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

4.2

Structural

Design

of

Dams

(I)

MesoNRy

Dau

.

Stability

Analysrs:

Th

IilJJ'

#:'!ffi.:i.#

l;';f;

zero

at

the

downsrream

,il

jf?n33'lr?5:

?:t',ro

#1

the

full

base

area

was

considered.

No

seismi"

Tor..t

were

considered

as

the area

was

not

covered

by

the

seismic

belt.

As

the

reservoir

has

got

u

turg"

-

dead

storage,

of

about-82.30

m

(270

ft)

height,

silt

pressure

up

to

the

sill

of

the

riveroutlets,

ile.,-up

to

about

56-39

m

(185

ft)

above

the

river

bed

levil

ivas

conside-

red

in

the

design.

No

tension was

aflowed

in

the

section

under

uotrr

full

reservoir

and

reservoir

empty

conditions.

However,

under

the

maximum

water-

Ievel

conditions,

a tension

up

to 0.35

fcg/

sq

cm

(S p.i.i.)

was

allowed.

stability

analysis

revealeZ

thut

th.

-criti-

cal

section

would

be-at

the

floor

level

of the

drainage

g?-llely a-nd

hence.

the

starting

elevation

of

the oprtr.u"rr

tillet

had

a

certain

relationihip

with

..rp..t'

to the

location

of

the

gallery,

especii.lly

under

ih-.-.a*itrum

water-level

conditions.

A-maximum

value

of

0.7

for

the

coefficient

of

friction

and

a

minimum

value

of

5

for

the

shear

friction

factors

were

considered

in

the

design

with

a shear

resistance

of 28.12

kg/sq

iro

(400

p.s-i.)

for

masonry.

-

Assumptions

made

in

the

Analysis

: The various

lorces

considered

in

the

stability

analysis

are

(t)

self-weight

of dam

excluding

the

weight

of

equipment

and

other

fixtures

as

well

is

live

load

on

the

roadway

on top of

dam,

(ii)

vertical

weight

of water

on

the inclined

portion

of

the upstream

face

of dam,

(iii)

horizontal

water

pressure

on

the

upstream

face

of

dam,

(iv)

silt. pressure

on

the upstream

face up

to the

sluice

level

of

*

l34.ll

m

(+440

ft),

and

(v)

uplift

pressure

rvith

full

head

at

the

upsrream

heel

reducing

to 50

percent

at

the

drainage

hole

and

reducing

to zero

at

the

downstream

toe

of tbe

dam

section.

Tail

water

effect

was

not

considered

as

it

rvould give

sater

results

and

seismic

forces

also

were

not conslde-

red

as

the

dam

was

not

located

in

the seismic

belt.

The

rveights

of

nraterials

considered

are

:

(f)

random

rubble

masonrv

24OO

ks/*'

(150

lb/cu

ft),

86

(ii)

cernent

concrete

(fii)

rvater

(iu)

submerged

silt

The

safe

values

and

analysis

are

:

at

toe

:240A

kg/mt (150

lb/cu

fr).

:100-0

kg/mt (62.5lb/cu

ft),

and

:900

kg/m'

(56

lb/cu

fr).

coefficients

considered

in

the

(t)

allowable

compression

in

masonry:32.g1

kg/

cmz

(30

tons/sq

ft),

(ii)

allowable

compression

in concrete:43.75

kgl

cms (40

tons/sq

ft),

(iif)

allowable

tension

in

masonry

and

concrete:

zero,

(l'u)

shearing.

strength

of

rl?sonr!:2g.12

kg/cm2

(400

p.s.i.),

(v)

coefficient

of

static

friction

of masonry:0.7,

(vi)

angle

of

repose

of

submer_eed

silt:30o,

(uii)

maximum

allowable

sliding

factor:0.75

to

0.g,

and

(viii)

Minimum

value

of

shear

friction

factor:4.5

to

5.0.

Procedure

Followed

srep

I : The

horizontal,

vertical

and

shear

stresses

at

.any^

point

along

any

horizontal

plane

section

in the

body

of

the dam

were

calculated

utiiising

the

following

equations.

(i)

Vertical

stress,

i.e.,

normal

stress

on

horizontal

plane,

or--alby,

(if)

shear

stress,

rza:ar{bry}-cry2,

and

(iii)

horizontal

stress,

i.e.,

normal

stress

on vertical

plane-

,

6,,:oz*bzy

*tczy,

+*dryt,

rvlrere,

y

is

the horizontal

distance

up

to

the

point

under

consideration

from

the

downstream

ena

of

the

plane

section.

The

dam

profile

rvas

divided

into

a n*tru.t

of

horizontal planes

according

to convenience

and the

equations

for

62,6!t

and

azs

are

obtained

for

each

plane.

