Kenney Lau Discussion Reply

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DISCUSSIONS

255

rnethods

f

analysis

avebeendeveloped

n recent

eius

o

deal

with

his

problem,

but the

results

of such

analyses

eported

n

the

iterature

re sometimes

uite

contradictory,

as evidenced

by

he

paperby

Azzouzzet

al.

(

98l

),

quoted

by Brand,

some

results

f

which

were

seriously

uestioned

y

La Rochelle

and

Marsal

198

).

lt seems

hat

more

experience

s

needed n

order

to

better

evaluate hree-dimensional

ffects

in

embankment

failures.

Conclusion

In

conclusion,

here

seems

o

be

a certain

consensus

elative

to

he

stability

analysis

f embankments

n

soft

soils:

-Total

stress

stability

analyses

are

quite

suitable

for

design

purposes

as

long

as

reliable

shear

strength

values

of

the

foundation

oils

are used.

For

this

purpose,

empirical

methods

such

s

the n

sita vane

correction (Bjerrum

Ig1-Z),

r the

cu

:

0.2?oo'

method

Trak

et

al.

1980)

may

be

quite

appropriate.

-Effective

stress

stability

analyses

are

an

efficient

tool

for

engineers

ho

have

o evaluate

he

stability

of

an embankment

during

construction

or

when

it

is

subjected

o reloading.

It is

important

hat

these

analyses

be

carried

out

using

"measured"

pore

pressure

alues,

and reliable

effective

strength

parameters

(determined

referably

at

large

strains).

It

should

be remem-

bered hat the determinationof the cohesionparametermay

present

ome

problems

and

that

a

value

of

c'

:

0 might

be

preferred

n

some

cases.

t is

also

true

that

the

present

practice

could

be

greatly

mproved

f more

cases

of failure

of

ernbank-

ments ncluding

effective

stress

analyses

were

reported

n

the

literature.

BILIsUSRAMANIAM,

.

S.,

SrvnNonnN,

., and

Ho,

y.

M.

1979.

Stability

and

settlement

f

embankments

n

soft Bangkok

lay.

Proceedings,

rd nternational

onference

nNumerical

ethods

n

Geomechanics,

achen,

ol.

4,pp.

1373-l4ll.

Brennuu. L. 1972.

Embankments

on soft

ground.

Proceedings,

SpecialtyConference n Performance

f

Earth and Earth-Supported

Structures, afayette,

N,

Vol.

2,

pp.

l-55.

Bnu, J.-P.,andDnvnux,

A. 1976.Rupture

u remblai 'essai Saint

Andr6-de-Cubzac.

n

Stabilit6 des talus: d6blais

et remblais.

Bulletin de Liaison

des Laboratoires des

Ponts et

Chauss6es,

Numdro

Special I I ,

Vol.

2,

pp.

145-148.

HeNznwA,

H. , Krsnron,

K. , and Mnrsuoe,

E. 1982.

Stabi l i ty

analysis with

the

effective s tress method for

embankmentscon-

stnrcted

on an alluvial marine clay.

Soils and Foundations,

22(3),

pp.32-a6.

JossenuuE,

H., BLoNDEAU,

., and hlor,

G.

1977. Etude

du

comportement

non draind

de

trois

argiles molles.

Application au

calcul

de remblais.

Proceedings, nternational

Symposiumon

Soft

Clays, Bangkok,

pp.

487-504.

Ln RocHer-LE,

., andMensnt-, R. J. l98l .

Slopestability-general

report.

Proceedings,Xth

International

Conferenceof

Soil

Mechanics

andFoundation

ngineering,

Stockholm,

Vol.

4,

pp.

485-507.

LERoUEIL,

., Tnvpxls,

F., TRAK,8., L^l Rocgeue,

P., andRoy,

M. 1978a.

Constnrction

pore pressures

n

clay foundationsunder

embankments.

Part I:

the Saint-Alban test

fills. Canadian Geo-

technical

ournal,15,

pp.

54-65.

LenourIL,

S., TnvexAs, F., MtEussnxs,

C.,

and Prtcxlup,

M.

1978b.

Construction

pore pressures

n clay

foundations under

embankments.

Part II:

generalized

behaviour.

CanadianGeotech-

nical

Journal,15,

pp.66-82.

SIuoxs, N. E. 1976. Field studiesof the stability of embankments n

clay foundations.Laurits Bjemrm Memoria l Volume.

Edited by

N.

Janbu,

F. Jorstad,

and B.

Kjoernsli. Nonvegian

Geotechnical

nsti-

tute,

Oslo,

pp.

183-209.

Sxeurrox,

A. W. 1964.

Long-terrn stability

of clay slopes.

G6otech-

nique,

4(2),

pp.

77

103.

TRAK,

B. 1980. De la

stabilitd des remblais

sur sols mous. Ph.D.

thesis,D6partement

e

g6nie

civil, Universit6 Laval,

Qudbec,

P.Q.

TRIK,

B., Le Rocgerle,

P., TevENls, F.,

Lenouett, S., andRoy,

M.

1980.A new

approach o the stability analysis

of embankments

on

sensitive lays,

Canadian

Geotechnical ournal, 7

,

pp.526-54.

&

q

t,

i

i

:

i

* :

*

j

i

i1'

t,

"

. .

Internal

stability

of

granular

ilters:r

Discussion

C.

F. Rrplry

50ll

HilariePlace,

Victoria,8.C.,

CanadaVSYA4

Received

October28, 1985

Accepted

October31, 1985

Can.

Geotech.

.23,255-258

1986)

The

purpose

f

this discussion

s

twofold.

Firstly,

the

writer

wishes

o encourage

.

C.

Kenney

and his

colleaguesn their

continuation

of

their

thorough

step-by-step

esearch nto

the

mechanics

f

particle

migration

and

particle

blockage within

soilmasses nderseepagelow conditions,andparticularlyto

encourage

hem

o

investigate

he relationship

betweenwidth

of

particle

ize

gradation

of a filter

material

and ts

susceptibility o

harmful

segregation.

Secondly, he writer

wishes

o emphaiize

that

designs

or

effective filter-drainage

components

of

dams,

have

been

used

successfully

ince

he 1940's,which

take nto

account

he construction

aspects

and

the field

behavior

of

embankment

ams.

Most

of the

piping

incidents

hat

have

occurred

t embankment

ams

n

the 1960's

and 1970's

would

not have

occurred

had filter-drainage componentsof

similar

design eenused.

The

nternalstability of

granular

ilters is one of a number

of

important

questions

o

be

consideredby designers

of filters

for

embankment ams. Numerous ncidents of seriousandcostly

piping

have occuned

within relatively thin core

rockfill dams

during

the

past

25

years,

where the filter zone and

(or)

the

adjacentdownstream ransition

zone have consisted

of

widely

graded

materials. f the filter and transition zones

at these

dams

had ulfilled their

intended

ilter function, none of the

ncidents

could have occurred.

