The Vajont Slide 1987 Engineering Geology

8/16/2019 The Vajont Slide 1987 Engineering Geology

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Engineering Geology, 24 (1987) 513--523 513

Elsevier Science Publishers B.V., Ams terda m -- Printed in The Netherl ands

Prepared Discussion

T H E V A J O N T S L I D E

L. MULLER-SALZBURG

Paracelsusstrasse 2 , A -5 02 0 Salzbu rg Austria)

(Accepted for publication December 1986)

I n t h i s p l a y h a l l, w e n o w w i l l p l a y t h e s t o r y o f V a j o n t . I w i l l n o t t r e a t o r

d i s c u ss t h e m a n y e x p l a n a t i o n s w h i c h c a m e u p a f t e r t h e d i s a s te r o f V a j o n t .

T h e s e e x p l a n a t i o n s h a v e b e e n p u b l i s h e d ( T a b l e I) a n d d i s c u s s e d a n d a r e q u i t e

w e l l k n o w n t o m a n y o f y o u , b u t l es s k n o w n , I t h in k , is t h e h i s t o ry o f h o w

t h i s d i a s te r , h o w t h i s s l i d e d e v e l o p e d , a n d i t is a f i rs t -c l a ss o p p o r t u n i t y t o

l ea r n h o w t o a v o i d s u c h th i n g s i n t h e f u t u r e b y s t u d y i n g t h i s c a se h i s t o r y ,

i n c lu d i n g : t h e d e v e l o p m e n t o f fi n d in g s a n d o f e r ro r s; o f i n t e r p r e t a ti o n s a n d

m i s i n t e r p r e t a t io n s ; a n d o f d e c is i o n s a n d m i s d e c is i o n s. S o I w o u l d r a t h e r

e m p h a s i z e t h e c h r o n o l o g y o f t h e e v e n t s b e f o r e t h e s l id e an d d e a l o n l y b r i e f l y

w i t h w h a t h a s b e e n e x p l a i n e d a f t e r t h e s l id e , b e c a u s e th i s w i ll b e d i s c u s s e d

b y o t h e r s p e a k e r s a l s o . I w i l l p r e s e n t f a c t s a n d f i g u r e s w h i c h a r e c o r r e c t b u t

a r e in s o m e c a se s d i f f e r e n t f r o m f ig u r es w h i c h h a v e b e e n p u b l i s h e d i n t h e

p a s t . A n d I w i ll t a lk a b o u t t h e h i s t o r y o f t h e r e c o g n i t i o n o f t h e d a n g e r , o r

t h e c o n s e q u e n c e s o f a s li de , w h i c h i n f l u e n c e d v e r y m u c h t h e i n t e r p r e t a ti o n s

m a d e a n d t h e d e c i s io n s ta k e n b y t h o s e r e s p o n s ib l e . I w i ll t a lk a b o u t t h e r o l e

o f d e c i s io n s in t h is ca s e m o r e t h a n a b o u t t h e r o le o f a n a l y si s, b e c a u s e i n o r d e r

t o l ea r n f r o m s u c h a m i s ta k e , a n d a ll o f u s k n o w w e o n l y l ea r n f r o m m i s t a k e s,

w e h a v e t o t r y t o u n d e r s t a n d w h a t h a p p e n e d n o t o n l y p h y si c al ly , n o t o n l y

m e c h a n i c a l l y a n d m a t h e m a t i c a l l y , b u t a ls o t o u n d e r s t a n d t h e p a r t i c i p a ti o n

o f h u m a n t h i n k in g . W e h a v e t o u n d e r s t a n d i t i n a r e t r o s p e c t i v e w a y , p u t t i n g

o u r s e lv e s i n t h e p l a c e o f t h o s e e n g i n e e rs a n d g e o l o g is t s i n 1 9 5 0 , 1 9 5 5 , 1 9 6 0 ,

b e f o r e t h e s li d es . O n l y t h e n c a n w e g e t t h e f e e l i n g o f w h a t s h o u l d b e d e v el -

o p e d i n o u r s o u l s t o a v o i d s u c h t h in g s i n t h e f u t u r e .

T h e t a s k is n o t e a s y b e c a u s e i n e a c h n e w c a se a d i f f e r e n t g e o l o g y a n d q u i t e

d i f f e r e n t p r o b l e m s w il l b e e n c o u n t e r e d . O u r k n o w l e d g e h a s i n c re a s e d v e r y

m u c h s in c e V a j o n t , o u r e x p e r i e n c e h a s i n c r ea s e d v e r y m u c h s in c e t h e n , b u t I

t h i n k n o t i n c r e a s e d i n t h e s a m e m a n n e r i s o u r a b i l i t y t o r e a s o n l o g i c a l ly ,

m a k e i n t e r p r e t a t i o n s , a n d t o t a k e d e c i s i o n s . I n f u t u r e c a s es , t h e s i t e i n v e st i-

g a t i o n s t h a t w i ll b e m a d e w i l l b e v e r y m u c h i n c re a s ed , c o m p a r e d w i t h t h e

i n v e s t i g a t io n s m a d e i n t h e p a s t , a n d a ll t h i s w i ll g r e a tl y i n c r e a s e t h e p o s -

s ib i li ti e s fo r a n a l y s is . B u t e a c h c a s e is d i f f e r e n t . W e c a n n o t a p p l y t h e e x p e r i -

e n c e f r o m o n e c a se t o a n o t h e r w i t h o u t h a v in g a n a ly s e d t h e f a c t o r s w h i c h

* Oral presentation.

