reservoir filling con-

t i n u e d , e n g i n e e r s m o n i t o r e d t h e

movem ent of the banks. In October

1960, whe n th e dep th of the reservoir

had reached 170 m, they observed a

rapid increase in the rate of displace-

men t to approx imate ly

3.5

cm/day.

At the same time, a

2-km

length joint

opened , de f in ing an a rea o f abou t

1700-m long an d 1000-m wide, sug-

gesting that a very large landslide had

been mobilized. O n November

4

with

the reservoir depth at 180 m, a failure

of a part of the landslide mass, con-

sisting of ab out 700,000 m 3, occurred

over a period of abou t 10 minutes. In

response, the reservoir level was gen-

tly drawn down, whereupon rates of

movement of the remaining body of

the landslide decreased from a maxi-

mum of about 8 mm/day at a level

of 185 m to less than mm /day at a

level of 135 m . Th e total displacement

of this main landslide body was about

m .

Site engineers realized that a large

mass of the left bank was inherently

uns tab le and p rone to c reep . They

decided to control the movement of

the sliding mass by varying the res-

ervoir s water level, while con trollin g

the joint water thrust within the rock

mass by means of drainage tunnels.

The designers realized that a section

of the reservoir could be blocked by

the landslide mass, but the volume of

water in the unblocked (upstream ) sec-

tion would be sufficient to allow the

generation of electricity. Therefore, a

bypass tunne l was constructed on the

opposite (right) bank such that if the

reservoir was divided into two sections

by

a

landslide of the left bank, th e level

of the lake could still be controlled.

The engineers assumed that by

carefully altering the level of the res-

ervoir, they could initiate and control



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Second Resemoir

Filling

and D

ra~s dwn

r ~ z 1 he beginning of October

- - ..

-:. cn ul early February 196 2, the

-.carer level was raised to 185 m, fol-

lowed

by

a phase of s low impound-

ment. By November 1962 the level

had reached 235 m . During the early

part of this phase, sla pe velocities did

not substantially increase; bu t by th e

end of the phase,

they

had increased

to 1. 2 cm/day.

In November 1962, th e dam s engi-

neers s lowly lowered the reservoir

level a second time, until the water

decreased to 185 m after four m onths.

Initially, displacements remained high,

but in December

they

began to reduce

and, by early April, the rate was effec-

tively zero. The experiences gained

from the second phase of filling and

the subsequent drawdown confirmed

to the engineers that control of the

landslide was possible by altering the

level of th e reservoir.

Third Reservoir Filling and

Drawdown

Between April and May 1963 the

reservoir level was rapid ly increased t o

231 m. Slight increases in velocity were

noted , but rates did n ot exceed

3

m m /

day. In June, the level was increased

to 237 m and the rate of displace-

ment increased to abo ut 4 mm/day.

In mid-July, the level reached 240

rn,

and some of the control points indi-

cated small increases in displacement

to abou t 5 mm/day. This level was

maintained through mid-August, but

during this time, velocities steadily

increased to 8 mmlday. In the latter.

part of August, the level was increased

once more, and by early September,

the water depth was

245

rn. In some

parts of the slide, velocities increased

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In late Septem ber, the water level

was s l owly d ropped t o b r ing t he

rates of creep back under control .

By October

9

a depth of 235

rn

was

reached, but velocities of movem ent

cont inued to s lowly increase , and

eventua lly rates of u p t o 2 m m /

day were recorded.

Catastrophic

Failure

The ca tas t rophic fa i lure of the

landsl ide took place at

22:38

GMT

on October 9, 1 963 . The entire mass

s l i d app rox ima te ly 500 m no r th -

wards at up to 30 rn/sec. The mass

com pletely blocked the gorge to a

depth of up

to

400 m, and traveled

M a r c h

/ A p r i l

2 0 0 6



2. Schematic cross-section through Vajont Valley.

from

Jaegar 198

u p t o 1 4 0

m

u p t h e o p p o s it e b a n k

from Jaegar 198

Chronology of events leading to the cata5trophic failure

T h e l a n d s l i d e s m o v e m e n t c e as e d

af te r abou t

45

second s. At the t im e

of failure, tb e reservoi~. ontained 115

mi l l ion m3 o f wa te r . The lands l ide

mass pushed a wave of wawr up the

o p p o s i t e b a n k t h a t d e s t r o y e d t h e

village of Casso-260 m above lake

level-and over- topped the da m

by

u p t o 245 m The wate r, e s t~ m at r d o

have had a volume of about 30 mil-

l i o n m 3 , t h e n c a s c a d e d m o r e t h a n

500 m do w n the valley to destrov the

villages of Longarone, Pirago, Villa-

nova, Rivalta and Fae, causing about

2,50 0 fatalities. Surprisingly, the d am

was not destroyed.

C USES O F

F ILURE

Since

1963

I l lany s tudies have

b e e n u n d e r t a k e n t o i d e r l t i f y t h e

causes of the failure. Initially, there

was cons ide rab le specu la t ion abou t

the

loca t ion o f th r s l id ing su r face ,

but i t is now establ ished that i t was

located in thin

(5 -15 cm) , over - ron-

solidated clay layers in th e lime stone.

To date , experts deb ate whe ther th e

landslide was caused by reactivation

of an old landslide or by a first-time

fa i lu re . I t s

now, however , f i rmly

established from detailed analyses of

the movement reco rd tha t the f ina l

Failure occurred in a brittle manner,

wi th the rap id acce le ra t ion resu l t -

i n g f r o m the s h a r p r e d u c t i o n i n



resistance as the clay weakened

peak to residual strength, and

om

the s teep shea r surface. Wh en

reservoir level was high, the pore

e t o s h e a r

ect of ind ucing hydraulic forces in

all shear stress as water dra ined

s. Thus, th e slo pe expe-

e time as shear resistance was

al. The catastrophic failure was

result.

LE RNED

While debate cont inues over the

om th e tragedy. Perhaps the

po rtan t lesson is the dangers

pts to control complex

spect ive, the at tem pts to co ntrol

, poorly u nders tood . Secondly, the

ure emphasizes th e impo rtance of

detailed ground

ly, those involved in th e very large

projects of today are fully aware

Finally

it is now well-established that

n movement rate would have allowed

final failure of

he slopes some 30 days or more before

the failure actually occurred . Wh ile it is

harsh to criticize those working on the

ite for their failure to d o this given

that these techniques were not really

Presto's Free Tech nical Resources Library D has the t o ds

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Load support,

slop

and chann el prote ction, earth retention and

understood at that time), hopefully,

engineers fa an g a similar challenge in

the future will use the m ovemen t record

to the fullest extent possible to extrapo-

late likely future behavior.

Dr. David N . Petley is Wilson kofes-

sor of Hazard and Risk in the Depart-

ment

o

Geography at the University

o

Durham, located in Durham,

UK.

His

research interests focus on understanding

landslides and landslide mechanics, espe-

cially in high mountain areas within less

developed countries. To this end, he estab-

lished and

is

director of the International

Landslide Centre

wuno.

andslidecen

h e.

org Dr. Petley can be contacted at d.n.

[email protected].

The Vaiont Landslide

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