Foundation of the Mseno dam; Analysis and the Study of Remedial Measures

Volltext

(1)

Dr.-

Ing.

L.

Satrapa,

Dr.-

Ing.

P.

Valenta,

(CZ)

CTU

Prague, Faculty

of Civil

Engineering,

Department

of

Hydrotechnology

Foundationofthe Mseno

Dam;

Analysis

andthe

Study

ofRemedialMeasures

1 Introduction

The curved

gravity

damwas constructedinthe

period

between 1906to 1909. The

dam is located on the Nisa river in the town Jablonec nad Nisou inNorthern

Bohemia.In the

past

themainpurpose of the reservoirwasflood

protection.

Now the reservoir is usedforwater

supply,

recreation

(winter

andsummer

sports)

and

flood

protection.

Theadditional

geological prospecting

of the siteofthe Mseno damwas

performed

in 1987 and in 1988. This

prospecting

wasthereactionto

unsatisfactoring

results of the dam surveillance. The

geological prospecting

discovered the

layers

ofnon

rock materials under thedam

body.

The

layers

of

granite eluvium,

alluvial

granite

eluvium, sandy

clay

etc.

probably completely

cover the base ofthe foundation.

Historical

drawings

and

design

documents do not

correspond

to the results of

geological

prospecting.

Itwasdecidedto

perform

themoredetailednumerical

analysis

of the dam andits

foundation.Thereasons for this

analysis

werethe

unsatisfactoring

results of the

geological prospecting,

the dam surveyand the

stability analysis

basedon

classi-cal classi-calculation methods.

Consequently,

the

technical

solution of remedial ineas-ures was

supposed

to be

suggested

thedetailed

analysis

of the dam.

2 The Mseno dam

Theconstruction material ofthe

body

oftheMseno damis

granite

masonry.Some

similar

dams,

approx. 90 years

old,

are located eitherinNorthern Bohemia and

other

European

countries. The

type

ofthese dams

(Intze

type)

ischaracterised

by

theearth

body adjacent

tothe

upstream

faceof the dam

(Fig.

1 and

2).

The dam is 420 m in

length,

the damcrest is 4.5 min

width,

the radius of the curvatureofthedamis350m.

The elevationof the

top

of the dam is 513.00ma.s.1. The

height

of the dam

(at

the

lowest

point

of the

foundation)

is

approximately

20 m. The width of the damis

15 m

(the

widthat the

toe).

The

evation

of thelowest

part

of thefoundation is

492.00 ma.s.1.

The contact between the

granite

masonry of the dam

body

and the

subgrade

is

treatedwith 1mthick

layer oflevelling

concrete.

(2)

/P':4

'tl

HUU,

r- 7

.'' '" ·*,·, ·,

A,-

A,·.·;

A

··.·;

a

Fig.

2:Msenodam crosssection

il

9

4=*• -•i.™,I

,„.4

1.„./

i sandyloam

lail/t.LE£j

?.andycle? (clluvialgronil.aw&4

alludal gman.Ituylum

W viath.redgranHI

kna

stightly weath ndgronm

Fig.

3:Msenodam

subgrade

''

'':':''li

EE,34

./'.'.'.'.'. '''.il-Ii-- .... ...

/F+Ii-F+Ei

i

1-]-ilr.P1

-4

-- ----

Fig.

4: --

Seapage

analysis -FEM mesh

3 The

subgrade

of the dam

The archive materials

(design documents,

drawings)

do not

probably correspond

to the real situationas for the

subgrade

of the dam. Thesolid rock under the dam is mentioned in olddocuments and

drawings.

_1,1·1n W

\

2

,

Fig.1.Msenodam-

plan

view

R

(3)

The thickness of soil

layers

between the dam and the bedrock varies from 7 to

11 m.The main

part

ofthenonrock

subgrade

consists in alluvial

granite

eluvium. Some

parts

arefilled

by sandy clay

and

sandy

loam

(Fig.

3).

The

rapid piping

failure ofoneboreholeatthe bottom ofthe

valley

(downstream)

occurred

during

the

geological

prospecting.

The

high discharge

of

running

sand

from theboreholewas

stopped

by

sand

bags.

The

pieces

of

wood,

whichwerefound in the material removed from the low

parts

of

geological boreholes,

canbeconsidered the evidencefor thepresenceof allu-vial

deposits

underthe dam.

4

Seepage

and

ground

waterflow

analysis

Theseepage

analysis

and the

ground

water flow

analysis

wasthe first

step

ofthe dam behaviour

analysis.

