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

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I ON

DETAIL-,X'

BHAXRA

DAM

CENTRAL OVERF

LOW SPILLWAY

MAXIMUM

SECTION

FIGURE

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PowER

PLANT

ELEVAT

IONS

FIGURE

7

(bI

El.

390.1 m

(1,280

ft)

pectively,

allowing

a

both.

The

probability

m/sec

(275,000

cusecs)

thirty

years.

and El.

367.3

m

(1,205

ft)

res-

free-board

of 2.7

m

(9

ft)

for

the

1947 flood

of

7,787.4

cu

worked

out

to about

once

in

The

Bhakra engineers

also recommended to the

Board of

Consultants

that

no

chances should

be

taken and a

solid

protection

of 101.6 mm

(a

in.)

thick

reinforced

concrete slab, well

anchored into the

rockfill, be

provided

on the top

of the

downstream

slope

of

the

upstream

coffer

dam and similarly

on

the top and upstream

slope

of

the downstream

coffer dam.

This

was thought

to be a safeguard against

a

spillover which

may take

place,

in case

a

higher

flood than

the 1947 type occurred

in

future.

This

proposal

was

also approved

by the

Board.

Later in early 1955, it

was

realised

that

it might

not

be

possible

to

provide

concrete

protections

on

slopes

against

overtopping in time and it

might

be

quicker

and

cheaper to increase

the

height

of

the

coffer

dams so Ers

to exclude

any eventuality

of overtopping

consequently,

it

was approved by the Board

of

Consultants

to raise

the height

of

the

coffer

dams

to

absorb a

flood

of

11,043

cu

m/sec(390,000cusecs)

(highest

ever

recorded)

which

occurred in

the

year

1875. No discharge

data

for

this

flood

was

available

but only its

peak

was known from flood marks,

etc.

For

routing

purposes

this flood

was

assumed

to follow

the

1914 flood

pattern

(60

hours

duration

the

maximum

on

record).

The routing

of

this

worst

possible

hypo-

thetical flood resulted in a

maximum

pond

level of

El. 394.4 m

(1,294

ft)

with

the two tunnels

discharging

6,513.1 cu m/sec

(230,000

cusecs)

at

the

peak,

resulting

in

a

tail-water

elevation

of 365.5 m

(1,199

f0.

This

hypothetical flood

had a

probable

frequency of 1 in

6,000

years

and

thus

designing the coffer

dams to

this

was

thought to

be absolutely

safe

and

free

from any

chances

of

a spillover.

The

coffer

dams

were located

slightly skew

with

the baseline

(Sta.

13+00) in

order to

have

their

abutments in

good

rock and

keyed

well

into

the

protruding

rock ridges,

and

thus

also to have

the

shortest

spans. The

bottom

cutoff trenches were also

Iqated

along

the claystone

band as far

as

possible,

so

as

to

reduce

grouting

operations

and to

have

a

more

tight

cutoff

(claystone

at

Bhakra is much more

impervious than

sandstone).

Design

of

Section

The

coffer

dams

being'temporary embankments

were

built

completely

out of

local materials.

While

selecting the

materials, economy,

stability and

pro-

tection

against

percolation \\'ere

the

main

considera-

tions.

As

such they

were

built as

composite earth

and

rockfilled dams.

Typical

sections

of these

are

shown

in Figure

6@)

A.6(b).

Heavy

rock,

sand,

shingle,

clay and

loam

used

for

the

construction

of

dams

were

all

available

in

very

good

natural

gradation

and

in abundance

within

i

short

distance

of the

site. In

the middle

of

the

section

an

impervious

clay

core was

provided.

The

core

was

rolled

in layers

of

152.4

mm

(6

in.)

and was

compacted

to a

dense mass

at

optimum

moisture.

To

reduce

the seepage

through the

UoOV

of the

coffer

dam,

the impervious

claycore

provided

in the

middle

of

the section

was taken

down

to the

sound

rock and

keyed

to

a

depth of

about 3.05

m

(10

ft).