0013 -795 2/8 7/$ 03.5 0 © 1987 Elsevier Science Publishers B.V.


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T A B L E I

E x p l a n a t i o n a f t e r t h e c a t a s t r o p h e

I Explanations

(A) con cern ing causes o f th e s lide

(Kenney , 1965 ; Kie r sch , 1964 ; Gruner , 1965 ; Skempton , 1966 ; Mi i l l e r , 1968)

(B) concern ing mechan ics o f the s l ide

(Miil ler , 19 64; Weif~, 1964 ; Jaege r , 1965; N onvei l ler , 1965 ; Mencl, 1 966 ; Sk em pto n,

1966; Broi l i , 1967; Haefel i , 1967; Kenney, 1967; Mii l ler , 1968)

(C) concern ing fa i lu re su r face

(Kiersch , 19 64; Sell i e t a l . , 1964; M uller , 1964 ; Sel l i -Trevisan, 1964; Weif~, 1964;

C a m b e f o r t , 1 9 6 5 ; Mi lli, 1 9 6 6 ; Me n c l, 1 9 6 6 ; S k e m p t o n , 1 9 6 6 ; V e n z o u . F u g a ti 1

Sem enz a, 1966 ; M ill i, 1967 ; Broi l i , 1967 ; Mii ller , 196 8)

(D) concern ing f r i c t ion va lues

(Nonve i l l e r , 1965 ; Menc l, 1966 ; Ske m pton , 1966 ; Nonve i l l e r , 1967 ; K enn ey , 1967 ;

Malina , 1968; Mii l ler , 1968)

( E ) d y n a m i c a p p r o a c h

(H aefel i , 1967 ; No nvei l ler , 196 7 ; Jaeg er , 19 68 ; Ren gers and Mii ller, 197 0; Scheidegg er ,

1 9 6 4 , 1 9 7 3 ; Ko r n e r , 1 9 6 4 , 1 9 7 6 ; C h o wd h u r y , 1 9 7 8 )

II Not attemp ted to explain

mechan ism tha t in i t i a t ed the s l ide

- - b a c k g r o u n d o f t h e d e c i s i o n - ma k i n g p ro c e s s

lapses in log ical r eason ing

mul t i - l aye red p rocesses invo lved

inab i l i ty to r econc i l e the va r ious p remises wi th a l l the obse rved fac t s

b a s i ca l ly i n f lu e n c e d t h e f i r st c as e ; o n l y t h e n m a y w e t r a n s f e r o u r e x p e r i e n c e

t o o t h e r s i t u a ti o n s . T o d a y w e a r e m u c h m o r e c le v er , m u c h m o r e e d u c a t e d a n d

t r a i n e d , b u t w e a r e n o t w i s e r. T h e r e i s a c e r t a i n p o s t - f a i l u r e c u r v e , a p l o t

v e rs u s t i m e o f o u r k n o w l e d g e , o u r j u d g m e n t , b u t a l s o o u r c o n f u s i o n . S o m e

y e a r s a f t e r su c h a fa i lu r e o u r k n o w l e d g e i n c re a s e s v e ry m u c h , b u t o u r a b i l i t y

t o r e a s o n a n d t a k e d e c i s i o n s d e c r e a s e s a f t e r a c e r t a i n t i m e . T h i s is m y

e x p e r i e n c e .

B A C K G R O U N D

O n f ig .5 o f m y p a p e r ( p . 4 3 3 ) y o u c o u l d s e e t h e t e r r i t o r y a f t e r t h e s l id e .

T h e V a j o n t r iv e r is c o v e r e d b y t h e h u g e s li d i ng m a s s , m e a s u r i n g a b o u t 1 7 0 0 m

in l e n g t h a n d a b o u t 1 k m in w i d t h . T h e t e r r i t o r y w i t h i n w h i c h t h e s l id e

o c c u r r e d , w a s m o r e o r l es s a p l a in , c a l l ed t h e " P o z z a " a n d t h e " P i a n d e l T o c " .