The research

work

reportedby Kenney and

his colleagues

n

the

paper

under discussionand

previous papers

Kenney

et al.

1984, 1985)

hascontributed

greatly

o a

better

understanding

f

both

particle

migration and

particle

blockagewithin

soil masses

under

seepage low conditions.

The laboratory

test

programs

rPaper

by

T.

C. Kenney

and

D. Lau.

Journal,

2, pp.

215-225.

1985.

Canadian

Geotechnical



256

CAN.

GEOTECH.

.

VOL.

23,

1986

have

been extensive

and thorough; so

also

have

been

the

analyses

f

the est

esults.

The

paper

deals

with the

potential or

internal

nstability

of filter

materials hat

havebeen

placed

under

controlled

laboratory

conditions in

order

to achieve

homo-

geneity

and

in order

to

prevent segregationand

horizontal

layeringeffects

within the

est specimens.

Hence he est

esults

and

the analysis

are applicable

o idealized

conditions

for the

group

of

widely

graded

filter materials

hat

were tested.

The

authorscorrectlyacknowledge hat the testconditionsof large

seepage

elocity

and

mild

vibration

were more severe

han

would

generally

be

expected

n

practice. However,

the

writer

believes

hat the

controlled

homogeneity

of the

test materials

was he

more significant

variation from actual

ield conditions.

Harmful

segregation

f

widely

graded

materials

during

place-

ment

n the

ield

is considered

y

the

writer and by

others

Leps

1979)

o be

nevitable,

and

measures

o eliminate

segregation

o

be at

least

uncertain

if not

impractical.

Even

with these

limitations,

the

findings

of the

test

program

provide new,

helpful

nsight

nto the

relationship

betweenshape

of

grain

size

curve and

potential for

internal instability

for filter

materials

in a homogeneous

tate.

However,

the

potential

for

internal

instability of

a

particular filter

material

can be significantly

higher than indicated by such laboratory tests, if the filter

material

s susceptible

o segregation

uring

field

placement.

The

question

of its internal

stability

is only one

facet of the

problem

of

design

of an effective

ilter

zone

within

a

dam. In the

writer's opinion,

the

most

critical

zones

with

respect

to

provision

of

effective

ilter

action are

he

downstream

ilter and

transition

zones

of thin

core embankment

dams,

particularly

thin

core ockfill

dams.

The subsequent

iscussion

s directed

o

zones.

The ability

of

a filter

zone to

block migration

of

particles

rom

the

adjoining

zone

upstream

of the filter

is an

equally

mportant,

f not an

overriding,

requirement

o that of

internal

stability,

for effective

filter

function.

In order to fulfill

this

function

the

zone

material

should

have the

following

properties:

segregation.

n upper

imit of

particle

size

o eliminate

oncern

|

for harmful

segregation

within a filter

sand

appears o

be 18

mrn

\

i*ffi

Til;fi".".:ft'dH;

fi #

NJ:;-iJ;."ij#;

D^ ^ ^ . , ^ ^ +L ^ - - ^ ^ ^ -^ ^ ^ f G-^ - ; - - ^ .+o . ' a r la c i ro L lo ^ ^ l ' o " i ^ - t ^ ' ^ \

Because

he

presence

f

fines mparts

undesirable

ohesion

o

a

I

filter

sand,

he upper

imit of

finei

content n order

o havo

^"î

I

;"":i::n I

stopper'

apability

ppears

o be

ZVominus

he

No. 200 siere

I

to.b?s

n*i,

althoughbTo

assinghe

No.100ieve

.

ii: n:l I

is

preferable. ecent

rograms f severe

aboratory

ests avs

t

verifiedheangef sandradations:),|;:j]t"T'":')t,ijlX,',-ll I

blockentry

of

tlhe inest 6re

particles

Sherar

et al.

1984a,

'.

,

Kenneyi

at.le8s).

i

enney t

al.

l9E5).

with respect

o he ransition

nd

drainage

ones o*nstr.am

I

of a

sand ilter

zone n a

rockfill dam,

he

major

gap

n.existing

|

knowledge

s he

ackof

proven

riteria

oncerning

he imitsof

|

widthof

particle

ize

gradation

hatcan

be olerated

ithout

he

I

occurrence

f harmful

segregation

uring

handling

;;.:

i

ment

r he

nu,..iur.-iil'";;"r,i'.T;il "'l'Gffi

t

;;

i

same

degree

of thoroughness

hat

the authors

have carried out

I

forprevious

ot: As mentioned

ubsequently,

he

proven

ilter

|

practice

f

the

1940's nd

1950's

n

NorthAmerica

or sloping

I

core

ockfill

dams

rovides afe

guidelinesn the

meantime

ar

f,

the olerable

width of

particle

size

gradation

o

prevent armful

|

forprevious

work. As mentioned

subsequently,

he

proven

ilter

*gffqi::""1-..-^.^

:-^--. ^r +L^ aaala ̂ r -^;^,,- -iî-- ianizro-r. i

Thi

unfortunate

ronyof the

spate f

serious

iping

nci^dents

[

of the

past

25

years

where ilter

and ransition

ones

ave ailed

I

in their

ilter

function

s that

widely

reported

revious

:*L:n- |

enceappears

o

have

gone

unrecognized

r

unheeded.

hat

I

experience

ncludes

he

ollowing:

)

(i)

the

admonition

f A. Casagrande

n

1950

s*9]:q

t

[

cornmon

ccurrence

f transverse

racks

n embankment

ams

i

and he need

or incorporation

9f

design

measures

o render

I

them

harmless

casagrande

950);

_-__.r^:_-^_, r ̂ ,^^_^^-. . ,

|

(ii)

the

proven effectiveness

f a

narrow

chimney

of clean

sand

as an

effective

crack

stopper'

zone

within homogeneous

lav

i

r^ . . r^r. i . ^ , ,^ : -^- , o<i . I

dams n

Brazil

(Terzaghi

1953;

Hsu

1963;

Queiroz

1963;

|

vargas nd

Hsu1970);

--, r- :__ _--^r:^^ r rL^

)

(i) a particlesizegradationwith 'controllingconstriction ize'

that

s

appropriate

n relation to

the adjoining

upstream

zone;

(ii)

low to

nil susceptibility

o segregation

uring the

practical

operations

f

handling

and

placement f the

material n

the ield,

otherwise

he actual

'controlling

constriction

size'

at areas

of

contact

of coarse

segregated

ilter material

with

the upstream

zone

will not be appropriate;

(iii)

'crack

stopper'

apability,

.e , the

ilter

material hould

be

incapable

of sustaining

an

open crack

within

the zone

as a

downstream

xtension

f

a transverse

rack

hrough he

more

cohesive

ore.