Srep 2 : The

constants

were

evaluated

from

the

following

equations

:

where,

6zD

analysis.

and 6zu

were obtained

from stability

fve

sign was

taken,

otherwise

-ve

sign.

Alternative

sign would give

the

value

of

crr, which

is

perpendicular

to

oor.

l_

,or:itan-r

f, =.

=""

I

t

l(

""

="" \l

(\

2

))

In

effect,

the

vertical

stress

on

a

horizontal plane

P7,

wofks

out

to

b:base

rvidth

of

dam

section,

and

e:eccentricity

of

the

resultant

force, and

the

principal

stress,

p,

at

the end

will

be

P:pv

sec2

{

where,

S:angle

of

inclination

of

the

face

of dam

to

the

vertical.

And

sliding

factor

:-W+

,

and

shear

friction

factor:

(w-

U)

tan

6+,qc

H

where,

l:base

area of

the

dam

section.

C:Cohesion,

and

H:horizontal

thrust.

dr:a

zltD,

,

ll->,v

Dt:

--Fl

o

z,

r\L

^

| (. Ev

-^- r

c1:@

(

o

=-f

Jazyu

t

a2uD:6zD

tan

$o,

and

ruzr:-(a"u-p)

tan

$"

*2t

"r"

*

Or

rrr),

3r,oo

).

whEre

Qe:6'aD

In

evaluating

the other

constants,

i.e., br,

cr,

and

dr,

the

following

assumptions were

made.

'c

zlr

Pv:!;L(-,+e)

where,

W:vertical

weight

of

dam,

U:uplift

head,

rvhich

at

the

ends-

+('*)xtan

S

Differentiating

the equation,

o

r:

ct

2*

b

zl

*

c

z!2

*

idu!"

dt

\

f

l

61t

):b'+"!*dil2

Equating (l)

and

(2)

d(

\

,tr

(

'",

)tan

$:6'*czY]-dr!2

. ..

(1)

...(2)

...(3)

...(4)

Putting

./:0

in

the

Equation

(4)

for

downstream

end and

differentiating

the

actual

equation of

r,o

already

obtained.

bz

was

evaluated.

The

other

two

constants,

cr.and

dr, were

obtained

by

solving Equa-

tions

(1)

and

(4)

simultaneously

rvith

the upstreim

end

value

of horizontal

stress,

i.e.,

oo,,

6!p:r.gD

tan

$o*p"

6.tu:Pn-Truu

tan

$u

vertical

planes

were

calculated

by

putting

the

values of

!', !2,

yrin

the respective

equations.

Srep

4

: The

principal

stress

and

their inclinations

were

then

obtained

for

each intersection point

with

the

following

assumptions.

orl:@--f...'.=l.'

2

f\i\

z

/*\'",/

In this

equation,

if

c"-or)0,

level.

The

front

batter

is I

in

20

berween

EI. i7.72

and,

166.29

m

(255.00

and

545.57

fi).

The

fronr

face is

vertical

above

El.

166.29

m

$45.57

ft)

and the

toe

below

El.

77.72

m (255.00

ft)

is

given

a batter

of.

I in

2. The

base

rvidth

of

the

section at

El.

64.00 m

(210.00

ft)

works

out

to

98.92

m

(324.53

ft). Similarlr..

the

maximum

spilhvay

section

analysed

'is

96.32

i

(316

ft)

high

between

the

lorvest

general

foundation

level

of

+70.10

m

(+230.00

ft)

and the crest

level

of

+.166.42

m

(+546.00

ft).

It

is

given

a

fronr

batter of

I in

20

above

El.

77.72

m

(255.0b

ftl. The

face

barter

of

the front

toe

below

E\.77.72

m

(255.00

ft)

is t

jn2

-'1rt-F

'ul

87

-{nHo

Top

of

rood

EL.+

t84:4om

L.l70-08m

Mox.W-L.

+l8t.O5m

F.R.L.

+

r;9€J4_.

8'53nr-

of mosonry

EL.+r66.2q

m

Fe.3ol

m

I

EL

EL.+ 174.29m

EL.

+

rSq.aom

m

.ta6.30

m

EL.+

t67,64

rn

{ I.pl

-T.P_E

L,

t61.74

m

EL.

r56.97m

8L.94'49

m

L.128'02m

-

tr8.26m

l12

.78

m

excovotion

TO|Om

eLi77

m

respectively.

Stress

analyses were

also

done

for the

construction

stages

with the

spillway and

non-overflow

blocks

raised

anA

kept at

different

llvels

and the

floods

passing

over

them

and

it

was found

that the

sections

were quite

adequate

for

all

possible

construction

88

EL.77.72m

lvloximum

spillwoy section

Scole'

?

'9

qo

-?