T h i s t e r r i t o r y s h i f t e d a b o u t 2 6 0 - - 4 0 0 m t o w a r d t h e r i g h t b a n k . T h e f i g u re

m e n t i o n e d b e f o r e s h o w s t h e p a r t o f t h e s l i d i n g p l a n e w h i c h is v i s ib le n o w a f t e r

t h e s l i de , b u t w h i c h n o b o d y f o r e s a w b e f o r e . T h e s l id in g m a s s r e m a i n e d

t o g e t h e r , v e r y m u c h l ik e o n e b l o c k . S o m e z o n e s s h o w e d t h a t d e t r i t u s s a n k

i n t o t h e g o r g e . I t m e a n s t h e s l id i n g m a s s j u m p e d o v e r t h e g o r g e l e a v in g t h e

g o r g e e m p t y , a n d o n l y l a t e r o n t h r o u g h t h e s e c o n e s t h e g o r g e w a s f il le d .

T h e d a m r e s is t ed t h e o v e r l o a d w i t h v e r y li t t le d a m a g e . O n l y t h e c r e s t o f


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the dam was slightly damaged , nothi ng else. Along the zone of th e sliding

mass the sliding surface was more or less horizontal.

Some weeks before the major slide to ok place, deta chme nt o f rocks

occurred a nd ind icated the mov eme nt was in the mass. For instance, a small

part of the rock came down some metres, later on it came down even more,

and farth er east abou t 700,000 m 3 of rock slid down . At this time the whole

mass was not motionless but, according to the measurements, showed very

little movement . Some weeks after this, when the lake was already filled to

abo ut half the elevation of the dam, which is 276 m high, a major slide

occurred and together with this on this plain of the Pian del Toc, a lot of

vertical or sub-vertical cracks, heavy cracks, could be observed. From this we

could see there w ould be a major slide, not a small one. The mai n event was

this sliding mass droppin g into the reservoir, and pushing the wat er very

mu ch in the vertical sense, but also laterally. The max imum height of the

wave came up to the village of Casso where some houses were destroyed,

260 m above lake level. The water came in throug h the roofs to th e people

laying in bed and they disappeared immediately. From there the water fell

over the gorge. The tunnels of the Vajont road were filled by gravel and stone

for some 100 metres and the wave pushed down through the main valley of

the Piave river against the small city of Longarone with a popul ation of ab out

10,000. Only a few houses remain there; noth ing fro m the church, no rail-

way, no roads, nothi ng, everything was washed away. Fo ur ot her smaller

villages were also washed away.

On the abut ment of the dam, which was subjected to a load eight times

greater tha n tha t for which it was designed, only two or three cracks occurred.

The rock mass remained in quite good condition thanks t o t he tyi ng together

of the rock b y some 150 roc k anchors, 50 metres long, and being prestressed

elastically, the reinfor cement could keep the rock tog ether after the shock.

Regarding the structure of the rock, horizontal layers of the Upper

Cretaceous limestone have been cut by a lot of vertical joints -- I may call it

faults -- and the layers started curving at the back, going parallel to the later

sliding surface. From such pictures, the mechanism of the slide has been very

early tho ugh t out. Before shear movement occurred at the bo tto m, tilting

prod uced clearly visible specks on the surface. Anoth er thing was the orienta-

tion o f the sliding path. There was an un dula tio n o f the layers in the direction

of the slide. According to this und ulat ion I could answer the question : when

the slide will occur wou ld it touc h th e da m or not ? and I said the slide will

leave 50 metre s bet ween the sliding mass and the dam. This could be predicted

by th e st ructure because the slide had to follow this structure in lanes.

Please consider the amou nt of energy involved in this slide. Twenty-five

million cubic metres of water spilled over the crest and dro pped abou t 500

metres. It means an energy of about 170 billions of kilowatt hours.

Now, let us consider why the Vajont slide became so catastrophic. We

could ask what have been the causes, because in such catastrophic events

never is there onl y one cause. There are always some causes playing together .

Of course, the slide came down on t he reservoir, but below th e reservoir


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there was a relatively dense population. Without this dense popul ation t he

event would not have been so catastrophic. The reservoir level at the time of

the major slide was relatively high, only 30 metres were left in freeboard, so

the main volume had t o go over the dam. The velocit y of the slide was very

high and this high velocity was unexpec ted. This is one of th e main reasons

why the slide became catastrophic. The height o f the wave, measuring in its

max imu m to 260 metres, was beyond exp ectation. The expect ation was 26

metres. Except these two deta chmen ts some weeks before the major slide,

there were no warning indications, no na tura l ones, not even artificial ones,

which were pro duce d at this time. Some facts and figures on the slide are

given in Table II.