This

combination

f

properties hat

are

equisite o effective

filter action

has

been

shown

o be

providedby

narrowly

graded

materials

f appropriate

radation.By

contrast,

umerous

ases

of damswith seriousnternalpiping ncidents unng hepast25

years

have

clearly

demonstrated

hat

widely

graded

materials

cannot

be

relied

upon to

provide

his

combination

of essential

propeniesor

effective

ilter action

Kjaernsli

and

Torblaa 968;

Kjaernsli

1973.

Wood

et

al. 1976:

Vaughan t al.

1970;

Boivin

and Seemel

1973a.

b:

Seemel

and Colwell

1976:

Chadwick

1976,

1979;

Vestad

1976:Sherard

973,1979.

1984;

Hoff and

Nilsen

1985;

Kjellberget

al. 1985).

Recorded

am

perforrnance

experience

nd rigorous

aboratory

est

programshave

clearly

demonstrated

hata clean

cohesionless

and

s not

only

capable

of effectively

ilteringeven

he

inestof

the

general

ange

of silt

and

lay

soils ound n

nature. ut

hat

t has he

necessary

crack

stopper

capability'

and

the

necessary

esistance

o harmful

(iii) the satisfactorynd proven ilter design ractice.gl*

1940's nd

1950's

n North

America

or

rockfilldams

ASCE

i

1960; omini

1954).

I

The

concept

hat a

I m

wide

vertical

chimney

of

clean

|

l l t

uu l tu trPt

tr ld [

a I l l l

w l r . ls

Y

,1 L l l -ar

v l rrrr l rvJ

vr

v

rvqrr

I

cohesionless

and

would

serve

as

an

effective

'crack

tlgqq:t'

I

appears

o

have

eenntroduced

y

K. Terzaghi

n thg

1940's.

I

and

as een

sed

xtensively

ince

hen

n

Brazil

nd lsewhere

i

no nas ocgl l

usgu

cxtcl lsl

vtrry

Srl lLs

tl lstt

l l l

t) | aLrr

4rrLr vrJe_w

rv.r

|

(Vargas

and

Hsu

1970).

None

of the

Brazilian

dams

with the

i

niurow sand

chimney

has

exhibited

piping in

spiteof

the

fact

I

that investigations

ave

shown

that

open

cracks

within

the

I

residual

clay

bodies

of

the dams

are common

(M.

Vargas'

I

personalommunication,

978).

he

successful

erfo...ul:.

uf

i

these

nternal

himney

rains

or

up to

40

years

hould

ispel

i

doubt

that

clean

sand

can be

relied

upon

to behav_e_::-

1

\cohesionless aterial, .e. as an effective crack stopper.

i

within the

confined

onditions

at

depth

nside

a dam.

. I

The filter design

concept

or

rockfill dams

hat

had become

I

virtuallystandard

ractice

n North

America

by

the ime

of

the

I

A.S.C.E. Symposium

n

Rockf i l l

Dams

n

1958

appears

o

[

have

een

nitiated

y J. P. Growdon

n

1940 t

Nantahala

alr.

|

" r - '

^ . - . I

a 250ft

(76m)

high sloping

core

rockfil l

dam.

Growdon

)

subsequently

sed

he

same design

or

other

rockfil l

{an1s

(Growdon

1960a-r').

t

was

widely adopted

y

others

n Noflh

i

UTOW OOn

y OUd - ( ' ) .

t t

wa s

w l0 e ly a o o p te t l

oY

o t l l c l s . r I r

r \ ( ' r t r r

I

America

during

the

1940's

and

1950's.

Growdon's

fi lter-

|

transition

component

between

he

clay core

and

the coarsc

I

dumped

ock

fill

consisted

f

thrcc

narrowly

gradedzonc.s

l

I

clean

and.

No.

2f i )

s ieve ize

o

i

in.

t0.075-12

mm).

-l

tn.



257

DISCUSSIONS

( l l -73mm),

and

3- l0 in.

(75-250mm).

To

the

wri ter 's

inowledge

none

of the

dams

using

this

design

or

the

filter-

iransition

omponent

as

shown

any

evidence

of

piping

within-

ih..or.

orthe

downstream

ilter-transition

omponent.

Many

of

thesedams

will undoubtedly

have

endured

transverse

ore

cracks

due

to

the

much

larger

Settlements

ssociated

with the

dumped

ockfill

dams

of

that

era,

as

against.lhoy

of

modern

compacted

ockfill

dams.

The

sand

ayer

provided

assured

ilter

,upuÛitity

gainst

migration

of

core

particles or

a

broad

array

of

coie

materiils,

ranging

from

tropical

clays

to

broadly

graded

elaciat

ill

soils.

T[e

ielatively

narrow

gradationof

the sand

i.y.,

prouided

assured

rotection

against

harmful

segregation,

,nO

ts

lack

of

fines

aisured

'crack

stopper'

capability.

The

narrow

and

appropriate

gradation

of

each

transition

zone

provided

assured

protection

against

harmful

internal

segrega-

iion

and

against

migration

of

particles

from

the

adjacent

upstream

one.

The

1960's

ushered

n an

era

of

accelerated

am

undertakings

throughout he world, which coincided

with

the

change

in

merh;d

of

rockfill

construction

rom

dumped

rockfill

to

com-

pacted

rockfill.

At

several

o.f

,J1l^.cases

f serious

.pigi"q

writer

believes

that

segregation

s

the

major

culprit

to

be

guarded

against,

and

hat

h;

risk

of

uncontrollable

segregation

in.r.ur.,

is

the

range

of

particle

sizes

becomes

wider.

The

comments

on

these"pointi

by

Leps

(

1979)

are

particularly

pertinent.

he

mech.ni.t

of

efminating

that

risk

is simple

and

ihe additionalcost is modest.The required nanowly graded

materials

can

usually

be

obtained

simply

by

the

washing

and

screening

f

availabie

widely

graded

materials

rom

quarry

or

pit

run

sour.es,

with

supplemental

rushing

being

necessary

n

io*.

cases.

A

rockfill

dim

has

nherent

structural

stability.

The

filter-transition

component

of

an

earth

core

rockfill

dam

is

the

most

critical

elemeni

o

its

continued

satisfactory

performance

as

a

water-retaining

structure.

Surely,

then,

the

processing

of

filter

materials

nt6

select

sizes

for

the

two

of

three

filter-

transition

zones

of

an

earth

core

rockfill

dam

is

as

mportant

as

the

universally

accepted

processing

of

aggregates

or

concrete

dams

into

a

i*g.t

nurb.t

of

sizls;

and

surely

the

costs

of

processing

hould

be

as

acceptable.

'

In

conllusion,

the

writei

hopes

that

this

discussion

will

encourage

he

authors o extend heir careful research n order o

develop

criteria

relating

the

susceptibility.

o

segregation

of

granular

ilter

materialsio

the

width

of

particle

size

gradation'

Finally,

even

though

gaps remain

in

our

fundamental

knowl-

edge

of

particle

migration

and

particle

blockage

under

seepage

Row

conOitions,

hE

engineering

profession

has

available

now,

and

has

had

since

1940,

a

proven

practice

or

design

and

con-

struction

of

effective

filterl

to

protect

against

internal

piping

within

dams,

which

takes

nto

iccount

practical

constnrction

considerations.