Moximum

non-orv€rflow

section

FIGURE 5:

Maximum

sections

of spitlway

and non-oyerflow

brocks.

d-ownstream

glacis

was

found

to

be

quite

suitable

for

all stages

of

construction

in

ensuring

the

overflowing

lSppe

to

cling

to the

downstream glacis

for a[

discharges,

and

provided.

Masonryfor

the

Dam:

The

entire dam

section,

excluding

the

spillway

crest,

glacis

and

bucket and

the areas

around

the

galleries,

sluices

and

other

Denver,

U.S.B.R.,

to determine

the

suitability

of

masonry

for

construcrion

of the

dam. A

minimum

every

care

was

taken

to

achieve

high

quality

of

work

by

-

carefully

filling

in

the

joints

in

between the stones

with

mortar

and

ensuring- good

bond

between

stone

and mortar.

For

this

purpbse,

an

independent

quality

bss

dlAeroge

lyeor

slrcnglh

of

mclor

h kglsq

cm

i*i.

f*o"orlsqfr.l

h.

If75r160T/sqft.l

tAl

l5l

t

l2O

T 7sqft.

I

t8l

75

ig

{-Gollery

r52X229

m

.::t---Ie

l. toa.sam

OZ:

F.-(/

H-5.O.29

m----+i

I

|

(..'.-.ru.P

gt?#

I

n.g-Eulo.zb#

.{

(AD"'

I

lf

rAsrrmed

tin€

"f

.r.""".fi"Yrcj

t-Srilt*ov

concrele

l'83

m

,\

-

1

admixture

of

surlfr.i,

the

mortar was

called

red

cement

mortar.

Air-entraining

agent

also

was

used

in

all

cement

mortar

and

cement

concrete mixes

to improve

section, as

indicated

below,

depending

on the

magni-

tude

of the

stresses

developed

in those

regions.

(i)

A,

type,

richer

mix of

I

: 3 by

weight,

in the

highly

stressed

regions in the downstream

toe

portion

where

the

stresses developed

are

more

lhan

21.87 kg/cm2

(20

tons/sq

ft)

as

well

as

in

the

front

2.7 m

(9

ft) thick

layer

for

imperviousness,

(tt) A tJpe,

rich

mix of I : 3.91-

by-weiglt,

in

the

regions

where

the stresses

developed

are

morc

than

16.40

kg/cm2

(15

tons/sq

ft)

and

less

than

21.87

kg/cmz

(20

tons

sq ft), and

(ttt) B type,

lean

mix of 1

:

4-71by

weiglt,

in

the

regions

where

the

stresses -developed

are

less

thin

16.40

kg/cm2

(15

tons/sq ft).

masonry.

-.-et/+d

*

89

Axis

of Oom--

-

- ln.'r^

Y1r-

EL.t82'9m

(EL.

+ 6oooori

EL.t8l

m'

r

EL

+

594'OOl

M.W.L.

l'22

m lhigk rip-rop

U/S

Rock-fitl

Conol

excovoted

moleriol

Grouf

hotes 9'lm

deep

ot

6'lm

C/C

5

O 5

l5m

|'-J--J-J

E

L.18l

rnox.

W.

(L

trpe."ious

R

ight eo

rth

dom

eu.oOo'o1

|

U/S

Fock

ftlt

ercovoled moleriol ?o'7m

3

m( ldt

fltler

-Turfino

EL.t75'26

m

t-

3m

*

3

EL.I

l'2m

dumpad

rip-roPover

-

O.76m

f

ilter

3m

EL.16ZOm

r

I

2

ffl*f-

r'2m+

ho|es

94m

deGP

6'l

m

C,/C

Left eorth

dom

FIGURE

6 : Maximum

sections

of

earth

dam.

3.,

gp

:i'--Grour

ol

masonry

from

the

front

face

of

dam

and

collect

in

the dra-ins

provided

in

the

three

drainage galleries.

In

addition,

the

foundation gallery

served

io carry

ou.t..high_pressure

curtain

grouting

and subsequent

drilling

of drainage

holes.

This

gallery

also

serves

to

measure

uplift

-pressures

and

read

the

other

measuring

instruments

embedded

in

the

dam.

The

toe gallery

was

originally,

intended

to

carry

out contact

{routing

at

the

junction

of concrete

toe

originally

provided

ind

the masogy.

-

By

switching

over

to

-masonry

in

this

toe

portion

also, this

toe

gallery

now

serves

for

the

location

of

the

terminal

boxes

of

the

instruments

embedded

in

this

porrion

of

the

dam.

All

these

galleries

serve

for

inspection

purposes

as

well and

they

are interconnected

by

means

of

shafts

and

suitable

adits.

In

addition,

two

elevator

towers

are

provided

at

either end

of

the spillway

dam,

connectin!

ali these

galleries

to

the

top

of

dam, with

electric-al

lifts

and

winding

staircases=

inside.

