Indian National Committee on Large Dams. Design and Construction Features of Selected Dams in India

Stages

Period

Monsoon

river

discharge

flowing

through

Progress

of

fill

placement

of

dam

Stage-0

Stage-I

Stage-Il

Stage-III

Stage-IV

Stage-IVA

October

October.

October

1970-June

1971

l97l-June

1972

1972-lune

1973

I973-June

1974

1965-June

1969

October

1969-June

l97O

Natural

river

bed

Tl,T2,Pl,

P2

& P3

and

over

partly

com-

pleted

dam

Tl,

T2, Pl,

P2 &

P3

Tl,

T2, Pl, P2

&

P3R

Pt,P2

&

P3R

and Spillway

\igh_t

abutment

raised

up

to Et. 365.76

m

simultaneously

with

constiuction

of

iunnits"

Right

abutment

up

to

El.

3g2.Sg

m

and

central

portion

up

to

El.

335.2g

m.

Central.

portion

up

to El.

390.14

m

(trunca-

ted

section).

Central

portion,

right

and

left

abutments

(truncated

section)

up

to

EL.399.29

m.

Central

porrion,

right

and left

abutments

truncated

section

up

to

El.

423.67

m.

Dam

substantially

completed

up

to

El.

435.86

m.

Bulk heod

Progress

>

Excovotion

line

IIITIIIITLrIT

Scnedd

guide

trus

Sli,ck

p,p"

for

invert

pour

?

50 m

working

room

L-SECTION

SHOWING

GENERAL

Overt

forrn

ARRAN6EI,ENT

Riser

Side form

Screed

trusses

PL

AN

208

FIGURE 13:

Tunnel

concreting-General

set-up.

Axi5

qf

66m

Normof

neservoir

il.

426.72

m

!93mrrop

of

dom

8t.435.96m

El.423.67 rn

2

399. m

m

-

q.-2.r

9-t

onoonr ntiroe weistrn

Tfr']'Ei

IOtOO

m

15 fOO

m

2OtOOm

2$OOm

Stolions

@23.76m

f

El.399.29 m

sE--fEt392.58m

covotion

line

X-SECTION

I

Rlght

obutmentl

FIGURE

14:

Beas Dam-Accomplishment

year

by

year.

FlGiJR3

15 :

Monsoon

flows

of

River

Beas

gushing

out through

iliversion

tunnels.

n'

4*'-+'4

t

365.76m

35O.52m

'58

m

l.423.67m

L

-

SECTION

THROUGH

DAM

Stogc O.

Pre-

dversion

1965-June

1969

Stoge

l,

OcloberrTO-.lne,7O

Stoge

2,OctobecTO-Sept 7 |

Stoge

3,

OctobecT

|

-Segt.72

Sloge

4,

Octobec

72-Sept

73

Slog

ArOclobecT3

J

rurr-7

4

Normol

reservoir

E,1.426',.72

n!_

a

EE

ssl

wvz,

Fslir

w

ts\\l

209

I

r

t

I

tunnels were

closed

in stages, after

these

had

served

the

diversion

requirements.

The

tunnel P3 was closed

for

second

stage

works, that

is, completion

of bulkhead

reach and

installation

of emergency

gate

in

October

1971. This tunne!

was re-opened

as

gated

tunnel

in

June

1972

for

passing

river

flows. Tbe

river bed

entries

of the

T-tunnels and

P-tunnels

(Pl

and P2 only),

were

closed

by

lowering stoplogs

in

the

upstream

portals

during September

1972

and October

1973

respectively

for

taking up

permanent

closure

plugs

and

second

stage

works. The

final

closure of P3R tunnel

for

conversion

into

penstock

was

planned

by

lowering

gate

through

bulkhead shaft with the

reservoir level at

El. 384.05

m. For

this

purpose,

a lined

construction

of

size

3.05

m

x6.40

m was constructed

in

the

tunnel.