ASCE.

1960.

ymposium

n

Rockfill

Dams,

1958.

ransactions

f the

American

ociety

f Civil

Engineers,

25,

Part

I'

Borvrx,

R.

D., an-d

EruEL,

R.

N.

1973a.

Churchill

Falls

power

deveiopment.

esignof thedykes.Paper,CANCOLDMeeting,

Quebec

ity.

lg'13b.

hurchill

Falls

power

development.

onstruction

f

the

dykes.

Paper,

CANCOLD

Meeting,

QuebecCity'

ClslcnnxpE,

A.

1950.

Notes

n

he

design

f

earth

ams.

ournal

f

the

Boston

Society

f

Civil

Engineers,

i.424-429'

CHa,pwrcr.

W. L.

19'76.

Disculsion

of

question

5. X11

COLD

Congress,

exico

City,

Vol.

V,

pp' 281-282'

1979.

iscussion

f

question

9.

XIII

ICOLD

Congress,

ew

Delhi,

Vol.

V,

PP.

10-414.

Gnowoox,

.

p.

fgOOa.

antahala

loping

ore

dam.

Transactions

f

the

American

ociety

f civil

Engineers,

25,

Part

I,

PP.

160-180.

|gffib.

Dams

with sloping

earth

cores.

Transactions

f the

American

ociety

f Civil

Engineers,

2S,Part

I,

pp' 207-225'

1960c.

Performance

f

sioping

ore

dams.

Transactions

f the

American ociety f Civil Engineers,25,Pan I, pp' 237-252'

Horr,

T.,

and

NlLsrN,

K.

Y.

1985.

Erosion

nd

eakage

roblem-s

n

some

Norwegian

ams.

Paper

R36,

Question

59,

XV

ICOLD'

Lausanne,

ol.

IV,

PP.

573-586.

Hsu,

S.

1963.

Residuui

tuy

earth

ams

onstructed

y

Rio

Light

SA.

Proceedings,

nd

Pan

Am

congress

on

soil

Mcchanics

and

Foundatioi

ngineering,

razil,

Vol'

II,

pp' 347-363'

JorvrtNt,

.

1954.

he

Kenney

am.

Engineering

ournal,

7(11)'

p'

6 -17

KexNry,

T. c. ,

Leu,

D.,

and

roeGBU,

.

I.

1984.

ermeability

f

compacted

ranular

materials.

anadian

eotechnical

ournal,

1,

pp.726-129'

KexxEv,

.

C. ,

CHeHnL,

. ,

CHtu,

E' ,

OroEcBU'

'

I ' '

OMnNGE'

G.

N.,

and

ulle,

c.

A.

1985.

Controlling

onstriction

izes

f

granular

ilters.

Canadian

eotechnical

ournal'

2'

pp'32-43'

K.riEnxsr-r.. lgi3. Discussion

f

question

2,y^1ICOLD

ongress,

Madrid.

ol.

V,

PP.416-419.

Kreenxsr-r.

. ,

and'To*rr,nn.

1968.

eakage

hrough

orizontal

incidents

uring

he

1960's

and

1970's,

surface

manifestati

rnc

ng

ifestations

of

piping

within

the

core

material

took

the

form

of

an

abrupt

inciease-of

seepage

discharge

aden

with

sediment,

and

the

appearance

f

sinkholes

upstream

and

(or)

downstream

of

the

.iist.

While

none

of

the

incidents

nvolved

a dam

breach,

all

;eated

a real

Sense

of

urgency

for remedial

action,

and

all

required

costly

repairs.

Several

of

the

incidents

have

been

widely eported

wiih

regard

o the

ncidents

hemselves

s

well

as hc invistigations,

analyses,

and

repairs

of

them.

The

focus

of the investfuations

and

analyses

n

the

reports

has

dwelled

mostly

on

transverse

ore

cracks-their

causes

and

mechanics

of formation-as being he primary problem to be overcome

n

the

prevention f

piping

(Sherard

1973).

Seepage

hrough

oints

in

he ock

foundaiion

was

he

assigned

ause

or one

case,

and

a

possible

assigned

cause

for

another.

One

wonders

why

this

nearly

otal

focus

of

attention

on

core

and

foundation

cracks?

The

obvious

and

undeniable

fact

was

that

the

filter

and

(or)

transition

ones

at

each

dam

failed

to

fulfill

their

design

unction

of

preventingerosion

of

core

fines

in

spite

of

either

core

or

foundation

iacks;

otherwise

piping

of core

material

could

not

have

occurred.

The

filter

failures

and

the

causes

of

them

received

ittle

attention

n

the

reports,

and

have

not

received

he

universal

ttention

by

dam

designers

hat

is

warranted.

The

reasons

or the

use

of

less stringent

defensive

measures

since

1960

by

some

designers

as

compared

to the

proven

Growdonpracticeare not clear and can only be_surmised.A

perceived

eduction

in

construction

cost

by

use

of

fewer

zones

with

little to

no

processing

was

Probably

one

reason.

The

concept

hat

core

materials

of

wide

particle size

gradation,

such

as

some

glacial ills,

could

be

relied

upon

to

be

self-filtering

or

self-healing

ppears

o

have

been

another.

Surely

the

occur-

rences

f surface

inkholes

ave

provided

sufficient

vidence

f

open

oles hrough

he

thin

glacial till

cores

at

several

of

the

cases

hat

this

concept

should

now

be

dismissed.

A third

and

more

ikely

reason

s

hought

o be

hat

he

danger

of

segregation

of

widely graded filter

and

transition

materials

was

not

universaliy

nderstood

or

accepted.

While

it

is

an

undeniable

fact

hat

n1uny

ams

have

performed

satisfactorily

with

widely

graded

filter

and

transition

zones,

surely

the

lesson

demon-

strated y most of the piping incidents s that use of such

rnrtcrials

incurs

a

real

riri

oi

piping

at

random

ocations

of

iir

-. . , r:ated

aterial

here

he

il iei

criteria

re

not

satisfied.

he



258

cAN. CEOTECH.

.

VOL.23.

1986

cracks

n the coreof Hyttejuvet

dam.

PublicationNo. 80, Norwegian

Geotechnical nstitute, Oslo,

pp.

39-47.

K.leunrnc,

R.,

NoRstEDT,V.,

and

FeceRsrRoM,

. 1985.

Lrakage

in

and

reconstnrction

of the

Juktan earth

and rockfill

dams. Paper

R35,

Question

9, XV ICOLD,

Lausanne,Vol.

IV,

pp.

553-573.

Leps, T.

1979. Discussion

of

question

49,

XIil

ICOLD,

New Delhi,

Vol. V, pp.414-415.

