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

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______--,/

Mon.

No.

of cables

on

No.

u/s side

No.

of cables

on

d/s side

TABLE

VIII.5

Position

of cables actually

installed.

8.15

Blocks

l8

and

24

were

half

overflow

and

half

8.17 The

stresses

developed

by the

prestressing

of the

blocks

across the

block

-

plan6

is

eitimated

td be

at

5.

R.

L.

!,.06-0-

upstream

fiber

20 T

I

m2

and downstream

fiber

I T/m2

8.!q The

problems

met with

in

drillin,e,preparing

cables,

homing

them

and

other

constructionai

dimcut-

ties

are details

beyond

the

scope

of

this

paper.

8.19

To

relieve

any

locked

upwater in

cracks,

fresh

drainagg

holes were

drilled

from

the top of

dam

into

the.

galleries

or

between

the

galleries-.

Length

of

drainage

holes

drilled

is

about

3593 m of

d'0

mm

diameter.

8.20 2330

tremors

were

recorded

from 12 December

1967

to

31

March

1969.

One was

of magnitude

5.25 on

29

October

1968. With

a

maximum

recorded

accelera-

tion

of 157

cm/secz

the

structure

stood

it

well.

The

lake level

was

645 m

(2714.50

fr).

8.21

The

general

tendency

noticed

was

that the

epicentres seemed

to

migrate

away from the dam

towards SSW direction.

Total

Cable

Force

tons

256s

2700

1890

2430

2700

Total

30

79

1

7685

to

be

not

less

than

3

percent

on

a

5

cm

gauge

length.

8.14

The

holes drilled were 90

mm

and

120

mm

diameter

according

as the

number

of

wires

per

cable

were

32

ot

64 and

to

avoid

concentration

of

stresses

in

one

zone

the anchoring

regions

of the

cables

were

staggered

in

each

block

by varying

drilling depths

from

472

to 577 m

(157

to

192

ft).

The

anchored

bottoms

are

not all at one

level

in

any

one block,

but

staggered.

Total lengths of

holes

drilled are

2750

m

of

115

to

120 mm

diameters

and

l4ls

m

of

20

mm

diameter.

64 Wires

32 Wires

carrying

270T.

t35 T.

64

Wires

32

Wires

carrying carrying

270

T.

135 T.

t8

l0

10

7

9

l0

l5

9

10

5

J

4

)

I

1

5

4

'l

)

_5

15

16

t7

I8

24

25

26

l9

9-

l There

was

little

choice

left except

to buttress

the

non-overflow

blocks

of the

dam

with

an

-ample

back-

ing of

concrete.

The

properties

of the

concrete

used

for

buttressing, has to

be

compatible

with

the rubble

concrete

in

the elastic

properties.

9.2

For the

purposes

of designs,

the

blocks

are

divi-

ded broadly

intb three

categories

(f)

the

end

(extension)

masonry blocks,

(ii)

the

shallow

heiglt

blocks

on the

flanks

rinicn did

irot

crack

in

1967 December

earth-

quake;

and

(1fi)

the

central

non-overflow

blocks

which

168

Seetion

IX-Inng

-Terrn

Strengthaning

(on

deep

and

deeper

foundations),

manifested cracks

in tbe

December-1967

earthquake.

9.3

The very

flank

blocks

1-B to

l-J

(Right

Bank)

and

blocks

40

to 43

(on

Left

Bank) were

not

streng-

thened

because

they are not

only

very

shallow

in height

but

also

were

not cracked.

and are more or

less

buried

blocks.

9.4

The middle

flank

blocks

(non-overflow)

l-A

to 9

on

Right

Bank and

35-39

on Left

Bank

and

full over-

|n

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r-3-

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D/S

face

of

exisiing dom

Dowel

bor

K.R L.2028.09

30-28

.2026.50

u

R.L

2O?5.OO

r

key

Zx^l

K.R.L.2030.OO

R.L.202t.25

/rK.R.L.202O.OO

R.L 20

t5

o

heor

key-

'#,

I

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Dowel

bor

oI 5 C/C

K.R

L.zO?l'Ogr_

g50

BUTTRESS

.L.2(.j-U.25

.R.L

20rO.CC

Sheor

key

*

\t,

Dowel

bor

ot

?4

Gop--l

Sheor'key

Dowel

bor ot

K.R.L.20t3.O9

K.R.1.20

flow

blocks

which

have

not cracked were

to

be

checked

with

a

design

seismic

coefficient 0.12

(g)

upstream to

downstream

with

reservoir

full

and

0.06

(g)

seismic

co-

efficient

from

downstream to upstream

with

reservoir

at

El. 2000.

There

should not be

any

tension,

and

if

any

it

should

be

insignificant.

9.5

The

deep and deeper

non-overflow

blocks 10

to

18

on

right

bank

and'24

to

29

on left

bank which

have

cracked should

be strengthened

to have

no

tension

with

design

seismic

coefficient

0.20

(g)

from

upstream

to

170

FIGURE 9.2

: Typical

cross-sectiorr

showing details

of shearkeys

and closing

gap.

l*_z'-g*4

R 99.97

_l

tor

3.00.

9.6

The

only

other

rigid

concrete dam

in

which

PHoro

e.r :

Tfl#""#$;,-:,1:f.r.nening

bv

buttressing

higher

acceleration

than

at

Koyla

were

recorded

in the

Ficoima

concrete

arch

dam

in California,

LI.S.A.

in

February

l97l

(1.0

g

was recorded).