However, during

operation

of

the

restricted

P3R

tunnel,

the

liners

provided

in the

bulkhead

reach

got

ripped

off

due

to high

velocity flows. It was,

thus

not

possible

to

block

the river bed

entry

of P3R

tunnels,

as

originally

planned.

The

final closure

of

P3R tunnel

was,

therefore, affected

by

lowering

stoplogs

in

the

upstream

portal

under

53.3

m deep

water with

the

help

of

divers and

derrick crane

in June

L976.

Lowering

of

stoplogs under

53.3

m

deep

water

was

an unprecedented

job

and

was

accomplished

by

meticulous

planning

and

devising facilities

necessary

for

successful

closure

of the

tunnel.

2. CONSTRUCTION OF DAM

The

construction of

dam involved

the

placement

of

nearly

35 million

cu

m

of

fill

materials

and about

nine

million cu m of excavation of

river

bed meterial

and

rock.

Tbis

was

a

stupendous

task

and required

thorough

pre-planning

of all operations, including

selection of equipment, facilities

to keep

the equip-

ment

in

good

working older,

arrangement

of spares,

tyres, fuels,

lubricating

oils, etc.

Section

of

Dam

Section

of the

dam and its

zoning

was

designed

to

facilitate

the

maximum

use of the materials

avai-

Iable

from excavation

of

various

appurtenant

works,

yfu.,

spillway,

tunnels,

approach

channels,

stilling

basins,

etc.,

for impervious core and

natural

river bed

deposits

for

filter

and

pervious

zones.

The final

maximum

section of

the

Beas

Dam

with

zoning

arrangements

as adopted,

is

given

in

Figure

6.

The

main

embankment of

the

dam

was

divided

into the

following

fill

zones

(f)

Iurnnvrous

Zolre

This

zone

comprised earth from stockpiles

of

materials

excavated

from

various

structures or

directly

from

excavation

of

foundations.

la-The

material

for

this

zone

was

primarily

the

clay

shale

containing

less

than

50

percent

sand

rock.

210

Ib-The

material

for

this

zone

was

primarily

the

sand

ro-ck

containing

less

than

-50

percent

clay

shale.

core.

(ff)

Prnvrous

ZoNr

This

zone

included

2a-Filter

zone

of selected

free

draining sand

and

gravel

wifh

not more

than l0

percent

of size

l.a-rger

than

75

ffiD,

but material

exceeding

150

mm

was

not

permitted.

2b..

Pervious

zone

of

selected

free

draining sand,

gravel,

cobbles

and

boulders. The

maximum

size

of

the

material

used

was

such that

no

stone

protruded

above

the

compacted

lift

of

0.6 m.

Rip-rap

of selected

gravel,

cobbles

and

boulders.

Upstream

and downstream

toe

weights of

random

material.

Excavation

and

Treatment

of Foundation

(i)

ExcevArloN

About

nine

million

cu

m

of excavation of river

bed

material

and

rock

was

involved

for foundations

of

the

dam.

In

the

river

portion

under the shell

zone

of the dam

embankment,

the

excavation

was extended

to

clean

sand;

gravel

and

cobble material having

same

characteristics

as

that

of

shell zone material. Tbe

excavation of

core

trench

on right

abutment

was

taken

up to

a

minimum

depth

of

6

m

below the

actual

bed

rock level

or to

the sound rock

level

whichever

was

lower.

However,

this criteria

was

dispensed

with

as

it was

considered

that no useful

purpose

was

served

by

excavating

the

sound

rock

first

and

then replacing

it

with the

same

material.

In

the

river

bed

portion,

the core

ftench

excavation

was, therefore,

extended

to sound

bed rock

only. The

key

trench was,

however,

excavated

6 m below

the

bottom of the

core

trench.

The

excavation

work

commenced

in

October

1965,

when the

excavation

on the right

abutment

was

taken

in ha

nd.