QuEInoz,

L. A. 1963.

Discussion:

Residual

clay

earth dams

con-

structed

by Rio Light

SA.

hoceedings, 2nd

Pan Am

Congress

on

Soil Mechanics and Foundation Engineering, Brazil, Vol. II, pp.

679-682.

SeEuet,

R.

N., and

CoLwerr,

C. N. 1976. Drainage

provisions

and

Ieakage

nvestigations

of the

Churchill Falls

dams and

dykes. Paper

R8,

Question

5, XII

ICOLD,

Mexico

City,

Vol.

II,

pp.

107-127.

SHERIRo,

J. L. 1973.

Embankment

dam

cracking.

/n Embankment

dam engineering.

Casagrande olume.

J.

Wiley,

New

York,

pp.

27 t -353 .

1979.Sink holes

n dams

of coarsebroadly

graded

soils. Paper

R2,

Question

49,

XIII ICOLD,

New

Delhi,

Vol.

II,

pp.

25-35.

1984. Trends

and debatable

aspects

in embankment

dam

engineering.

Water

Power

and Dam

Constmction,

36(12),

pp.

26-32.

SHEnlno,

.

L.,

DuNxrclN,

L. P., andTlLsor, J. R. 1984a.Filters

for silts and clays. ASCE Journal

of

the GeotechnicalEngineering

Division,110(6),

p.

701-718.

1984b. Basic

properties

of sand and

gravel

filters. ASCE

Journal

of

the Geotechnical

Engineering Division, 110(6),

pp.

685-700.

TeRzlcHr,

K.

1953.

Discussion.

Proceedings,

Third

International

Conference n

Soil Mechanics

and Foundation

Engineering,

Zurich.

Vol.

I I I ,

pp.2l7-218.

VeRGns,M., and Hsu, S. 1970. The use of vertical core drains in

Brazilian

dams. PaperR36,

Question

36, X ICOLD, Montreal,

pp.

s99-608.

VlucHlx,

P. R.,

KtutH,

D.

J., LEoNlno,

M.

W.,

and h,loouRe.

H.

H. M. 1970. Cracking

and erosion

of

the

rolled clay core

of

BalderheadDam and

he remedial works

adopted or

its repair. Paper

R5,

Question

6, X IC OLD,

Montreal,

pp.73-92.

Vesrao,

H. 1976.

Viddalsvatn

dam. A history

of leakages

and

investigations.Paper

R22,

Question

45, XII ICOLD,

Mexico

Citv.

pp.

369-390.

WooD,

D., Kllennslr,

8., andHoec,

K. 1976.Thoughts

oncerning

the

unusualbehaviourof Hyttejuvet

dam. PaperR23,

Question

45,

)il

ICOLD, Mexico

City,

pp.

391-414.



cAN. GEOTECH.

.

VOL.

23. 1986

Internal

stability

of

granular

filters:l

Discussion

Can.Geotech.

.23,414-418

(1986)

The authors

have carried out a detailed and most interesting

examination

of the internal stability of

granular

materialswhen

they are

subjected o seepageorces and

vibration.

On the basis

of their

test

data,

which

demonstrate hat seepage

lows in

certairt

coarsely

graded

filter

materials tend to wash

"fines"

through

he

coarse raction

of the

material, hey

propose

riteria

to evaluate

uch

potentially

unstable

gradations.

From

the data,

these

criteria

in themselvesappear ogical but,

as the authors

state,

"are

neverthelessdisquieting" in

that they imply that

many

existing

dams could have filters that are

potentially

un-

stable.

This implication s so mportant hat t must be

examined

most

critically.

Comparison

of the

laboratory

tests

with field

experience

The

materials tested by the authors and determined

o be

stable

are,

with

one

exception, sandy

gravels

with

a maximum

size

n the

medium

gravel

rangeand a coefficient

of uniformity,

U

(=

Doo/Drc),

generally

ess

han

0

but occasionally

s

high

as

about

5;in comparison,

he

unstablematerialswere

coarser,

but more

broadly

graded

with a maximum size consistently

n

the coarse

gravel

rangeor larger, had ess han30Vo

andsizes,

and had

a coefficient of uniformity

generally

n

excessof 20.

Such materials,both

stableand unstable,are

generally

coarser

than he

uniformly

graded

sand-rich"

filters ong

advocated y

Ripley

(1983)

and

discussed

y Sherard t al.

(1984);

neverthe-

less, many

dams do have such broadly

graded

coarse ilters

adjacent o the core.

The

authors determined that the most significant

factor in

internal

stability of the filter materials

was

the

particle

grada-

tion:

"the

absolutesizesof the

particles

are of little

importance

in

comparison

with the shapeof the

grading

curve."

This forms

the basis

or

assessingtable r unstable

radations

s ndicated

in Fig.

l.

This

simple shape elationship s used n

Fig.

2

to

examine

he

grading

stability of

coarse ilter materials

n dams

where adverse eepage ffects havebeen

eported,

by Kjaernsli

and

Torblaa

1968)

or Hyttejuvet Dam and by

Vestad

1976)

for

ViddalsvatnDam; the

gradation

of an unstable

ap-graded

filter

material eported y de Mello

(1975)

s alsoexamined.

Al l

plot

definitively as

potentially

unstable

ilters.

In contrast,

agreement

s

poor

for

even the

finer filter

gradations

for

Balderhead am as eported y VaughanandSoares1982)and

for the

Churchill

Falls

dykes reportedby Seemel nd

Colwell

(1976)

and

which

are

plotted

in Fig. 3. The mechanism

of

distress

eported n all casessolely related o

"fines"

from the

core

materials

iping

throughcoarser

ortions

of t he filters;

no

direct evidence

s

given

of the

filters

in themselves

eing

unstable.

t is interesting o

speculate

f

such could have

occurred,

articularly

n thosecases

where

cracking

of the core

was

marked, as

in

BalderheadDam, thus

producing

possible

critical

lows directly

across

ocal

filter zones.

rPaper

by T.

Journal.

2 ,pp.

C. KenneyandD. Lau. 1985.Canadian

cotechnical

2

5 -225 .

V.

Mrr-lrceN

GolderAssociates,

5

WhartonWay,

Mississauga,

Received anuary 7, 1986

Accepted ebruary

,

1986

Canada IAX 286

The method

of

testing, as described

n a companion

paper

n

this

ournal

(Kenney

et al. 1985), nvolved seepage

elocities

higher than

those

generally

experienced

n

the filter zones

of

most dams;

the samples

were

also

subjected o mild

vibration

during

testing

and, as the authors state,

"vibration

had

a

profound influence

on the behaviour of

some

of

the

tested

materials

or

which

even he

mildest

vibration

ncreasedhe

oss

of

fines."

This

combination of high seepage

elocities

ogether

with lengthy

vibration is not likely to be encountered n most

dams;

however,

even

if

the

laboratory conditions appear o

be

stringent

in

comparison

with

probable

field conditions,

the

writer agrees

hat

"potential

for instability" could exist

n

local

zones n many dams where such coarselygradedfilters have

been

used.