Enfiedded

Instruments

:

Resistance

thermometers

to

measure

the temperature

rise

in

masonry,

stress

and

strain metres to

record

the

stresses

in

ihe

dam

and

joint

metres.to

measure

the

extent

of

openings

at the

contraction

joints_and

other

places

and-

pore

pressure

cells

were

embedded

in

the

body

of

ihe dim with

E

L.

t58'5m

Rock-fill

--Axis

of

dom

r

19'l2nt

try

their

terminals

led

into

terminal

boxes located

in

the

galleries.

Plumb

bob

wells

were

provided

in

two

blocks

of

the

dam,

one on each

flank,

to

measure

the

deflection

or

tilt of the

dam.

Uplift

pressure

pipes

were

installed

in

the

foundation

gallery

to measure

the

uplift

forces

on the dam.

Piezometers

were also

embedded

to

measure

the

pressures

on

the

spillway

glacis

and

sides of river

training

walls.

All told, 129

numbers

of instruments were

embedded in

the

masonrv

dam-14 stress

meters,

28

strain

meters,

14

joini

meters,

63 thermometers

and

10

pore pressure

cells.

(2)

En

nu{Deus

(Frcunn

6)

Left

Earth

Dam: The left

earth dam

is a zoned

embankment

with central

impervious

core covered

with

semi-pervious

earthfill

on

either

side.

There

are

three

hills,

with

two saddles

between,

on the

left

flank

of

thc

dam.

Earth

dam

was

constructed in

these two

saddles

and in continuation of

the

masonrv dam

towards

the

flanking

hill. The lowest

foundation

level

of

this

eafth dam

is

at EI.

158.50 m

(520

ft)

and

the

top of

dam is

kept at

El. 186.00

m

(610.00

ft),

1.52 m

(5

ft)

higher

than the top

of

masonry

dam. The

maximum

height of this

earth dam is

27.4 m

(90

ft).

The

top width of

dam

is

kept at 9.30

m

(30

ft

6

in.).

The

front slope is

kept at

3

:

I and rear slope

at

2

:

1.

Trvo

berms

of 5.49

m

(15

ft)

width

each are

provided

on the

rear slope, one at

El.

175.30 m

(575.00

ft)

and

the

otherat

El. 166.10

m

,(545.00

ft).

Rock

toes

are

provided

both

for upstream

and downstream

slopes.

The

upstream slope

is

protected

with

1.22

m

(4

f0

thick

dumped riprap

provided

over

0.76

m

(2

ft

6 in.)

thick

f.lter,

while

turfing

is

provided

on

the

down-

stream

slope above

rock toes. Longitudinal drains

at

9.14

m

(30

fD intervals

and transverse sloping

drains

at

30.48

m

(100

ft) intervals

are

provided

on the down-

stream

of the dam

section

to drain the

rain

water.

The

impervious soil

and

gravel

required to form this

earth

dam

were

brought

from the

water

spread

area

above

the dam with

a

lead of

about

6.4

to 8.0 km

(4

to

5 miles).

The

soils

were

tested for suitability

and

the following

are

their

properties.

Gravel content

Sand

content

Silt

content

CIay

content'

Impervious Semi-pervious

soil

15

percent

36

percent

52

percent

42

percent

11

percent

14

percent

22

perceht

8

percent

Total

fines

content

33

percent

22

percent

Angle

of

internal

friction,

t'

200

250

Cohesion

in

kg/cm,

(lb/sq

ft)

0.245

(500)

0.196

(400)

Percentage

moisture

at o.M.c.

Coefficient

of

permeability

I

in cm/sec

1ft/year) |

J

11to14

9to11

percent

4.8

x l0-G 67

x

10-6

to

9.6

x 10-6

135

x

10-6

(5

to l0)

(70to

1a0)

t

181.05

m

(

+

594

ft),

the

factor

of

safety for

the

downstream

slope

was

worked

out

to 1.28.

Right

Earth

Dam:

The

right

earth dam

is

also

a

zoned

embankment

with

central

impervious

core.

But,

as suitable

semi-pervious

soils wert

not

available

within

reasonable

leads,

the

impervious

core was

covered

on

either

side

for

this

earth dam with

the

canal

and tunnel

excavated

rock

muck available within

short leads,

unlike in

the case

of

the

left

earth

dam,

which

resulted

in

considerable

economy in the

cost of

construction.

The

top

width

of thii dam

is

kept at

9.14 m

(30

ft). The

upstream

slope varies

from 2: I

above

El. 181

m

(594

ft) to

3 : 1

below.