CHRONOLOGY

There are some important figures with regard to time that are very instruc-

tive. The first time I suspected tha t a slide would occur was 4% years before

the catastroph e. Evidence tha t the slide would be a big one, i.e. having a

volume of more than 200 million cubic meters, was produced nearly 3 years

before the failure. Observation of measuring points, starting with 8 points

and finally 23 benchmarks, was executed and evaluated for more th an three

years. It should be kept in mind tha t dur ing the period of first filling of the

reservoir to half of its anticipated de pth, t he slope behaved regularly; meaning,

according to expect ation . The rising water level was expected to increase the

rate at which the benchmarks would move. With lowering water levels we

expected not a sudden stop but a gradual slowing down of the movement.

During the second filling period t he sliding mass behaved c ompl etel y differ-

ently . As the level of the reservoir was raised, no m ovement ,

n o m o v e m e n t a t

a l l ,

to ok place until the reservoir had risen more than 50 metres above the

elevation where a small slide had occurred during first filling. Only then did

the sliding mass start moving, but at much slower rate than the first time. In

the thi rd filling period , t he slide behaved nearly regularly again. We will come

back to this; it is an important fact.

Please take into account that the dam was started to be built at 1957. My

appo intm ent had not hing to do with the slides in this time. My job was the

foundat ion of the dam and the reinforcement of the abutment. The first

time I suspected that a slide was occurring was about April or May 1959

when the dam was already built to more than half elevation. The famous

dam builder, Carlo Semenza, with w hom I discussed the fou ndat ion problems

and the need for rock reinforcement of the abutments, between the end of

the discussion and the time m y train was leaving asked me, "di d yo u already

know our beautiful reservoir?" I said, "no, I would like to see it". So he

too k me aroun d the reservoir and at a point about 1 km upstream of the

dam o n the right abut ment , we stopped to look against the left abutment .

Immedi ately I asked him, "Ho w m any cubic metres of slides do y ou expe ct?"

He was very shocked. He said, "We do not expect a slide because we have

been p romised by geologists, we have been promised by sliding experts, and


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T A B L E I I

F a c t s a n d f i g u r e s

V o l u m e o f s l i d e m a s s

v o l u m e o f r e s e rv o i r

f r e e b o a r d

v o l u m e o f r es e r v o i r a t t h e t i m e o f c a t a s t r o p h e

v o l u m e a t f lo o d o v e r th e d a m

H e i g h t o f a r c h d a m

d i f f e r e n c e i n h e i g h t b e t w e e n r e s e r v o i r le v e l

a n d m a i n v a l le y

E x t e n t o f s li d e

t h i c k n e s s o f s l i d e m a s s

h o r i z o n t a l d i s p l a c e m e n t , a v e ra g e

m a x i m u m v e l o c i t y

H e i g h t o f w a v e

F i r s t s u s p e c t e d s l id e i n r e s e rv o i r

e v i d e n c e t h a t a m a j o r s l i de w o u l d o c c u r

S u r v e y r e f e r e n c e p o i n t s i n s t a l l e d

2 7 5 m i l l i o n m s

1 6 9 m i l l i o n m 3

1 3 m

a b o u t 1 1 5 m i l l i o n m 3

2 5 m i l l i o n m 3

2 7 6 m

5 0 0 m

1 7 0 0 m × 1 0 0 0 m

a b o u t 1 3 0 to a b o u t 3 5 0 m

a b o u t 2 4 0 m

2 5 m / s

2 6 0 m

A p r i l , 1 9 5 9

J u n e , 1 9 6 0

M a y , 1 9 6 0

R e g u l a r b e h a v i o u r d u r i n g f i rs t f i ll in g p e ri o d . P a r a d o x i c a l b e h a v i o u r d u r i n g s e c o n d f i ll in g

p e r i o d . N e a r l y r e g u l a r b e h a v i o u r d u r i n g t h i r d p e r i o d .

b y o t h e r s , t h e r e w i l l b e n o s li d e s" . I s a id , " I d o n ' t w a n t t o i n t e r fe r e w i t h

o t h e r e x p e r t s b e c a u s e i t is n o t m y j o b , b u t p l e a s e I w o u l d s u g ge s t a n

i n v e s t i g a t i o n " .

S e m e n z a i m m e d i a t e l y s t a r te d i n v es ti g a ti o n s. H i s s o n , a n d o t h e r s , w e r e

i n v o lv e d . B o r i ng s w e r e m a d e f o r n e a r l y o n e y e a r . T h e r e p o r t o n t h e i nv e st i-

g a t i o n s w a s m a d e i n J u n e o f 1 9 6 0 , s h o r t l y b e f o r e t h i s sm a l l s li de o f 7 0 0 , 0 0 0

c u b i c m e t e r s o c c u r r e d . A t t h is t im e , t h e o u t l i n e o f t h e m a j o r s l id e m e a s u r in g

1 .7 k m b y 1 k m b e c a m e ap p a r e n t . T h e r e p o r t p o i n t e d o u t t h a t t h e w h o l e m a ss

b e l o w P i a n d e l T o c c o u l d s l id e a n d , f u r t h e r m o r e , i t p o i n t e d o u t t h a t t h e s l id e

c o u l d b e c a u s e d b y r e s e r v o i r fi ll in g . T h i s w a s t h r e e y e a r s b e f o r e t h e c a t a -

s t r o p h i c s li d e t o o k p l ac e

A s e c o n d g e o l o g ic a l r e p o r t w a s g iv e n b y P r o f . D a l P ia z a t t h e e n d o f J u l y

1 9 6 0 i n w h i c h h e s ai d t h e r e w o u l d b e n o m a j o r s li d es , o n l y s u p e r fi c ia l o n e s .

