Technical Visit - Vajont Dam

8/11/2019 Technical Visit - Vajont Dam

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Venice, 13thApril 2013

THE HISTORY,

THE TRAGEDY.

VAJONT DAM


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Venice, 13thApril 2013

THE HISTORY,

THE TRAGEDY.

VAJONT DAM

9 th October 1963

Dam of Vajont


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View and postcard of

Longarone, some daysbefore...


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and the day after


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A stonefell into a glass full of water,

and the water spilled out

on the tablecloth.

That is all.

Dino Buzzati


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1937: Second project:dam at

Colomber bridge

1926: First project:dam at Casso

bridge

Vajont river

Massalezza river

Pian del Toc

La Pineda

Monte Toc0 500 m


8/42

The geological reports were focused on

the problem of the stability and

impermeability of the abutements of the

dam.

The stability of the slopes of the

reservoir was considered, but not in

detail.

The dam regulation in force at the

beginning of the construction (1957)dated back to 1931.


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The Real Decree n.1370 on October 1st

1931 was substituted in 1959by the DPRn.1363

It required that the geological study must

include objective elements relative to

slope stability


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PONTESEI

reservoir

Hydroelectric system

Piave-Boite-Ma-

Vajont

Beforeoctober 1963VALLE

reservoirVODO

reservoir

Pieve di Cadore

reservoir

PONTESEI

GARDONA

Pressure tunnel

Pieve-Val Gallina

COLOMBER

VAL

GALLINA

reservoir

SOVERZENE

PIAVE RIVER

VAJONT RIVER

VAJONT

reservoir

Regione

Veneto

Piave river

high basin


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PONTESEI

reservoir

Pieve di Cadore

reservoir

PONTESEI

GARDONA

Pressure tunnel

Pieve-Val Gallina

COLOMBER

VAL

GALLINA

reservoir

SOVERZENE

PIAVE RIVER

VAJONT RIVER

VAJONT

reservoir

Regione

Veneto

Piave river

high basin

VALLE

reservoirVODO

reservoir

Hydroelectric system

Piave-Boite-Ma-

Vajont

Beforeoctober 1963


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Inlet from Gardona

power plant

By pass tunnel

bridge

Outlet to Val

Gallina

Colomber

power plant

Outlets of the dam

Inlet from Pieve

di Cadore


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Main geometric data

Elevation of top of dam 725,50 m aslArches thickness

At elevation 720 m asl

At elevation 480 m asl

21,5 m

Heightabovelowerfondation

261,6

0m

3,9 m


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On march 22nd, 1959, a landslide fell into the

reservoir of Pontesei. The landslide was estimated

about 3 x106m3. This potential instability was

known by technicians

At the end of july 1959, Edoardo Semenza, son of

Carlo Semenza the designer of the dam, began a

program of geological-technical studies of the entireimpoundment zone of the Vajont dam in order to

discover any phenomenon of potential instability.


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Isolated

hill

Prehistoric

landslide

Gorge of Vajon

cut through

paleoslide

Paleoslide

Clays lenses Vajont limestone

Vajont valley

Isolated Hill

Mt. Salta

Mt. Toc


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RECONSTRUCTION OF THE PALEOSLIDE:

This is the situation

before the paleoslide and the paleoslide

Thenew stream-bed

of Vajont river

This is the geologic

situation preceding

November 9th, 1963

Vajont

limestone


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Drillholes

Drillholes

(piezometers)

Drillholes

(piezometers)

Geoseismic

survey

Supposed extentof the landslide

Dam

Investigations carried out


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At the end of 1959

the paleoslide

was NOT recognized

by all geologists.

A monitoring system was installed on

the slopes in order to control the

movements of the ground


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Comparative diagram

of water levels in the reservoir and rate of

the movement from 1960 to 1963

J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

19611960

First infill

1962 1963

A large M-shaped perimetricalfessure appeared

On 4thNovember 1960 a landslide of

700000 m3fell into the reservoir.

The drawdown of the reservoirbegan


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Dam at Ponte di

Colomber

Vajont river

Massalezza river

Perimetrical

crack appeared

in november

1960 The

boundary of the

landslide?

Landslide of 4thNovember 1960

4thNovember 1960


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Landslide of 4thnovember 1960

(700 000 m3)


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Perimetral crack


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Pineda

S.MartinoErto

Massalezza

Casso

Monte Toc

Extension of the

paleoslide

Stereogram of the vajont reservoir

DAM


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After november 1960

it was clear

the existence of the landslide

but the geologists

did not suggest abandoningthe project

and therefore it was decided to

allow the movement of the landslide

into the reservoir and to controll it by

controlling the reservoir water level


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In order to continue to use the

reservoir in case of the fall of the

landslide

1) A tunnel was built on the right

slope, in order to prevent the

division of the lake by an eventual

landslide (summer 1961).

2) A physical-hydraulic model wasbuilt at Nove in order to evaluate the

effects of the fall of an eventual

landslide


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Landslide By-pass tunnel

Scheme of the by-pass tunnel to reconnect the two lakes


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The hydraulic model at Nove

22 tests were carried out with

various fall times and reservoir

levels.The minimum fall time was 60

seconds


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Comparative diagram



Second

infill

1960 1963

First infill

Building of the

by-pass

Piezometers

1961 1962

At the end of November1961 the drawdown began



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Comparative diagram



Second

infill

19611960 1962 1963

The third infillstarted...

At the level of 650 m asl, the

movements stopped.

First infill

Building of the

by-pass

Piezometers


At the end of November1961 the drawdown began


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Comparative diagram



Second

infill

196319611960 1962

The third infillstarted...

At the level of 650 m asl, the

movements stopped.

First infill

Building of the

by-pass

Piezometers

Seismic tremors were felt


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Comparative diagram



Second

infill

196319611960 1962

Seismic tremors were felt

It was decided to reach

the level 710 m ....First infill

Building of the

by-pass

Piezometers


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The Vajont

reservoir.

Before

and after


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View of the mouth of the Vajont river

before and after


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before. and after

The town of Longarone


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Outlet

Planimetry of the

Vajont Reservoir

Outlet

Outlet

Tunnel to Val Gallina dam

From Pieve di Cadore

and Valle

From Gardona

By pass

Residual

lake C


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Tunnelby

pass

New outlet of the

residual lake C

VAJONT DAM

Planimetry

Before 1963 Nowadays

From Pieve

di Cadore

and Valle

From Gardona

Tunnel

bridge

by pass

Colomber

powerplant

Outlets

Upstream

Downstream


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Why did the hydraulic model fail ?

Was it wrong ?

The first doubts about the model were two:

1) the fall time

2) the material used to simulate the landslide


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Why did the hydraulics use such a long

fall time?

The fall time for the landslide of Pontesei was

estimated to be about 2 minutes or even longer

For the Vajont mass the technicians agreed on afall time between 2 and 8 minutes.

Even if the time length of 1 minute was thought tobe too short and too conservative, it was used by

the hydraulics involved in the model.


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Why was the fall time so short?

Some research carried out many years after theevent explained this very short time of fall with

the production of a vapor cushion and with the

strong reduction in the mechanical characteristics

of the clay along the sliding plane during the

movement.

Only with the wisdom of hindsight

the time of 1 minute could be

considered too long.


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The technicians during the trial stated that :

The use of the real fall time in the model could

have reproduced the same results correspondent

to that of reality .

thegravel at such an elevated velocity reacts like

compact material .

It is possible to conclude that the

physical model was not wrong

but was wrongly used.

Vi f th d d l d lid


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View of the dam and landslide


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Technical Visit - Vajont Dam