The base

trench

in sandstone and

the

right

abutment

were

also

grouted

to

a

depth

9.

t m

(30

ft) all

along

to

coffer

dam was

made. To

collect the seepage

oilen

drains filled

in

with

free

draining rock

and

sumps

at

suitable

points

for

pumping

were

provided

along the

inner

toes of

the

two coffer

dams. Actually during

the time

coffer dams were

in service

practically

no

seepage was

observed and

the

working

area

remained

fairly

dry.

2.4

Design

of the

Dam

The

type

of

structure

adopted

.

was

slrai-eht

gravity

dam.

Multiple

arch dam and other

forms of concrete

structure

were

not

considered suitable

due

to

the

hydraulic height of

about

155.4

m

(510

ft), the

nature

of

the

foundations and the vulnerability of the

area

to

earthquakes. Earth/rockfill

structure

was

ruled out

due

to

the

great

height

and considerations

of

availabi-

lity of material and

cost.

The

section of

the

dam adopted

consisted

of the

basic

triangularsection

with

its apex

at El.

518.2

m

abovemean sea

level

(1,700

ft)

with

a

downstream

slope of

0.80 horizontal

to

1.0 vertical

and

upstream

face vertical. However, due

to

proximity

of

upstream

clgystone

ban{, a fillet

with

0.35 horizontal

- to

1.0

vertical

slope

was

provided

on

the

upstream

face

below

El.

411.5 m

MSL

(1350

ft)

(Figure

7).

Stability Analysis

(1949

Studies)

Before

adopting

the

final

gravity

section

for

the

dam

in the 1949 designs, a

detailed

stability analysis

by the

trial load method of

analysis

was

carried

out.

t7

The'main

conclusions

from.

the

trial

load

1949

were

:

,i' :

,,

:'

.

limits

:,,

-'

l:'i

"

'

".

a

.,,

(t)

"$?l

ues

of

stresses

i

and

.

-sqbil

ity faqtors

.

gbta

ined

: ,

':nby

trial

lqad

arialysis"

showed considerable

difference

fromgravity

analysis

r.qqults. ,

i; I

(ii)

Comnressive stresses.in the

dam

appeared

to

.i'

'."6e

'witnin

allowablU iimits thrbtrlLout

the

'.,

t!

i- :

strdcture

:.

,.

.r_..

ir

).

'[

,,

-t

(;;i)

Tensile

principal

stri:sses'

6t-

and''

near the

abutments:in'the

upper

portiogsi

bf

rhe

dam

indicated that

some cracking'

ijf

the concrete

may occur

in these regions.

If'such

cracking

were

to

occur,

the beam

compressive

stresses

.

considerably.

The

resu{ting

compressive stresses

'

would, however,

proba,bly

be

within

allowable

(4)

Two-dimensional

mathematical,or

photo-

,

.elastic

stress studies

of

local

areas

in the

dam or in foundation;

particularly

.an

:

'r

:

investigaJion

of

strengthening

of

claystone

seams.

(5)

Studies

to determine the

effectiveness

of

.i

,

:'

twistslots. :

"

;;

19s2s1Y{es

:

.:

.',

such

studies

were

carried out

.

by the

United States

Bureau

of'Reclamation

in Denver, U,$,A,'

under

an

agreement

with Government of In"dia.' At

the- outset

it

was

felt that from a structural Ctand-point

it

would

result in considerable

saving

if

the

upstream

slope

of

0.35

horizontal

to

1.0

vertical

below

El.

411.5

m

(1,350

ft) MSL were

omitted.

The first

stability

analysis

by the

trial

load

twist

method

carried

out

in

U.S.A.

was

on

such a section

with

a

vertical

upstream

face.

However, in November 1952

it

was decided

that

the

slope of 0.35

horizontal to

1.0

vertical

below

El.

411.5

m

(1,350

ft) MSL

was necessary

from

considerations

other

than those of structural

requirements,

namely

the

proximity

of

the upstream

claystone

seam and

its

treatment.