T h e n , a n e x p l o r a t i o n a d i t w a s m a d e i n t h e h i g h e r p a r t o f t h e s li d e a n d p i e z o m -

e t e r s w e r e i n st a l le d . M y r e p o r t , m y o n l y r e p o r t o n t h e s li d es d u r i n g t h i s

p e r i o d o f e n g a g e m e n t , w a s g iv e n a b o u t t h e e n d o f M a r c h 1 9 6 1 . A f t e r t h is I

w a s n e v e r c a ll e d a n y m o r e .

I f w e l o o k o n c e m o r e a t t h e t i m e t a b l e , c o n s id e r t h a t i n 1 9 5 7 d a m c o n -

s t r u c t i o n w a s s t a r t e d ; i n 1 9 6 0 t h e d a m w a s f i n i s h e d , b e f o r e f i r s t f i l l i n g ; i n

1 9 6 1 , t h e M a l p as s e t f a il u re o c c u r r e d . I n 1 9 6 1 , a f t e r M a l p a ss e t, I r e m e m b e r

t a lk i n g w i t h t h e d a m d e s ig n e r , C a rl o S e m e n z a i n M e x i c o . W e w e n t t o s o m e

d a m s i n M e x i c o w h i c h h e h a d d e s i g n e d , a n d h e l o o k e d l i k e an il l m a n , a n d

q u i t e t i r e d . B e c a u s e m y r e l a t i o n t o h i m w a s l ik e f a t h e r a n d s o n I a s k e d ,


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"Wha t is wrong wit h y ou ?" He answered, " I have the illness of Malpasset".

He was so depressed abou t this event, as he said, I have built ma ny, m any

dams bu t never before did I recognize how big is the responsibility of a dam

designer. Consider, also, that only in 1962 the International Society of Rock

Mechanics is founded, and in 1965 the first finite-element computation was

adapted to rock discontinuities.

EXPECTATION OF DANGER

If we want to learn somethin g from this event, it is necessary to examin e

how the expectation of danger developed. My first guess in 1959 to Semenza

was at least 20 millions, but I was not sure if I should say 20 or I should say

70. As I did not want to shock him too much in the first moments, I said

between 20 and 70 millions. The first small detachment in 1960 made those

responsible inclined to believe my predicti on, against the pred iction of

geologists, of soil mechanists, and of seismologists. A seismic investigation

was made by Prof. Caloi with the result that 10 to 20 meters below the

surface the rock mass was loosened but below 20 meters it was very strong

rock. No indications of slides. Then, the superficial detachment occurred,

but still, nobody except Eduardo Semenza saw it at this time in terms of a

prehistoric slide or some foregoing big event. A fter the tho ro ugh investigation

of Giudici and Semenza, which I could influ ence in the first weeks, came the

result tha t probabl y 200 million cubic meters woul d be involved in this slide

and later on a fter some discussion with Giudici and Semenza we came to a

figure of 250 million cubic meters: of this I was convinced, Giudici and

Semenza and Broili were convinced, but n ob od y want ed to believe it because

famo us experts said it was a foolish idea. Lat er on when t he larger slides

to ok place the meani ng changed. Fro m this time on everybod y was convinced,

everybody, except the experts. Everyb ody on site, Semenza, and all the other

very good engineers working on site, they were convinced the slide would

occur. In my report which I gave at this time, Febr uary or March of 1961, I

expressed the opinion that there will be no way to prevent the slide. This

probabl y was the reason why I was not called any more. It was quite impossible

to turn away the precipitation water. At that time nob ody would take the

responsibility to make a drainage tunn el, which was the only wa y at least to

reduce t he velocity of the slide, maybe to stop it. As the whole mass was

already broken and divided into many faults and many joints, and was really

damaged , no bo dy would have taken the responsibility to start a 2~/~ km long

tunnel and from this tunnel to make some form of drain screens. This was

not considered to be feasible at the time. Anot her seismic survey was made

with the result that the loosened rock was not 10--20 m but 350 m thick. So

it became the general conviction that the slide could not be prevented.

As all the thou ghts were going in this direction, the question now became:

how can we bring down the slide before filling the reservoir? The sliding

mass was moving. This was know n. N ob od y could d ou bt it, it was measured.