The

down-

stream

slope is kept

at2:.

l. Suitable

filters

of

1.52

m

(5

ft) thickness

are

provided

on

either

face of

the

impervious

core. Suitable

rock

toes

also are

provided

for both the slopes.

The upstream

slope

is

protected

with 1.22 m

(4

ft)

thick

riprap

and the downstream

slope

with

0.46 m

(1

ft 6 in.)

thick

revetment. One

berm

of

3.05 m

(10

f0

width

is

provided

on

the

downstream

slope at

El.

176.8

m

(5S0

fr).

This

earrh

dam

section

also

rvas

desi_ened

b.v

Swedish slip circle

method.

Impervious

Semi-pervious

soii

soil

Liquid

limit

Plastic

limit

Saturated

weight

in

ks/*'(lb/cu

ft) 2150

(134)

30

percent

38

percent

16

percent

20

percent

Gravel

content

Sand

conteni

Silt

content

Clay

content

Total

fines

content

Liquid

limit

Plastic

limit

Saturated

weight in

kg/m'

(lb/cu

ft)

0

percent

52

percent

l4

percent

-14

percent

48

percent

36

percent

17

percent

2150

(

r34)

40

percent

44

percent

4

percent

12

percent

l6

percent

-19

percent

20

percent

2res

(r37)

9l

2r9s

(r37)

Angle

of

internal

friction,

t'

Cohesion

in

kg/cm2

(lb/sq

ft)

Percentage

moisture

at

o.M.c.

Coefficient

of

permeabitity

in

cm/sec

(ft/year)

Impervious

Semi-imper-

soil vious

soil

10"

300

0.342

(700)

0.195

(400)

16

percent

ll

percent

13.5

x

l0-o

(14)

Embedded

Instruments

:

Cross-arms

and

piezo-

meters

were

installed

in

the

right

earth

dam

at

km 4.084

(ch.

134.00).

l0

piezometers

were

installed

here,

3

of foundation

type

in

the foundations

and

l0

of

embankment

type, I in

the semi-.pervious zone,

5

in

the

impervious

zone

and I in the excavated

rock

muck

zone. No

instruments

were

embedded

in

the left

earth dam.

5. Design

of Spillway

5.1

Design

Flood

Estimation

of spillway

design

flood

of Nagarjuna-

sagar Dam

became

a

problem

due to the

absence

of

any observed

data of

the

damsite. The

daily discharge

available

at

Vijayawada

anicut

which

is

situated about

176 km

(ll0

miles)

downstream ofthe

damsitewas

utilised in

arriving

at the

design flood.

Such

daily

discharge data was

available

for

a

period

of

60

years

from

the

years

1894

to 1954. Based

on

this

available

data,

frequency

studies

were

made

and

the 1000-year

design

flood was

computed

based

on

Gumbell's

method

taking

into

consideration

the

upper

confidence

limit

of

Ven-te-Chow

method.

The maximum

observed

flood

at

Vijayawada

anicut was

30,050 cumecs

(10.61

lakh

cusecs) in

the

year

l90l

and

the

corresponding

mode-

rated

flood rvas

conside

red

as 29.085 cumecs

(

10.27

lakh

cusecs). The

1000-year

flood

considered

in

the

original

desi-etr

of

spillway

was 39,220 cumecs

(13.85

lakh

cusecs) and

26

spans of

15.24

m

x

I 2.19 m

150

ft x

40 ft)

size,

rvith

3.05

m

(10

ft) thick

piers

in

berween,

were provided

with

crest at

*167.64

m

(+550.00

ft)

to surplus

an

unrouted

flood of the same

rnagnitude

at

F.R.L

+179.83

m

(+-590.00

ft). Subsequently

it

rvas

considered

reasonable

to

account

for

the valley

storage

obtaining

in

between the

damsite

and

Vijayawada

anicut

and

provide

for

a higher

flood

of

grearer

magnitude

at

the

damsite.

Accordingly, the design

flood

for Nagarjunasagar

spillway

was refixed

at

43,360

cumecs

(15.31

lakh

cusecs)

and the spilhvay

increased

to

26 spans

of 15.24

m x

13.41

m

(50

ft

x

44

ft)

uith-

the

same

3.05

m

(10

ft)

thick

piers

in betrveen,

but

$'ith

the

crest

lowered to

*,1

66.42

m

(+546.00

ft).

But,

later

during

construction,

stress

analysis

made

in

the region

around

the

reinforced concrete

pier

in

the

masonry

spillway

based on

photoelastic

studies

revealed

thar

a

4.57 m

(15

fr)

thick

pier

was required

and

provided.

With

this

increase in

the thickness of

pier

from

3.05

to

4.57 m(10 to 15 ft),

the span length

was reduced

from

15.24 to 13.72

m

(50

to 45

fr),

92

tion

effects

of

both

the

reservoirs

for integrated

operation

and

it

is

found

that

the

maximum

oltfloru

Nagarjun.asagar

Dams

of

the

order

of

about

g,500

cumecs

(3.00

lakh

cusecs).