Potential for segregation

It is

of

interest to the

writer

that the

gradations

of the

"unstable"

coarsematerials ested end o

be ypical of materials

that segregate

eadily.

Such

materialsas reportedby Sherardet

al.

(1963),

Woodward

et al.

(1969),

and Sherard t

al.

(1984)

are

plotted n Fig.

4,

only the ast being appropriate o a

granular

filter.

It

may be

noted that the boundary between he

"stable"

and

"unstable"

gradings

tested by the authors

(Fig.

I of the

paper)

approximately

coincides

with

the coarse boundary

suggested

y Sherard

et al.

(1984)

o limit segregation f

filter

materials.

In

practice,

such segregationcan

produce

serious

seepage roblems.

Optimum

density

is not optimum

for internal stability?

It may

also be

inferred that the

factors affecting

hydraulic

instability

or stability

of the

material are apparently

quite

different

from those

affecting

density.

(The

same

actors

also

relate o

mechanical

tability suchas rutting

and he

ike.)

The

ideal

gradation oroptimum density

s shown n Fig.

5 and

given

by the

equation

Fuller

and Thompson

1907)

Percentage

assing

given

sieve

100

(aperture

ize

of

thi sieve/size

of

lutg.tt

particle n ihe material)r

Fuller's

curves

are similar o the

gradations

f

sandymaterials

containing

optimum

gravel

content

given

by

the United States

Bureau

of

Reclamation

1963).

n highway

practice,

however.

maximum

densities

have been

produced

more

readily using

material

on

the

fine side of the

optimum

gradation

Asphalt

Institute

1958X

his

ange

s also

ploned

n Fig. 5.

lt may be

seen

that such

gradations or optimum density

colrespond

with

shapes

hat

are ypical of

potential

hydraulic

nstabil ity.

This

trend

s even

moreapparent

whenconsideringhe

gradation nd

compaction

characteristics

f

pervious

ill for

Oroville

Danr.

reported

y

Miller

(

1965) ndshown

n Fig.

6,

and

which s

no t

dissimilar

o the

coarsestmaterials ested

y the authors.

As

the

percentage f

material iner han

he

No.

4

sieve

size ncreases.

the

material

apparently

becomesnlore

hydraulicallyunstable

(Fig.

6b),

but

n

contrast,

he compacted

ry density

markcdly



DISCUSSIONS

G RA O ING

l

IA

5

o.q

)

z

0 . 8

F

(E

lrj

J

J

z

VT

z

o

F

E

o. q

t

l

0. 2

0.2

S HA P E

C U R V E

UN

TA

LE

G RA O IN

S

H = ( 1 . ? ) F

/

,ro"rry'

"'x

G R A D I

G S

N A

0. q

0 . 2

:

I

I

q n

D

I+D

GRlrr.r

tzE

D

(LOG

SCALE)

Frc.

l. Method

of describing

he

o .

l . o

l 0

1 0 0

G R A I N I Z E ,

m m

/

I

/

,52:

H

ma s s FRACTIo N

ETwEEN

D n Ho

tO

shape

of

a

grading curve

(after

the

authors)'

Frc.

2.

Problem

filter

gradations-I.

z

-

F

u 6 0

z

tr

F

z

lrj

5

'ro

o-

increases

Fig.

6c).

(It

is also

nteresting

o

note hat

even

a

short

period

of

vibiation

densifies

he

compacted

material

urther.)

It

is

difficult

to

accept

hat

materials

of

gradationso appropriate

for

optimum

deniity

are

apparently

so

suspect

n

terms

of

hydraulic

nternal

stability.

Does

instability

only

result

under

high

seepage

lows?

Is

there

a

critical

level

below

which

the

materialsare

stable?

Application

to

practice

In

most

seepage

problems

in

dams,

it

is the

silt

(or

finer)

particle

sizes

thit

-are

lost;

seept*,le

velocities

are

rarely

sufficiently

arge

o

transport

sand-sized

articles.

Conventional

methods

ideslgning

filters

for

dam

cores

are

herefore

directed

to

inhibiting

the

movement

of

such

"fines"

and

to

ensuring

an

adequate

degree

of

permeability

of

the

filter

material,

say

in

.i..5 of lg'-am/s. The hydraulicconditionswithin the filter

are

not

considered.

This

thb

authors

have

carefully

examined,

and

determined

he

potential

hazatdof

the

internal

instability

of

coarse,

broadly

grid.O

materials

when

used

as

filters.

In

practice,

in

a

uroaoly

graded material

under

a

high

hydraulic

gradient

some

t.*ungt-ent

of

particles

is

inevitable;

this

iesults

n

a

local

increaie

n

permeability

of

the

material

with

a

consequent

eduction

n

the

hydraulic

gradient,leading

n

turn

to u

nr*

equilibrium

state.

Under

a

new

hydraulic

gradient

he

process

may

be

repeated-

Such a

process

s exemplified

by

iepeated

oises

of

i'fines"

as

in

samples

A

a1d

As,

shown

n

fiir

4 and

5

of

the

paper.)

However,

with

each

uccessive

oss,

inJreasingly

largei

quantities

of

water

tend

to

be

required.

i

o

^

x

F I N E L l t ' t l T

I v T O D A L S v A T N

O A M

F T L T E R ( S )

C O A R E

L I I , I I

I

5 A N 0 Y

R r v E L

1

H y T T E J U v E T

D A r . 1

T L T E R ( S )

G R A V E L

J

6 A P .

G R A O E D

I L I E R

(

D E

I ' I E L L O

1 9 7 5

H

= (

. ? )F



416

CAN.

GEOTECH.

.

VOL.

23.1986

F I L T E R

S )

r 0 0

z

r

F

f r o o

z

L

F

z

lr,

S q o

o-

:

; X l : l . * l iRocEssED

"

l

cxuRcx rLL

ALLs

DyKE

a

Ft i lEST

.l

-

- /A V E RA G E "

I

B A L DE RHE A o

A r . , t

I L TE R( s

.

COARSEST

/

t . 0 t 0

GRAI I {

|ZE

n m

0 . 2

F

Ftc.

3.

hoblem

filter gradations-Il.

z

F 6 0

E,

lr,

r

F

z

14,

v q 0

G

l4l

o-

r 0

r 0 0

0

0. 1

S l Z E , m m

Flc. 4.

Materials

subject

o

problems

of segregation.

0.2

F

t . 0

GRAI N

3

]

, , t a o * a c A B L E , , f i A T E R T A L .

H r L L s

c R E E K

D A , . l

I

t

R A N 6 E

F

G R A D A T T o N s

F

, ,

s E G R E 6 A B L E '

. T A T E R T A L s

r { A G 6 0 N E R

TA L .

r 9 6 9

X

C O A R S E S T

I T 1 I T

O R

F I L T E R S

O

L I I . t I T

S E G R E G A T T O N

S H E R A R D

T

A L .