The volume of about 200,000,000 cubic meter was known. It was not really


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519

i g n o r e d , b u t i t w a s d o u b t e d b y s o m e e x p e r t s . B y t h e n , a s I h a d w r i t t e n , n o

c o u n t e r m e a s u r e s w e r e p o s s i b l e b e c a u s e i t w a s t o o l a te t o c a r r y o u t d r a i n a g e

m e a s u r es . T h e m a s s w o u l d c o m e d o w n , i t w a s k n o w n t o e v e r y b o d y . T h e r e

w e r e t h e i n v e s t i g a ti o n s o f G i u d ic i a n d S e m e n z a , w h i c h s h o w e d a h i gh p r o b -

a b i l it y o f a p r e h i s to r i c s li de , a n d I w o u l d s a y t h e e v i d e n c e t h e y s h o w e d w a s

c o n v in c i n g . I t w a s a b s o l u t e l y c o n v i n c i n g b e c a u s e s o m e r o c k o n t h e r ig h t

a b u t m e n t , w h i c h w a s q u i t e d if f e r en t f r o m t h e s u r r o u n d i n g r o c k o f t h e r ig h t

a b u t m e n t , c o r r e s p o n d e d e x a c t l y t o t h e l ef t a b u t m e n t a n d b e y G n d t h is ro c k

t h e r e w a s s o m e g r a v e l a n d s o m e s o i l . I t w a s c l e a r t h a t t h e r e w a s a p r e h i s t o r i c

s li de . T h i s w a s a c c e p t e d b y m a n y p e r s o n s , b u t i t w a s m a i n l y i g n o r e d b y t h e

e x p e r t s . E v e n i n t h e l a s t g e o l o g i c r e p o r t i t w a s d e n i e d . T h e s li d in g p a t h w a s

a c c e p t e d t o b e k n o w n . L a t e r t h e r e w e r e m a n y d i s c us s i o n s o n t h e s h ap e o f

t h e s li di ng m a s s, b u t g e n e r a ll y it w a s k n o w n t o b e r a th e r d e e p a n d m o r e o r

l es s h o r i z o n t a l i n t h e l o w e r p a r t . B u t e v e n s o , i t w a s n o t f u l l y a c c e p t e d t h a t

t h e s l id in g su r f a c e w o u l d b e a t a d e p t h o f m o r e t h a n 1 5 0 m .

PROGRESSIVE FAILURE

T h e p o s s i b i l i t y t h a t a p r o g r e s s iv e f a il u r e c o u l d b e i n v o l v e d w a s n o t r e c o g -

n i ze d . I n 1 9 5 9 a n d 1 9 6 0 , o n l y a sm a ll n u m b e r o f p e o p l e t h o u g h t i n t e r m s o f

p r o g r e s s i v e f a il u r e s. A t V a j o n t i t w a s n o t r e c o g n i z e d t h a t c h a n g e s i n sh e a r

s t r e n g th c o u l d o c c u r w i t h t i m e , n o r w a s i t r e c o g n i z e d t h a t t h e s h e a r r es i st a n ce

c o u l d b e m o b i l i z e d i n s o m e p a r t s o f t h e s l id e b u t n o t i n o th e r s . T h e g r o u n d -

w a t e r t a b l e w a s m e a s u r e d , b u t i t s i n f l u e n c e o n t h e m e c h a n i c s o f t h e s li de w a s

i g n o r e d b y a ll p e r s o n s. T h e p o s s i b i li t y o f a r te s ia n w a t e r w a s n o t k n o w n . T h e

v e l o c it y , an d t h e d e v e l o p m e n t o f th e v e l o c i t y t o b e e x p e c t e d , w a s u n k n o w n .

T h e m e c h a n i sm o f t h e s li de w e c o u l d s a y w a s k n o w n r o u g h l y , b u t u n k n o w n

o r u n k n o w a b l e w a s t h e p o i n t in t im e a t w h i c h t h e s li de w o u l d o c c u r . A n d

t h e e x i s t e n c e o f in i t ia l s tr e s se s , w h i c h p l a y s a c e r t a i n r o l e a c c o r d i n g t o m y

c o n v i c t i o n , w a s n o t d i s c u ss e d a t th e t i m e o f V a j o n t .

EXPLORATIONS

W h a t w a s d o n e f o r t h e e x p l o r a t i o n o f g e o lo g i c a n d r o c k m e c h a n i c a l c o n d i -

t i o n s ? D e t a i le d g e o l o g i c e x p l o r a t i o n w a s d o n e b e f o r e a n d a f t e r th e b i g sl id e .