The

maximum

outflow

5.2

Spillway

Profile

-

The

spillway

profile

consists

of

a compound

circular

curve of radii

equal

to

2.69

m

(g.g

ft) and

6.71 m

{22

ft) on

the upsrream

side

of

the crest

at Et. 166.42

m

(546

f0, connected

to

the

downstream glacis

of 0.7

to

! -with

an o_gee

_curve

of

parabolic

shape

of equation

X2:101.436

f.

Hydraulic-model

studiei were

carried

cut

-on

this

profile

to study

the

stability

of the

crest

profile, pressures

on

the

crest

as

well

as on

the down-

stream

glacis

and

discharges

for variorrs

depths

of flow

and

various

gate

openings.

These

studies

indicated

that

the

coefficient

or

discharge

for

this

profile

varies

from

2.7

to 3.95

depending

o;

the depth

of

florv

over

the

crest

and the maximum

value

of

3.95 is

attained

at

of 6.71 m

(22

f0 thickness

in

the

cresr

and

1.83

m

(6

fr)

thickness on

the

glacis

slope,

in

view

of the

very

high

velocities

of florv

of not

less

than 8.23

misec

(27

ftlsec)

obtainin_e in

M.W.L.

conditions.

5.3

Spillv'ay

Bucket

Energy

dissipating

arrangements

provided

below

the

spillway

section

were

evolved

with the aid of hydraulic

model

experiments.

Several

types

o[

energy

dissipating

devices

were

considered

and.

out

of

alt

of

them,

three

types

were selected

for the

initial trials,

viz.,

the

stilling

basin,

the

interacting

jet

dissipator and

the

flip

bucket.

Preliminary

studies

carried

out revealed that the

deflector

or trajectory

type

of

flip

bucket

would

be

the

best

suited of

all the

three, considering both

economy

in construction and

efficiency

in

perflormance.

The

guiding

factor

in

selecting

this type of

bucket

for

Nagarjunasagar

spillway

was

the

presence

of

sheet

rock

of

hard

granite

variety,

which

is

not

easily

erodable,

downstream

of the

spillway,

which

does

not

justify

the

much

higher cost involved

in either

of the

other

two

types.

Even

this

flip

bucket

was originally designed

with a

radius of 27.43

m

(90

ft), keeping the invert at

the

average

river

bed

level of

+73.15

m

(+240.00

f0

at

the

site

with

an angle

of

projection

of 30"

and

flip

elevation

at

+76.81

m

(+252.00

ft).

Again

hydraulic

model

studies

carried

out

on the

flip

bucket

with

different

radii,

different

flip

angles and

flip

elevations

and

different

invert levels,

indicated

that

a

bucket

rvith

a radius

of

2I-34

m

(70

ft) and angle

of

projection of

34"

with

the

same invert

and flip

elevations

is

as

efficient

as ihe 27.34

m

(90

ft) radius

bucket,

the

flow

features

observed

in both the setups, such as

the height

and

range

of

the

trajectory

of the

nappe,

the

pressures

obtaining

on

the

bucket and

the

erosion

characteristics

in

the

mobile bed

simulated below

in

the model,

being

almost

identical.

It

was

also

observed that the

pres-

sures

exerted

on

the

bucket

were

quite

within

the

practicable

limits and

that

the

sheet

of rvater

projecteQ

from

the

bucket

was

quite

stable up to a tail

water-level

of

+85.34

m

(+280.00

ft),

and

free

from

negative

pressures

even

at low head conditions and

that the

sheet

tended

to

become unstable resultin-e

in a submer-

ged

bucket

only

u'hen

the

tail

water-level

exceeded

+88.39

m

(*290.00

fil.

The

water

jet

was

thrown

off

to

a distance

of

about

12I.92 m

(400

ft).

away

from the

lip

of the

bucket

and impinged

on

the

water

cushion

built

up by

the tail

water.

The

adoption

of

21.34

m

(70

ft)

radius

flip

bucket

without

sacrificin-e,

in any'

way,

the

efficiency'

of

the energy

dissipation,

resulted

in a'savings

of

about

Rs.

12 lakhs. The

maximum

velocity of

rvater

in the

bucket

will

be

of the

order

of

46.O2

m/sec

(l5l

ft

sec)

and

high

pressures

of the

order

of

32,891

k,eim2

(6,740lb1sq ft)

will

be developed

on

the

bucket under the

maximum discharge

conditions.

The

bucket

was

built

in

reinforced

cement

concrete

with

a

minimum thickness

of

1.52 m

(5

ft) at the

invert

pornt.