It

was

then

decided

that

the

section

of

the

dam with an

upstream

slope

be retained

and

that

additional analysis be conducted

with

the

object

of

improving the

shear-friction

factors

of

safety

and

reducing the tensile beam

stresses, with

special

reference

to item

(3)

of 1949 study

mentioned

above,

i.e.,

the

possibility

of raising the

reservoir

water

surface

to

elevation

higher than

El.

411.5 m

(I,350

ft)

MSL

at the

time

of

grouting

the transverse

contraction

joints.

It

was

also

decided

that the

analysis

be

directed

towards

determining

the feasibility

of

eliminating

the

grouting

'of

transverse

contraction

joints

in

the dam.

The

programme

for the

additional trial

load

an-aly-

sis

was

finalised

and the following

studies

were

made

:

Study

2

-Transverre

joints

ungrouted,

earth-

3i;If"f?ilio'lj'il3*..''.'.*oiratEl.

Study

2A-Transverse

joints

ungrouted,

earthqu4g

effects

not

included,

reservoir

at

El-

512.1

m

(1,680

ft) MSL.

studv2u-r;:a::'"'ff"i?l$"'i:"iriii:i'#i"',"r"6

ft)

MSL and

held at that

elevation

during

the

period

of

grouting, earth-

"

quake

effects

included.

analysis of

(iv) The

occurrence

of the

low values

of

shear

friction

factors

of

safety

and

high

values

of

stresses

at the right abutment

near the

'hump'

in

the

profile

indicated a concentration

of

load

in this region.

While shear

friction

factors

at

this

location

were

slightly below

the

desired

minimum value,

the overall

value

for

the

right

side

of

the dam

appeared

to

be

satisfac-

torY.

(v)

The stresses

and stability

factors

at the

base

'

of the

maximum

section and

in

the

lower

portions

of

the

dam

were

very

consetva-

tive.

(vf)

The raising

of

the

reservoir

water

surface

to

El.

411.5 m

(1,350

ft)

MSL at

the

time of

;

grouting

the

contraction

joints

did

not

have

.

'

is

great

a

beneficial

effect

as

desired.

Values

of

ihear.

friction

factors

at

the

-

base

of the

spillway

section

indicated that

more load

could

'b;

cariied

effectively

by

vertical

cantilever

action

in this

region

of the

structure.

As

a

result

of

the

trial

load

analysis

and in

order

to

establish

a

basis

for

final

structural

adequacy

of the

monolithic

dam,

the following

additionil

investigations.

werd suggested

:

(

l) Trial

load analysis

for

normal

full

reser-

voir

without

earthquake

effects.

(2)

Study

of removal

of

excessheat

and

the

effect

of

the

sub-cooling

of the

mass

con-

crete

on stresses

and

stability

factors.

(3)

Trial

load

analysis

with the

reservoir

water surface

elevation

at time

of

grout-

ing

contraction

joints

15.24

m

(50

ft)-

1o

"

30.5

m

(100

ft)

-higher

than

assumed

in

1949

study.

l8

r

.--

''"

'

The main design

criteria

and,-basic

data for, these

trial

load

analysis

were

:

De.sign

lrlteria,

(i)

Top

of dam El.

,',:i1

,

t*:

518.2

m

(1,700

ft) MSl,.

(tt)

Base

of

maximum

section,

El. 320.00

m

(1,050

ft)

MSL.

'(iri)

Normal

reservoir

water surface, El.

512.1

m

(1,680

ft) MSL.

('!

'

':

(fu)

Minimum

tail-water

surfaceo,

El.

355.1

m

:.:

(1,165

ft) MSI-.

:

(v)'sustained

modulud

of elasticity

of concrete

in

,

{

, tensig,n

aqd gompression

175,732

kg/cmz

(2,500,000 lblsq

in.).*

-j

(ui)

Sustained modulus

of

elasticity'bf

foundation

abutment

rock

175,732 kg/cmz

(2,500,000

lbisq

in.).