A v e r y g o o d s t r u c t u r a l a n a l y s is m e a s u r i n g a l l t h e j o i n t s a n d t h e f a u l t s w a s

d o n e ; e x p l o r a t o r y d r il li ng s an d p i e z o m e t e r b o r e h o l e s w e r e m a d e ; a n d t w o

s e is m i c s u rv e y s an d a g e o d e t i c s u r v e y o f l ev e li ng w i t h a n a c c u r a c y o f a b o u t 2

t o 2 1~ c m w a s ca r r ie d o u t . N o m e a s u r e m e n t s w e r e m a d e t o i n d i c a t e h o w

d e e p t h e s li d in g s u r f a c e r e a l l y w a s . N o s l o p e i n d i c a t o r w a s u s e d a t th i s t i m e ,

n o e x t e n s o m e t e r s t o m e a s u r e d i s p l a c e m e n t s d i r e c t ly , o r e v e n r o c k t a l k

i n s t r u m e n t s , o r g e o p h o n e s , f o r w a r n in g .

N o w , t h e g e o l o g i c k n o w l e d g e , o r a s s u m p t i o n s , h a d a n in f l u e n c e . T h e

l i t h o l o g y , i f t h e r e w e r e c l a y l a y e r s {o r c l a y in t e r c a l a t i o n s , o r c l a y s e a m s ) o n

t h e s i l t ed jo i n t s w a s d i sc u s se d b u t n o b o d y c a m e t o a d i s ti n c t o p i n i o n o n h o w

t h i s i n f l u e n c e d t h e s h e a r a n d f r i c t i o n a l r e s i s t a n c e a n d t h e o v e r - a l l b e h a v i o u r


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TABLE III

Limitations of calculations in avoiding the catastrophic event

The slope was unstable since it was detec ted in 1960

Static stability analyses were tried in order to check the possibility of countermeasure;

any result could be obtained depending on assumptions

The paradoxical behaviour of slide during 2nd filling could not be explained by calcula-

tion at the time

Creep calculations were impossible at the time; no prediction of movements on a time

scale was possible

Progressive failure could not be considered because of unknown properties of the sliding

mass

of the paradoxical behaviour of the slide could not be explained by the

calculations, which were don e b y th e owner. This is very sad to say because

it was obvious that refined calculation was not possible at that time.

THE ROLE OF DECISIONS

A main decision was to store the reservoir at a certain freeboa rd below t he

crest. As I was tol d ma ny time s by dif fer ent persons, this was considered a

small risk because the wave would no t be higher tha n 26 metres, according

to large-scale mode l tests. A "con soli dat ion hy pot hes is" was accepted by all

the experts. It means that a part o f the rock which came under water is more

consolidated than it was before. This is true for many t ypes of soil but is not

true for rock. It is wrong in principle. It was quite a decision to rely on this

hypothesis because there was a fou rth opinion which opposed it. There was

the decision to follow a set storage program, despite the f act that the hyp oth -

esis did no t hold tru e durin g the thi rd filling. Then there was the decision

to favou r a diversion tun nel instead of bringing down the slide progressively,

although the diversion tunn el allowed only a small variation of the water

table. I suggested in my report of 1961 to vary the water table in a manner

which w ould bring the sliding mass dow n piece by piece at low wa ter levels

and to create at least a certain resistance at the toe. This was tried only to a

small exte nt; instead it was decided to have a diversion tunn el.

C O N S I D E R A T I O N S T O A V O I D T H E C A T A S T R O P H E

It is very interesting to examine th e circumstances unde r which the slide

would no t have resulted in a catastrop he. These circumstances are summarized

in Table IV. The items are self-explanatory. I will not at te mp t to relate the

many, ma ny explanations given after the slide o cc ur r ed .. , all very good

explanations but, un fortuna tely, they do not conform with one another,

which shows the co mplex ity of the system. An explanation of the mechanism

which

initiated

the slide has not ye t been att emp ted . The decisions, the lapses

in logical reasoning, the one


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5 2 2

T A B L E I V

U n d e r w h a t c i r c u m s t a n c e s w o u l d t h e s l i de n o t h a v e r e s u l te d i n a c a t a s t r o p h e ?

--

i f t h e sl i di n g e l oc i t y c o u l d hav e b e e n p r e d i c t e d

- - if mo r e f r e e b o ar d h ad b e e n l ef t

if m o d e l e x p e r i m e n t s h a d i nd i cat ed t h e d r ast ic w a v e h e i g ht t ha t w o u l d d e v e l o p

if an early stage extensive drainage was pe rf or me d

- - if mo r e t o e r esistanc e had b e e n c r eat e d b y ear l y c o nt r ol l e d m o v e me n t s

if w a r n i n g s y s t e m s h a d b e e n d e v e l o p e d [ ]

- - if d e ci si o ns had n o t b e e n base d o n t h e c o nso l i d at i o n h y p o t h e sis o n l y

if decisions ha d no t be en ma d e so difficult y differences in expertise of the ma n y

advisors involved

- - if e x p e r t s had n o t i n it ial ly r o mise d t h e r e w o u l d b e n o maj o r sl i de

-- if the project reservoir level er e re du ce d

both, the inability to draw conclusions when many factors are influencing

one an other . These are the principal reasons the slide was so catastrophic .