5.4

Crest

Gates

(Figure

7'1

The spillway

crest

at

El. 166.42

m

(546

ft)

was

fitted

with

radial

.eates

of size

13.72

m x 13.41

m

(45

ft

x

44

ft),

with top of

-sate

at

El.

179.83

m

(590ft)

and

with

rope

drum

hoist

and the

operation

hoist

platform

at El, 187.11

m

(613.875

ft).

No

separate

large

scale

outlet

arrangements

are

provided

fbr

the

reservoir

for

operation

at higher

reservoir

levels,

except

for

the

diversion-cum-irrigation

tunnel

and chute

sluices

at

lower levels.

These

crest

gates

are

intended

for

not

o-nly

surplussing

high

floods

over

the

spillway

but also for regulation

of the

outflows

from

the

reser-

voir

when

the reservoir is

at higher levels. The

effects

of

partial

gate

openings

and

unequal

gate

openings,

with

adjoining

vents

closed,

were

studied by hydraulic

model

experiments

and

it was

observed

that

there

would

be

no

adverse

effect

on

the

spillway

glacis,

bucket

and the

flip

due

to

unequal

gate

openings.

Fressure

studies

were

also

made

on

the

spillway

piers

for

the

conditions

with

one

venr fully open

and the

adjacent

vent

closed

and with

both the adjacent

vents

fully open

and

the

pier

was

found

to be stable and

safe

for such

unequal

pressures.

The

gate

structure

and

the

over

bridge

were

to

be safe

for

the

normal condi-

tion

with

the reservoir

at F.R.L.

of

+

179.83

m

(+

590

ft),

i.e., water

standing

up to top of

gate

and

the

overtopping

condition

with

the

reservoir

at M.W.L.

of

+

181.05 m

(+594

ft).

But,

inasmuch

as

the

permissible

stress

in

the

members

for the

overtopping

condition may be

increased

by

33+

percent

while the

loading

goes

up by about

l0

percent

only, the

gate was

designed

for

the normal

condition.

The

rvei_eht of

gate

is

86.5

tonnes

(85.14

tons) and

a

6

H.P. motor

at720

r.p.m. is

provided

for

each

_qate.

6.

Design

of

Outlet

Works

6.1

The various

outlets

provided

in the

Nagarjuna-

sagar

Dam, in

addition to the

spillway.

ate

chute

sluices

at either

end

of spillway,

right canal

head sluice

in

the

right

non-overflow dam, with

a separate diver-

sion-cum-irrigation

tunnel bored

through

the Ieft

flank

and left

canal

head

sluice located separately

in

the

foreshore

of

.the

reservoir, apart

from

the

penstocks

and

power

sluices

provided

for

purposes

of

porver

development.

6.2

lrr\atiort-cum-Dit'ersion

Tunnel

(

Figure 8)

In

order to

facilitate

release of

irrigation

waters

for

the

Krishna

delta

both during the

period

of cons-

truction

of

the

dam

and after completion

of

the

dam,

it

rvas

earlier

proposed

to construct

18

river

sluices

at

E[.

129

54

m(a25 ft)

and 8

at

El.

l02.ll

m

(335

ft)

in

the

dam. But.

due

to

the

difficult

foreign

exchange

position

obtaining

at

the

time

of

the construction

of

the

dam

for

purchase

of the high

pressure

slide

gates

and

the

embedded

parts

required

for

these

sluices,

all

these

river

sluices

were

eliminated

and.

in

their

place, a diversion

tunnel

was

provided

on

the

left

flank

of

ttredam at

El.

91.44

m

(300

f0'

This

tunnel

was later

connected

with

a

goose neck

tunnel

at El.

121.92

m

(400

ft)

and

the

entrance

to

the

El.

91.44

m

(300

ft)

plugged

so

that

the

El.

121.92

m

(400

f0

tunnel can

p-ass

the

required

disclarg-e

of

i66..t*."s

(20.000

Cusecs)

undei

a

head

of

15.2 m

93

3

m

dio.drum

l.O3m

senl

position

of drum

W

3

t-5

O38

r

of drum

h8L.613875-gg.oposed

{op

of

pier

_E_L.185€3

m

existing

fop

;

of

pier

s,tD

V

ie wi-A'

r.-of

f runnion

(

Dirnensions

in

metres

unlese

olherwise

gtoldd

t

Scole

o

|

2

3

4m

l-,ll-

|

r

.

L.'t

82.29

m

Flow

+

Silt

tevel

.lA

op

of

s90'oo

"\

t.Au-

EL.l66'27m

FIGUR.E

7 :

Spilf

.vaV

crest

gates.

94

EL.193'55.3q-

tI

Hoi3t chornb"r-jf_,i

r37.!6

E

:.