9.7

For

densities

to

be

similar

(of

old

new

concrete)

maximum size

of aggregate

used was 150

mm. Uplift

forces

were

to be

assumed

as full at heel,

reduced

by

50

percent

or

66

percent

of the difference

between FRL

antl

tail-water level at the

drainage

gallery and

ttren

linearly

reduced to tail-water depth

on

foundations

at

the toe. As

actual

uplift

n'leasurements

were

even

very

much

less

downstream of

the

fouidation

and

drain-

age

gallery,

a

66

percent

reduction was finally

adopted

at the

gallery

position.

9.8

Accordin,ely the

following

typical sections

of

strengthening

were

evolved,

Figure

9.1.

9.9

Complete

backing of the block

was

done upto

El. 2,000, in the

flank

blccks,

and up

to

El.

l97O in the

deep

and deeper

blocks.

9.10

Cement

content was

about

177

kg/mt,

i.e., about

the

same as

in rubble concrdte

using

270kglcu

m

(450

lb/cu

yd)

matrix,

with 37

percent

rubble

sunk,

no

precooling of

aggregates

was

resorted to.

22

percent

of

the

mixing

water was

ice.

9.ll

To

enable

monolithic

bond between

old and

new

work, a

1.2 m

(4

ft)

gap

was left

between

the

new

work

and

old

work,

to be

filled

in after

new

work has cooled

down

(almost

wholly)

and

when the

lake is at

the

low

level in

summer.

In addition

to normal

preparation

of

surfaces

for

good

bond,

shear

keys were

formed

in

the

PHOTO

9.2

:

Koyna

Dam-strengthening

by buttressing

after completion'

t7l

old

concrete

and

new

concrete

[on

either

face

of the

1.2

m

(4

ft)

gapl

at

3.0

m

(10

f0

vertical

intervals

but

staggered

as in

Figure 9.2.

9.12

The

strengthening

work

was

taken

in

hand

in

September

1969

and

completed before

monsoon

1972.

10.1

Density

of

Tremors

and

Amount

of

Energy

Release

The

density

of tremors,

amount

of

energy

released

are illustrated

in

Figure 10.1.

The

tremors

have been

classified

according

to

magnitude

below

2,2 to

3 and

3 to 4

and above 4.

The

annual record

of shocks

above

magnitude

3

and

above

are as

under

(from

C.W.P.R.S.)

volume

of

concrete

involved

was

about

2,50,000

mB

(about

8.7

M

cuft).

Photos

9.r

and9.2

"t"'or

the

strengthening

work.

9-

13

The

lake

was

thereafter

allowed

to be filled

up

to

FRL

(top

of

spillway

gates).

153

58

34

55

46

Similarly

these

above

4 magnitude

were

1968

1969

1970

r97l

t972

1968

t969

1970

1973

r974

1975

t976

r977

t97l

r972

30

45

44

30

2

t0

5

n

8

t2

1973

5

as

earlier reported

by C.W.P.R.S.

The

statistical

laws

governing

the fortnightly num-

ber

of foreshocks and

aftershocks

are

8.81

(0.0053)-t

and

1342t-o'?7

respectively

where

't'

is

in

fortnights

starting

from

the

first

fortnight

of

December

1967 for

both

foreshocks

and aftershocks

sequences.

"Similarly

the fortnightly

energy release, of fore-

shocks

and aftershocks

are

respectively

given

by

3.188

(0.OOS)-'

and 1656

1-1'05 in units

of

101?

ergs-

The

formula Log

N:a-bM

(where

N is the

num-

ber

of earthquake

of magnitude

M

and

where

a

and

b

arc determined using

the

least

square

method)

was

used

to study the

b values

and

whether

the

b

values

'

could

lead

to

a

better

understanding

and anticipation

of

the

probability

of occurrence of large

earthquake.

172

Koyna

Dam

(Top

of monolith

12)

263

Koyna

Dam

(I-B

Gallery)

104

Koyna

Dam

(Downstream)

on

rocky

95

ground

Pophali

(Underground

Power

House)

48

EW

330 130

205

100

147 68

Alore

52

NS 18

r84

13

The

magnificati-on

from

the

ground

to

top

of

dam

is

graphically

presented

in

Figure

10.2.

By 1973 the buttressing

work was

completed

and

was

one

year

old.

The

lake

was at Et.2158.50.

No new

cracks

were

noticed

even

after

a detailed inspection.

35

percent of

transverse acceleration

is

quite

heavy

(much

more than the

September

1967 recorded acceleration).

55

Seetion x- Post-Strengthening

Experienees

A-bove

magnitude

5

_only

two

shocks

were

experi-

enced-one

on

29

October

-1968

and

the

second on

tZ

October

1973.

The

maximum

accelerations

recorded

on

l7

october

1973

are

as under (Table

X.l).

TABLE

X.l

shows

the

maximum

accelerations

at

different

locations

in

Kovna

Dam

and

"i;"ffJ,r"f#"r.r-rl:'Koyna

earthquake of

Seismological

Observatory

Maximum

accelerations

cm/secz

Longitudinal

Transverse

Vertical

IN FLOW

HYOROGRI

I.

I

r9

to

2too

eooo

r7

lo

t?

lo

rl

to

t6

to

l4

to

lrl

vr

2'O.

LAI(E

LEVEL

)ROGRAPH

WEEKLY

ENERGY

RELEASE

3}:

gARTHGUAi{ES

OF

VARIOUS

MAGNITUOE

GR(

3'O<

M

<

2.O<-;

<

5111

.asuala

900(

-

l-

300c

I

JDE

OROUPS

tun33c

t{

to

lrl

rZ

lo

I

WEEKLY

I

FR

EOUENCY

OF

OCCURRENCE

foo

to

I

z

o

lrf

t

f

o

t

F

G

lrl

IL

o

.c

Irf

D

I

3

z

loo

to

I

too

to

I

r965

3.O<

M

12

FIGUR"E

l0.l

:

Group-wise weckly eoerglr

releasc,

frcquancy

of

occru

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

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