C O N C L U D I N G R E M A R K S

After the disaster of Vajont, many people were saying, and writing, that it

would have been easy to avoid this catastrophe . Ot her people were saying

that it was an unavoidable misfortune, "un cas de force majeure". For me,

bot h meanings are wrong The disaster was the result of human error, in this

case defin itel y of scientific error, and a consequence of lack of knowledge.

R E F E R E N C E S

B ro il i L . 1 9 6 7 . N e w k n o w l e d g e s o n t h e g e o m o r p h o l o g y o f t h e Va j o n t s li d e sl ip su r fac e.

R o c k M e c h . En g . G eo l . V : 3 8 - - 8 8 .

Cambe fo rt H. 19 65. Gli ssemen t de terrain. Sols-Soils 5/IV.

C h o w d h u r y R. 1 9 7 8 . A n al y s is o f t h e Va j o n t sl id e - n e w a p p r o a c h . R o c k M e c h . 1 1 :

29--38.

Gruner, E., 1965. Prozesse um Seeausbriiehe. Schwz. Bzt. f. Verm. Wes., 63 4): 100.

Haefeli, R., 1967. Zum progressiven Bruch in Schnee, Boden, Fels und Eis. Rock Mech.

Eng. Geol., V: 117--137.

Jaeger, Ch., 1965. The Va]ont rock slide. Water Power, March/April.

Jaeger, Ch., 1968. Discontinuouscreep of masses. Water Power, pp.197--198.

Kenney, T.C., 1965. Causes of the Va]ont reservoir disaster. Civ. Eng. -- ASCE.

Kenney, T.C., 1967. Stability of the Vajont valley slope. Rock Mech. Eng. Geol., V 2):

10--16.

Kiersch, G.A., 1964. Vajont reservoir disaster. Cir. Eng. ASCE, pp.32--40.

Kbrner, H., 1964. Schnee-und Eismechanikund einige ihrer Beziehungenzur Geologie.

Rock Mech. Eng. Geol., II: 45--67.

K6rner, H., 1976. Reichweite und Geschwindigkeityon Bergstiirzen und Flie~schnee-

lawinen. Rock Mech., pp.225--256.

Mencl, V., 1966. The influence of the stiffness of a slidingmass on the stability of slopes.

Rock Mech. Eng. Geol., IV: 127--131.


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523

Milli, 1966. Sondaggi geognostici sulla frana del Monte Toc. Rel. Tecn. 18.4.1966 (not

publ. ).

Milli, 1967. Sondaggi geognostici sulla frana del Monte Toc. Rel. Tecn. 15.3.1967.

Miiller, L., 1964. Therock slide in the Vajont Valley. Roc Mech. Eng. Geol., II: 148--212.

Miiller, L., 1968. New considerations on the Vajont slide. Rock Mech. Eng. Geol., VI:

1--91.

Miiller, L. and Malina, H., 1968. Schubspannungsverteilung im Progressiven Bruch. Rock

Mech. Eng. Geol., VI: 216--224.

Nonveilter, E., 1965. The stability analysis of slopes with slip surface of general shape.

Proc. 6th Int. Conf. Soil Mech. Found. Eng., Montreal, Vol. II, Div. 6, Toronto.

Nonveiller, E., 1967. Zur Frage der Felarutschung im Vajont-Tal. Rock Mech. Eng. Geol.,

V(1): 1--9.

Rengers, N. and Miiller-Salzburg, L., 1970. Kinematische Versuche an geomechanischen

Modellen. Rock Mech., Suppl. 1, pp.20--31.

Selli, R. and Trevisan, L., 1964. Studio geologico della frana del Vaiont. Allegato alla

relazione della Commissione d'inchiesta sulla sciagura del Vaiont. Genn., 1964.

Semenza, E., 1966. Sintesi degli studi geologici sulla frana del Vajont dal 1959 al 1964.

Memorie del Museo Tridentiono di Scienze Naturali, A. XXIX--XXX, Vol. XVI,

Fasc. I, 1966--67.

Skempton, A.W., 1966. Bedding-Plane-Slip, Residual Strength and the Vajont Landslide.

Corr. Inst. Civil Engineers.

Scheidegger, A.E., 1964. Tektonische Spannungen und deren Einflu~ auf geologische

Verschiebungen. Rock Mech. Eng. Geol., II: 100--105.

Scheidegger, A.E., 1973. On the prediction of the reach and velocity of catastrophic land-

slides. Rock Mech., V: 231.

WeirS, E.H., 1964. Vajont -- Geologische Betrachtungen zur Felsgleitung in den Stausee.

Steir. Beitr. Hydrogeol., H. 15/16, Graz.

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The Vajont Slide 1987 Engineering Geology