(vif)

Coefficient of

internal friction

for

concrete

or

rock, 0.80

(shear

friction

factors

have also

been computed

for' coefficient

of

internal

friction

values

of

O.75 and

0.65).

(viii)

Unit

shear

resistance'

fior concrete

on

concrete

or concrete

on

rock,

35.15 kg/cm2

(500

lb/sq

in.).

(fx)

Poisson's

ratio

for

concrete

and

abutment

rock 0.20.

(x)

Unit

weight

cu

ft).

(xi)

Unit weight

cu ft).

of

concrete,2466

kg/mt

(154

lb/

of

water

1,001

kg/mt

(62.51b1

(xfi)

Total weight of

four

spillway

radial

gates,

816,300

kg

(1,800,000

lb).

(xiii)

Total

weight

of

spillway bridge 969.148

kg

(2,t37,040

lb).

(xfv)

Maximum

horizontal earthquake

was

assumed

to have an acceleration of

0.15

of

gravity,

a

period

of

vibration of

1.0 second and a

direction

of vibration

normal

to

the axis

of the

dam.

(xv)

Maximum

vertical

earthquake

was

assumed

to

have

an

acceleration of

0.075 of

gravity

acting upward

and

a

period

bf

vibration of 1.0

second.

,

i

(xvf)

Inciease

in loads,

if

any,

on

the

lower

portion

of

the sloping

upstream face

of the dam

due

to

rock

protection

of claystone seam

or

due

to

the accumulation on

sand,

g{a.vel

or silt

was

neglected

in

the

analysis.

Basic

Assumptions

'(i)

Effects

of

galleries

on

deflections

and

stress

load

studies.

.

,

(tt)

Etrects on stresses

due

io the

Columnar

method

'

of construction

were included

in

the

trial

load

.

.gtudies

4s

foflows-:

,-

.

_

Lohgitudinal

contraction

joints

were

assumed

''(200

ft)

abbve the base

of the lift

being

'.

'

grouted.

-

It

wAjg,.slsumed

that vertical

shears

would

not

be

transferred

across

,the

longitudinal

joints

until after

they

were

grouted.

In other

words,

the

dead

load

stress due

to each

61 m

(200

f0

of

concrete

would

be carried

to

the

base

of the

blocks

individually

instead

of

being distributed

linear.'ly

across

the

entire

section.

All subse-

quent

loads

due to

the

weight

of concrete

were

assumed to

add

linear

increments

of stress in

the

grouted portion

of

the dam.

(fii)

Except

as

noted

in

Basic

assumptions

G't)

above,

normai

cantilever

stresses

on

horizontal

planes

were

assumed to

have

a

linear

variation

from the upstream

face

to

the

downstream

face

at all elevations.

.(fv)

Horizontal.

beam

stresses

(if

the transverse

contraction

joints

are

grouted)

normal

to

vertical

transverse

planes,

have a

linear varia-

tion

from

tbe

upstream

face

to

the downstream

face of the

dam

at

all

elevations.

(v)

For

purpose

of making

the trial

load

adjust-

ments, both

beam and cantilever elements

were

assumed

to be

capable

of carrying

loads by

tensioa without

cracking

the concrete.

(v;)

Uplift

pressure

at the base or

horizontal

sections above

the

base of

the

cantilevers

to

be

computed

according

to the

following

criteria:

'

(l)

Two-thirds

Area

Upltft

Criterion

:

The

uplift

pressure

varies

from

full

reservoir

water pressure

at

the upstream

face' to

zero

or

tail-water

pressure

at

the

down-

stream

face and

acls

over

two-thirds

of

the

area.

(2) New Uplrft

Criterion:

Uplift

pressure

has

an intensity

at

the line

of

drains that

exceeds

the

tail-water

pressrue

by one-

third of

the

differential

head between

head-water

and

tail-water

head, the

gradient

then extending

to

head-water

and

tail-water

respectively in straight

lines.

If

there

is no

tail-water,

then

the

downstream

19

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

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