This was

almost

completed

prior

to the

river diversion

in

October

1969. Afrer

river

diversion,

excavation

operations

were

extended

to the

natural

river

channel. Excavation

of material was

done

in the river

bed

portion

in

order to expose the

sound

foundation rock

(Figure

16).

The

task

of

excavation

in

river

bed

portion

was

accomplished

in

a

very

tight

working

schedule

despite the fact that

adverse

geological

features,

actually

encountered

in

the

foundations needed

complex

foundation treat-

ment. Subsequent

to the flow

of the river

over

the

partly

completed

dam,

that

is,

afier

the monsoon

of

1970,

the

excavation

for

the

core and key

trenches

of

the lefi

abutment

section

was

carried out.

(fi)

PaonLEMS

nNo

FouNDArroN

TnrerueNr

During

course

of foundation

excavations

in the

river

bed

portion,

a

number

of adverse

geolo-eical

features,

viz., seepage,

glide,

cracks,

piping

conduits

and cavities,

plastic gougy

seams, etc.,

were

encoun-

tered.

Such features

were

successfully

tackled

and

necessary

treatment

(Figure

5)

provided

to ensure

safety

of

the

dam

embankment.

Seepage

Control

Perforated G.I.

pipes

embedded

in

gravel

filter

drain

Were

provided

where

seepage

was encountered.

Glide

Cracks

The

presence

of open

glide

cracks

in

the

foun-

dation was

detrimental

to the

stability

of

the

dam

and safe

functioniqg of

the core.

Since

grouting

was

not

considered

effeltive for such

open

-cracks]

t.tS

lakh

cu

m of unsound

rock

affected

by

such

cracks

rvas

removed

flastic

Seams

A

number

of

exposures in

the foundation

area

of

the dam

highlighted

the

presence

of

the

plastic

gougy

seams

under

the

dam section.

Such seams

had very

low shearing

resistance.

These

plastic

seams

required

special treatment

as under

:

(a)

Excavation

of

six

small

cut-off

trenches 3

m

wide

in a

length

of 4.5 m,

extending

0.6 m

below the

plastic

seam

in

clayshale

band 44,

and

3

m deep baby

trench

for

treatment

of

FIGURE

16 : Excavated

river bed

(Right

abutment

of

dam

is

seen in the

rear).

2lr

plptlc

seam

at

the base

of

clayshale

band

8A. The

trenches

cut

the

plastic

seams

and

provide

a longer path

of

percolation.

(b)

Placement

of

upstream

and

downstream

toe

wgielts

.-requiring

additional

fill

placement

,

of 2.8 million

cu

m.

This

additional

dll

became

necessary

for

safety

of

the

dam

under

seismic

condition

in

view

of low

shearing

strength of

gougy

seams.

(c)

lxcgvation

and

concreting

of

eight rigid

buried

shafts

3 m

diameter

and 2t.l

m

aeef

at bench

elevation374.9

m

for safety

of

hilislope near

penstock

intake

shafts.

Piping

Conduits

In

the key

tretch

excavation,

active

seeoage was

obscrved

in sand

rock band No.

2

through

i iumber

3. DAM

EMBANXMENI

of

openings-

As

excavation

for

key

trench-progressed,

a

nu4be1

o{

piping

conduits and

cavitiei wire

also

FiIl

Materials

trouced

on

rts

downstrcam

side. The

followins

measures

wcre adopted

to

reduce

thc

prospect

of

eas-y irosion of

The

quantity

of fill

materials

utilized

for the

sand rock

and of seepage

inlhe

itam

found'ation:

constroctii;

"fit

"

aa--i*!.-riii"r-i"

(a)

Extension

of

k€v trench

between

stations

Impervious

core

zone

(fc

and 16)

10-97 nillion

cu

m

17*37.3 m

to 17

+52.6

m

so as to cross sand -.-''.''

rockband

No.2 and

an

auxiliary

key

trench,

trltet

zone

2a

1.67

rrillion

cu m

5.5

g

long,.

gltenping

one metre

ieep Loto-"tgy-

pervious

zone

2J

22.40

million

cu m

shale

band No.