The FEM

technique

was

applied (2D

quadratic

isoparametric

elements).

The2Dmesh of finite elementsconsistsin 1305elementsand 4088 nodes

(Fig.

4).

Some results ofseepage

analysis

are

presented

in

Fig.

5. The

drawing presents

path

lines and

equipotential

lines in thecaseof fullreservoir.

Hydraulic

gradients

were the most

important part

of obtained results.

Fig.

6

presents

the

large

area of

high hydraulic

gradients

(higher

than

1.0)

located near

the downstream

part

ofthe dam toe.The

shape

ofthisarea canbe characterised

by

the width of4mand the

height

of2.5 m.

5 Statical

analysis

-

present

conditions

The results ofthe

ground

water flow

analysis

andthe results ofthe

geological

prospecting

were used for statical

analysis.

The FEM

technique

was used

(3D

quadratic

isoparametric

elements).

The detailed3D mesh

(2D

view)

of finite elements consists in720elements and

5323 nodes

(Fig.

7).

The

following

loads have been taken into consideration:

hydrostatical loads,

hydrodynamical

loads

(uplift,

seepage

pressure)

and volume loads.

Some results in the caseoffull reservoir areshown

by

Fig.

8aand 8b. The

con-centration of

high

tensile stresses

(Fig. 8a)

andthe concentration of

high

shear

stresses

(Fig. 8b)

is

presented.

6 Thedam

safety

-

present

state

The

high hydraulic

gradients

concentrated in thearea

adjacent

to the downstream

toeof thedam areconsideredto be the

warning

that the dam isnotsafe

enough

from the

hydraulical

point

ofview. The external

impulse

can cause the

rapid

piping

erosion of the nonrock

layers

under the base of the foundation. The

following

restrictions were

applied

to reduce the

dangerous

effects of

high

hydraulic gradients:

(4)

1/the

operating

waterlevelwaslowered

by

2 m,

2/ itis forbiddento

operate

heavy vibrating

machinesatthedownstream face of the dam

(in

the

strip

20 m in width

along

the downstream

face)

and open any

excavationatthesame area.

'14 >...

. , "

Fig.

5:

Seapage

analyss

equipotential

lines and

path

lines(full reservoir)

26= 9-

'7·I

1

M

-L ;

29

Fig.

6:

Seapage analysis

-

hydraulic

gradients higher

than1

Fig.

7: Staticalanalysis-FEM mesh

GD analysis)

thI

K

/:.'1.1 J

7.,1.

.

9:-:

Fi .Sa:Staticalanalysis present state,

Fig.

8b:Statical

lysis

presentstate,

full reservoir(areaof

li'gh

tensile

stresses)

full reservok(areaof

high

shearstresses)

stresses)

(5)

Itis

prescribed

to focusthe damsurveillanceon the

changes

of the downstream

landsurface

(to

observe

springs

and their

turbidity,

terrain

deformations),

The

stability

of thedamisnot

endangered

now

(from

the statical

point

of

view).

The

present

state of the Mseno dam does not

require

any

rapid

and extensive remedialmeasures but theresults of

geological

prospecting

and theresults ofFEM

analysis

showthat

safety

coefficient of the dam isnearto 1.0now.

The above mentioned facts are reflected in the effort ofthe owner ofthe dam

(Povodi

Labe, a.s.)

and the

supervisor

ofthe dam

(VD TBD, a.s.)

to

plan

and

design (in 1997)

and to

perform (in 1998)

necessaryremedialmeasures.

7

Subgrade

treatment

(sealing)

-alternatives of the solution

The

technology

basedonthe construction ofthe cut ofwallhas been selectedto be used for the

design

of remedial measures. The

analysed

alternatives are

presented

in

Fig.

9. All thesealternatives were

analysed

by

2D FEM

modelling.

The

changes

ofstress-strain behaviourwere

analysed

to compare themin all

alter-natives and with

present

behaviour ofthedam.

The behaviour of the dam was

analysed

taking

future conditions into

considera-tion:

1-the

change

caused

by

the construction of

seating element,

2-the

analysis

in thecaseofafull reservoir

(after

remedial

measures),

3- the

analysis

in thecaseofan

empty

reservoir

(after

remedial

measures).

8 Deformation of dam

body

-3Deffects

The threedimensional

analysis

of the behaviourof the dam

body

was

performed

tofindan informationabout the relationbetweenresults of 2D

modelling

and the

real behaviour of the dam. The

analysis

was

performed

for

straight

and curved dam

(Fig.