I 9 8 . {

ry

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: l

, ,

P RA CTICA L

RA NG E

F

G RA O A TIO NO RO P TIHUHCO I lP A ( T IO N

, ,

IDE A L

I I

G RA DA TIO N

O R

O P T I I . I U I . IO T . IP A CTIO N

"

FUL L E R 'S CURV E

T

z

-

- 6 0

G

lr,

z

lt

z

U q o

(E

t

o-

r . 0

t 0

G R A I N l Z E , m m

1 0 0

0

0 . t

0 . 2

F

Frc. 5.

Materials

ideal"

for optimum

compaction'

o

A FTE R

5

I ' , I I N U T E S

I E R A T I O N

.

AS

COI. I

ACT

D

NO VIBRATION

5 1 0

1 5 2 0

F I N E R

HA N 9 S IE V E

(

5 MM )

o

r < n

E

to

l t {5

z

* 6 0

CE

U

=

\

F

z

U q o

E

trJ

r . 0 t 0

G R A I NI Z Em m

0 4

f 0 0 0

0 . t 0 . 2

F

Frc.

6. Compaction

data-Oroville

Dam

fill.

0.?

0. ' {

0

P E RCE NT

(1

blcu

t

:

16kglm'.)

Repetition of this

process

n the

filter adjacent o the core of

a

dam

can

ead

o

progressive

ore

damage.

When

hecore tself

s

of

suspect

nternal stability,

as n Balderhead

Dam,

the

damage

is

exacerbated.

o

prevent

suchdamage, t is

generally

accepted

acceptable

ilter

gradationhas

been

postulated y Sherard

et

al.

(1984)

and

is

plotted in Fig.

7;

it

is compared

with the

filter

gradingsexamined

by

the authors

n

their

Fig.

l3

and

also with

the

suggestions

y

Lowe

and

Binger

(1982)

o ensure

dequate

self-filtering

of

a

widely

graded filter

material.

Perhaps

not

surprisingly,

a

"coarsest"

envelope

can be

drawn

hrough

all the

dati.

It

is

suggested

by

the

writer

that

this

represents

he

"coarsest"

acceptable

grading for

filters;

for most

cases,

he

"finest"

acceptable

grading

corresponds

o that

of concrete

o

I

p E R v r o u s

f i g A N K r . r E N T

r L L , o R o v r L L E o At i

o J

( b )

.{

I

I

I

I

i r

( ) %

<

r {

t E v E )

H = ( 1 . 3 ) F

(

20%

>

,

{

SIEVE

(c )

o o

e e

i o

3 l

o 1 9 I

+ .

5

o l i

" l

a

' . 0

i

o i .

? l

r T

. i

a



4 1 8

CAN.

GEOTECH.

. VOL.

23.

1986

- . -

F I L T E R

xA MtNE D

N

F t G .

?

O F

p A p E R

-.-- ,,LOI'EST

"

ACCEPTASLE

FIG.

13

OF PAPER)

x

,,

FLATTEST

.

GRADATIoN

LowE

a EINGER

982)

O

"COARSESTI

LII,ItT

S}IERARD

TAL. I98,{

Sixth Regional

Conference

for

Africa

on soil

Mechanics

and

Foundation

Engineering,

Durban,

South

Africa. Vol.

2,

pp.

2g5_

304.

1977.Reflections

n

design

decisions

f

practical

ignificance

to

embankment

ams.

G6otechnique,2T(3),

p.

2gl-355.

FurLrn, W.

8.,

and THor',rpsoN,

.

E. 1907.

The

aws

of

proportion-

ing concrete.

Transactions

of

the

American

Society

or

clt'it

Engineers,

9,

pp.

64-143.

KENNEy,

.

C. , CHnHeL,

R. ,

Cntu,8. ,

Oroecu,

G. I . ,

OueNcr.

G

N.

,

and unar

c. A. I

985

controlling

constriction

sizes

of

granular

filters. CanadianGeotechnicalJournal, 22, pp. 32-43.

K-lepnNsu, B.,

and

roRBLne,

I.

1968.

Leakage

hrough

horizontal

cracks

in the

core

at Hyttejuvet

Dam.

Norwegian

Geotechnical

Institute,

Oslo,

Norway,

No.

80,

pp.

39-a8.

LowE,

J., III,

and

BrNcEn,

W.

V.

1982.

Tarbela

Dam project.

Pakistan.

second

Annual

uscol-D

Lecture.

United

states

Com-

mittee

on Large

Dams,

Atlanta,

GA,l03 p.

MnLrn, R. K.

1965.

Discussion

n vibratory

maximum

density

est.

compaction

of soils.

American

society

for

Testing

and Matirials.

SpecialTechnical

Publication,

No.

377, pp.

23-29.

RrpLEy,

c. F. 1983.

Discussion:

Design

of filters

for

clay cores

of

dams. ASCE Journal

of

Geotechnical

Engineering,

109(9),

pp.

l r93- r

195.

Sreurr,

R. N.,

and

CorwELL,

C.

N. 1976.

Drainage rovisions

nd

leakage investigations

of the

churchill

Falls

dams

and

dykes.

Transactions

f

the Twelfth

Internationalcongresson LargeDams,

Mexico City,

Mexico, Vol.

2,

pp.

107-127.

SHERIRD,

. L., WooDweRD,

R.

J.,

GHrzrrNsxr.

S.

F.,

and

CrrveNcrR,

W.

A. 1963.

Earth

& earth-rock

ams.

ohn

Wiley

and

Sons,

nc. New

York,

NY, pp.

629-631.

SurRlRo,J. L.,

DuNNrcAN,

.

P.,

and

T,c,r-Bor.

.

R.

19g4.

ilters

or

silts and

clays. ASCE

Journal

of

Geotechnical

Engineering,

l0(6),

pp .

701 -718 .

UNtrpp Sreres BuRrnu

or Rpcr-euArroN.

1963.

Earth

manual,

pp.

42-44.

VeucH.lN,

P. R.,

and

Soenrs,

H. F.

1982.

Design

of

filters

or clay

cores of dams.

ASCE

Journal

of the

Geotechnical

Engineering

Division, 108(GTl),

pp.

17-31.

VEsuo,

H. 1976. Viddalsvatn

Dam.

A history

of leakages

and

investigations.

Transactions

of

the

Twelfth

International

Congress

on Large Dams,

Mexico

City,

Mexico, Vol.

2,

pp.

369-390.

WaccoNen, E. B., SHrReRo, . L., andCrevENcER,W.

A.

1969.

Geological conditions

and

construction

claims

on earth

and

rock-fill

dams

and

related

tructures.

n Engineering

eorogy

ase

histories.

Edited by

G. A. Kiersch

and

A.

B.

Cleaves.

Geological

Society

of

America, Boulder,

CO, No.

7, pp.

33-44.