3.

The

excavared

deDth of this

auxiliary

key

trench varied

from

7.6 m to

Selection

of Material

10.7

m.

GroutinglPrbcedure

(b)

Drilling

of

25

drainage

wells,

300

mm diameter

and

25

to

77.2

m deep

filled

with

sand and

gravel

at the toe of

impervious

core.

In the

river bed

portion,

area

grouting

was

done

at 6 m

spacing, within

30

m, on either

side

of the

key

trench

and cover the

old

perennial

channel

section so

as to

seal

off the

cavities

in

sand rock

band No.

2.

Area

grouting

at

6 m

spacing was

extended

over

the

done

in

order

to cover

the

possible

extension

of

the

cavities

in sand

rock

band

No.

2.

The

area

grouting

under

gravity

head

was further

extended

on

the left

abutment,

under

the

core trench

212

IMpsnvrous

MATERIAL

In

the

meanwhile,

excavation

for

diversion

tunnels

and construction

of approach

roads

had

started.

It

was

observed

that

the

excavated

material

crumbled

down

into

-fine

earth

in

the

process

of blasting

and

subsequent

dozing.

This

material

was

tested

and

found

ideal

for

use

in

impervious

core.

The

test

results

are

summarized

below

:

(t)

The material

was,

generally,

finer

than

2 mm.

(tt)

The

_

material

was

easily

compactible

and

provided

-on

average

density

of- 1.94 gmlcc

with

specific

gravity

of 2.69.

t7

ct

q)

t

00

f-

r-1

D

wos+8'ots

oo+8!ts

213

khod

orto

Av

E1.338.32n

Sibo khod

Av.81.381.6

m

c

20o

f

L

a?1

O

!v

?

o

60:

c

o

(t

80b

(L

o

I

6

4

c

o

E

L

o

.g

b

c

o

(,

L

o

(L

A,8,C,D,E.F,G

ond

H

llnpervbut

cora

lo

&

lbl

Solhono,

GojrKhonpur.

Sibo.

Lr.

M.N,

pond

O

t Filtcr

ond

pervi'6ui

Zo

&

Zb

t

Eorrow

or?og

oo

9s

(ii)

Unit

submerged

weight

(iii)

Unit

saturated weight

(iv)

Coefficient

of

permeability

(v)

Angle

of

internal friction

(vi)

Cohesion

o

I

o

666

o@

Size

in

millimelre

s

LIMITING GRADING CURVES

FIGURE 18.

q90

no

r}rlFo

o

(iii)

The

permeability was of the

order

of

l0-?

cm/sec

(lv)

Average value

of

cohfsion was

0.4

kg/cm2

(u)

Pore

pressures

were almost

negligible.

On

the

basis of

the test

results, the

soil

charac-

teristics assumed

for the

impervious

material

were

:

(i)

Unit

moist

weight

214

l.l6

gm/cc

2.16

gmlcc'

10-G

cm/sec

or less

26.5

degrees

0.35

kg/cmz

BORROW

AREAS

2.08

gn/cc

FnrsR Mlrentnr,

In the

initial stages,

the

placement

was

started

from

SiUa

fnaO,

a rjvulet

about

5

km

upstream

of

the

dam

axis.

The

material

contained

small

percen-

Penvlous

Ma.remA.r

action.

on

the

basis

of

extensive

field

and

laboratory

tests'

tne

fodowing

design

parameters

for

the

pervious

material

were

evolved

:

could serve

as

a

natural

passage.

The

local depres-

sions

were

filled

up

manually

with

'la'

material

and

compacted

with

pneumatic

tempers. The

foundation

surface

was

moistened

to

obtain a suitable

bond

with

the embankment.

Conn

Tnsxcn

After

the

key

trench

had been

filled, the

work

of

filI

placement

was

extended

to

the core

trench.