10-3D FEM

meshes).

The effect of threedimensional

shape

ofthe dam

body

is

possible

to

clarify,

for

example, by

the

displacement

ofsomereference

point

atthe

upstream edge

of the

crestin the centreof the

valley

(the

displacement

between fulland

empty

reser-voir):

2D

analysis

- horizontal

displacement:

29.2mm

upstream

vertical

displacement:

5.0mmsettlement 30

analysis

- horizontal

displacement:

24.9mm

upstream

(straight

dam)

vertical

displacement:

4.3mmsettlement

3D

analysis

- horizontal

displacement:

22.6mm

upstream

(6)

Fig.

9: Alternativesofthelocation ofcutoffwall

r

942

/

Fig.

10:Staticalanalysis-FEM mesh(3D

analysis)

9 Statical

analysis

-thestateafterremedialmeasures

Afterthe

analysis

of allalternativesthe

acceptable

alternative has been selected.

Figures

llaand lib

present

the resultsin the caseof full reservoir

(concentration

ofhigh compressive

stresses-

Fig.

1

la,

the concentration

ofhigh

shear and tensile

stresses-

Fig.

1

lb).

Thecut off wall islocatedunderthe

upstream

toe.This

alter-native isconsideredtobe

dangerous

for the dam.

r

r

F--r

gl

Ii|l,

.1J

1.1,

,]

Jj

'.1

Illi

111"

1

11

(7)

Alililm

'4*pgil

liliir„n,

/

1<

.-Fig.ilb: Staticalanalysis

cutoffwallunder theupstream

toefullreservoir(area ofhigh

shear and tensilestresses)

Fig. 1la: Statical

analysis

cutoff wall under the upstreamtoe,

fullreservoir(area ofhigh

compressive

stresses)

1

L.¢ip

wmm#

44)51d

/424

1

rad,

'1'192U

F. Apt.

%

6

1

Tll

lilI

Illli1l

'MI!11

Ililli

Fig.12b: Staticalanalysis

cutoffwall in front

ofupstream

face,fullreservoir(areaof

high

shearandtensilestresses)

Fig.

12a:Staticalanalysis

cutoff wall in front

ofupstream

face

fullreservoir

(area

ofhigh compressive stresses)

*

'.1

"'r:

Lar:EL

-2 -.

11

Im

IT

i

Y

N.

l/ti

d

i

ifuj

i

..,4

4

R

-9-721%

T'

m:

-;i .::=:=. : :=11 .. .. .... .. t•T•-*

T.1

411

*13

% / 89

(8)

Figures

12a and 12b

present

the results forthecaseof full reservoir

(concentration

ofhigh compressive

stresses-

Fig. 12a,

the concentration

ofhigh

shear and tensile

stresses-

Fig.

12b).

Thecutoffwallislocatedin front ofthe

upstream

face.This alternative is considered to be convenient for the

design

of remedial measures.

The

strip

betweenthe damand the

top

ofthe cutoff wallcanbe sealed

by

flexible membrane.

10 Back

analysis

ofthe dam behaviour

The back

analysis

of the dam behaviour will be

performed

before the detailed

design

ofremedialmeasures.

The

supervisor

ofthe dam

(VD

TBD, a.s.)

is

responsible

forthestatistical

analysis

of the

displacement

measurementsoftheMseno dam. The main

objective

ofthe statistical

analysis

isto find the correlationbetween the dam movementand the water

level

movement andtoexcludethe

thermal

effects (May

1997).

Theownerofthedam

(Povodi Labe, a.s.)

is

responsible

for the

emptying

of the reservoirand the simultaneousmeasurementofthe dam

displacements (September

1997).

The 2D and 3D back

analysis

willbe

performed

and the mathematicalmodel of the dam will be verified.

Consequently

the verified models will be used for detailed

design

ofremedialmeasures.

11 Constructionof remedialmeasures,dam surveillance

The construction ofremedialmeasuresis

planed

forthenextyear

(1998).

The measurementof dam movement will be

performed during

the construction. The measured values will be

compared

to

predict

values calculated

by

verified2D

and 3D numericalmodels. Thesemodels will also beusedfor the

analysis

ofpos-sible differences between calculated andmeasured

displacements.

The

procedure

of numerical modelverificationwill continue

during

the

construc-tion

period. Consequently

the

relationship

betweenthe movementofwater levelin thereservoir and the dammovementwill be

analysed (critical

values

including).

These information will be used for the surveillance of Mseno dam after the construction ofremedialmeasures.

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