Or

,E

v

o

E

rf

o

;

c

C

2

E

!

j

o

J

lri

rEL

{;

3

(,

L

I

,

3

:

o

I

lnloLe slruclurc

forELt2l:92m

funnrr--!

I

for

divcrsion

FIGURES :

Irrigation-cum-diversion

tunnel.

(Not

to

scole)

Finol

plug.ging

ol diversion

lunnll

EL.

to

{50

ft),

as a

permanent

arrangement,

after com-

pletion

of

the

dam.

In the

initial

reaches of

about

106.68 m

(350

ft)

length of rhe

El.

91.44

m

(3C0

f0

tunnel,

no

concrete

lining

was

provided,

whereas

in

the

reach

beyond the

goose

neck tunnel,

it

was con-

crete

lined

with

nominal reinforcement

of 19.05

mm

(B

in.)

diameter rods at

30.5

cm

(12

in.)

centres

as

this

portion

was

to serve as

the

permanent

tunnel for

the El.

12I.92

m

(400

f0

goose

neck tunnel. The

tunnel section is

horse

shoe

shaped

with

a

diameter

of

9.14

m

(30

ft) in the unlined

portion

and 8.23

m

(27

ft)

in

the lined

portion.

The

goose

neck tunnel

is

again

concrete lined

with

a

diameter

of

8.23

m

(27

ft),

The

discharge from

the tunnel

cascades

down

a

chute

and energy dissipating

arrangements

are

pro-

vided

at the river

bed

level

to

prevent

scour

down-

stream of the chute.

The

intake

structure for

this

El. 121.92 m

(400

ft) tunnel

consists

of

2

vents

of

3.05

m x

7.62

m

(10

ft

x

25 ft) size.

The

service

gates

for this

tunnel

of

25 tonnes

(24.6I

tons)

weight

are operated

by

rope

drum

type electrical hoists of

T2tonnes

(71.00

tons)

capacity,

located

on the top of

the

intake

structure.

A

set

of fixed

wheel type

emergency

gates

of

size 3.05

m x

8.53

m

(10

ft

x

28

ft)

is

also

prvided.

The service

gates

are

to

be

operated

to

regulate

the

discharge

of 566

cumecs

.

(20,000

cusecs) thr.ough the tunnel

to

meet the

require-

ments

of Krishna

delta. In

order

to

reduce the

operating head

on

these

gates,

it

was

proposed

to

regulate them

only when

the

reservoir

water-level

comes

down

below El.

170.69

m

(560

ft).

Above

this

level,

the delta requirements

are

to

be

passed

over

the spillway

crest by regulating

the crest

gates.

All

these

gates

and

hoists were

supplied

by Messrs

M.A.N.

of West

Germany.

An

interesting

feature of

the service

gate

is that

a stainle.ss

steel

siiding

plate

is

provided

on the

upstream

face

of

the

gate

shaft

to

serve

as sliding

plate

for

the seals

and

prevent

water

jetting

over the

gate

and

thus

reduce

the

gate

vibration

throughout

the range

of regulation

of the

gate. The

wheels

are

made

of chromium

alloy

steel and bearings

are

of

needle

type,

the

needles being made

of

chromium

steel. An

air vent

of 1.98

m

(6

ft 6

in.)

diameter

is

provided

to

ensure

aeration and

prevent

cavitation

and vibration. Extensive

hydraulic

model

studies

were

carried

out

in

the

laboratories of

Messrs

M.A.N. in West Germany

for

finalising

the

desiges.

6.3 Chute Sluices

The

original

proposal

was

to

provide nine

river

sluices of size

1.52 m

x

2.74

m

(5

ft x

9

ft)

in

the

dam

with

centre line at

El. 134.11

m

(440

ft) to serve

as

river outlets. As such

high

pressure

slide

gates

with hydraulic hoists

required

for

operation

of

these

sluices

at a maximum head

of 45.72 m

(l50ft)

were

not

manufactured

in the

country and

would

involve

considerable

foreign exchange

besides

long

delivery

period,

if

obtained

from foreign

firms,

which

was

likely to

handicap the

construction

programme of

the

dam,

two

outlets only

were

provided

in

the

dam.

These

outlets of

size

3.05 m

x 7.62 m

(10

ft

x 25 ft)

were

provided,

one in

each

of

the

non-overflow

portions

of

the two

end

blocks

of

the

spillway

(blocks

25

and

5l) at FI.

137.16

m

(450

ft)

with sloping

chutes

downstream to let down the waters

into

the

river

below.

These

chute

sluices

can

also

meet the

Krishna delta

requirements,

as an

alternative

to

the

irrigation-cum-diversion tunnel

to

provide for

unfore-

seen

contingencies in the operation

of

gates.

Each

aa*'

_-qPygvs#

95

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

Подождите немного. Документ загружается.