Since

the

working

area had

widened,

operations

were

divided

into

different

lanes of about

30

m

width each

parallel

to the

axis

of

dam

so

that entire

operations,

viz.,

dumping

the

fill

material,

levelling,

stone

picking

and

compaciion

could

be carried

out

in

a

continuous

cycle.

Pl,nceueNr

rN

LnYgRs

tyred

equipment

which

crossed

the

fill

embankment

for

dunipiirg

operations,

the surface

of

the

was siarified

before

the

placement

of

the

Morsrune

CoNlnNr

Coupe,crtoN

(i)

Unit

moist

weight

(r'i)

Unit

submerged

weight

(iri)

Unit

saturated

weight

(iu) Angle

of

internal

friction

38

degrees

(v)

Cohesion

Nil

(vi)

Perme

abilitY

1 x

l0-4

cm/sec

The

limits

of

grading

curves

for

fill-materials,

are

shown

in

Figure

18.

4. FILL

PLACEMENT

Impervious

Core

Zone

Kev

TnetIcH

The

final

cleaning

of

the

surface

was

done

by

.ru^-

o?

uit

jets

so

as

to

remove

sand

content

which

L.92

gmlcc

1.20

gm/cc

2.20

gmlcc

215

rock

was

carried

out

by means

of

pneumatic

compactors.

Figure

l9

depicts

various

operations

of impervious

€ore

construction,-viz.,

material

production,

fill

place-

ment

and

compaction.

Filter

Zone

(2a)

PraceueNT

AND

Morsrune

CoNrenr

During

the

initial stages,

the

placement

was

carried

out

in

0.6 m

thick

layers and

equal

amount

of water

workable

moisture

content. The

placement

was

done

in

two lifts,

each

of

0.3

m.

Each

of the

two litts

yls sufficiently

watered

prior

to

the rolling

of

total

trift

of

0.6 m.

CoupecuoN

The

compaction was

done

with

5 and

l0 tonnes

vibratory

compactors. For

proper

compaction,

four

coverages with

5

tonnes

and

two

coverages with

10

tonnes

compactors

were

given.

Prior

to

receipt

of

vibratory

compactors on

the

project,

the

compaction

of

0.3 m thick layers

was done with D-9

crawler

tractor.

Sixteen

coverages were

given

to

ensure com-

paction

of

the material.

The

material was

originalty

compacted

to

achieve

a minimum

relative

density of 80

percent.

The

corres-

ponding

values

obtained

for

dry

density of

the

material

ranged

between

2.16 to 2.40

gmlcc

against the design

value

of

1.92

gm/cc.

This

indicated that the

material

was being

over

compacted,

thereby decreasing

the

permeability

of the

material.

The

specified criteria

for

compaction

of the

filter

material

was

revised

to

70

per-

cent

and

this was

followed up to

the end

of

1968,

when

it

was revealed

that

the

relative

density tests

perfor-

med

yielded

inconsistent results.

In some

of

the

tests

for

the

same fill,

density was

of

the order

of 2.16

gm/cc.

The

relative

density,

varied

from

32 to

86

percent.

2t6

fhe

.compaction

criterion

was

modified

from

rerative

l.::ril^"l

/u

percent

to

a

minimum

fill

dry

density

of

z

gmlcc.

Pervious

Shell

Zone

(2b)

Preceugxr

MorsruRp

CoNreNr

coverages

required

to

attain

the

requisite

relative

density

varied

between

6

and

22.lt

w'as

thus

establi-

shed

that

the

addition

of

water

did

not appreciably

affect the

density

results.

As

such,

the

addition

oi

water

on

pervious

fill rvas

abandoned.

CoupacrroN

With

the

dry

compaction

of fill

material,

the

specified density

results

were

achieved

with

four

passes

of

a

S-tonne

compactor.

The

number

of coverages

was

two, when

a l0-tonne

compactor

was

deployed.

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Indian National Committee on Large Dams. Design and Construction Features of Selected Dams in India

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