F

r r 6 0

lr,

z

L

F

z

lr,

Y e o

U

c

cnarrutzE.mlrl

lo o

Frc.

7.

criteria

o

define

coarsest"

gradation

imit

for

filters.

sand.

The

writer

agrees

hat

broadly

graded

ilters

should

be

avoided.

.

I

is

strongly

advocated

hat

the

authors

continue

and

extend

their

work.

In

addition

to

questions

aised

n this

discussion,

possible

aspects

ould

inclube

the following:

-The

testing

of

"sandier"

gradations.

-The

examination

of

a

lower

range

of

seepage

elocities/

gradients.

critical

or

"threshold"

levels

n

meihinically

stable

materials?)

-The

examination

of potential

or

segregation-without

water

flow.

(Does

vibration

inc.ease

segregation?) what

degrees

f

segregation re tolerable?)

-

AspHer-r

Nsrrrure.

_r959.

Asphalt

handbook.

sphalt

nstutute,

Construction

eries

l.

or

MEuo,

V.

F.

B.

1975.

Some

essons

rom

unsuspected,

eal

and

fictitious

roblems

n

earth

dam

engineering

n

grazit.

proceedings

Internal

stability

of

granular

filters:r

Discussion

Jnues

L.

SHennno

P.O.

Box

1416,

San

iego,

CA 92073,

U.S.A.

AND

LonN

P.

DuNNrcnn

soil

Mechanics

Laboratory,

soil

Consertation

service

usDA),

Lint'oln,

NE

6g50g.

u.s.A.

Received

March

26. 1986

Accepted

March

27. 19S6

Can.

Georech.

.23 ,41g_420

l9g6)

The

authors

describe

basic

esearch

n a

subject

f

practical

importance

hat

has

not

heretofore

eceived

much

attention.

at

-

-

rPaper

by

T.

c.

Kcnney

nd

D.

Lau.

19g5.

anadian

eorechnical

Journal.

2, pp.

ZIS-22i.

least

n the U.S.A.

and

Canada.

Researchers

n

Europe

have

studied

n a

general

way

the nrovement

f

sandparticles

nside

sand-gravel

mixtures,

rom

which

activit ies

ave

ome

hene

w

terms

suffusion"

and

"colmatation"

(Wittmann

97tt).

Suffu-

sionhasbeendiscussed

n

the

iterature

n Russia or

more

ha n



DISCUSSIONS

years,

generatinga

volume

of

theory

that

comprisesa

tantial

part

of

east

European

books on seepagehrough

media

Kovacs

98l).

Some

embankment

ams

have

damaged

y migration

of fines

out of the

voids

between

particles n zones

of

coarse,

broadly

graded mpervious

(Sherard

979).

However,

o

our

knowledge,

here

have

no studies

imilar

o

thoseof

the authors,

.e., careful

xperiments

irected

oward

he

practical

roblem

of

he internalstabilityof compacted iltersof sandy

The

authors'

work has reached

he

rather

surprising

onclu-

hat

some

sandy

gravels,

apparently

airly

well

graded,

re

unstable.

Substantial

migration

of sand

particles

within the

densely

compacted

specimens

n

the

ests

mployed,

with thesand

oming

out he

bottom.

fhe

unstable

materials

ested

were sandy

gravels

of

fairly

grain

size

distributions,

generally

considered

s

completely

stable

and suitable

for

use as

filters

in

ams.

For example,

he sandy

gravels

A,

As,

and

are

representative

f

materials

hat

have

been used

as

filters

ransitions

n

many

dams,

and

heretofore

here

hasbeen

no

hat

hey

had

questionablenternal

stability.

fhere

are

several

main

reasons

or surpriseat the authors'

Sandy

gravelsof this

general

gradation

are

common

as

iver

alluvium

n

dam

oundations.

n

deepexcavations

the

water table,

unwatered

by sump

pumping, large

nflows

of

groundwater

can

be seen

seeping

out

of

the

and

inclined

side

slopes.

In

these there

is no

ndication

of

sand

migrating

out

of the

gravel voids and

rmulatging

n the

excavation

bottom as

would be expected

f

material

were

nternally

unstable.

Also, as

part

of studies

or

am

design

t

is fairly

common

o make

aboratory

permeability

ests

on

materials

of

similar

gradation, with no observations

f

significant

migration

of sand

out

of the

bottom.

In fact,

the

authors'

est

appalatus

and

procedures le

very

similar

to those

used for

routine

permeability tests

(sample

ength,

hydraulic

head

applied,

and

details

of

drainage

layer at

the bottom),

except

hat

it

is

not the

practice

to

vibrate

a

permeability est

specimen

by

tapping

t

with a rubber

hammer.

-

We have

been

engaged

during

the last several

years

in a

general aboratory

esearch

rogram

on

filters ordams

(Sherard

et al.

1984a,

b;

Sherard

and

Dunnigan

1985).

While this

research

did

not

include

any

tests

exactly

like those

of the

authors, ur

experience

with similar

tests

Sherard

1985)

would

havecaused

s

to

predict hat all the

sandy

gravels

shown

o be

internally

unstable

n the

authors'

ests

in

their

Fig. l)

were

highly stable.

After

puzzling over

this apparent

conflict

for

severalmonths

ollowing

publication of the

authors'

paPer,

we

madea series

of

laboratory

ests

o see

f we could

duplicate

he

authors' esults.

The

authors'

unstable

materials

A, As,

and

D are

all

very

similar,

with

a nrllrow

range

of

particlesizedistribution.

For

our

tests

we chose

a sandy

gravel

with

particle

size

distribution

n

about

he

middle

of

this nilrow

range,

a

well-gradedmaterial

rvith

particle size

distribution

as shown

n Fig. I

. A total

of

five

tests

were

made,

all on

this

material,

using

the same

general

.rocedures

as

described

or

the authors'

tests,

n a

254

mm

iin.)

diameter,

508

mm

(20

n.) long

plastic cylinder.

A

of

sample

hicknesses

127-254mm

(5-l0in'))

an d

hydraulic

gradients

3-45)

wasused.One

est

wasmade

with

an

additional ffective

stress

f about

138kPa

20

psi)

applied

with

a

spring

load.

The

cylinder

was

mounted

vertically

with

downward

flow.

As

in the

authors'

ests,

a

"drainage

layer"

about 152mm 16 n.) long, or filter, was

placed

underneath

on

4t9

So n d wh rc h c n le rs

d ro tn o g e

mo le r io l

v o rd s

Si

es

Dro inoge

Moler io l

D . ' u :

0 m m

0 . 5

1 . 0

5 . 0

l 0

Por t ic leDiomeler ,

mm

Frc.

l.

Sandy

gravel tesred,

which

is similar

to the

authors'

unstable

aterials

,

As,

and

D.

downstream

side)

of the

material

being

tested.

This

drainage

layer

material

consisted

of uniform

gravels,

retained

on the

19mm

(i

in.; sieve

nd

passingtheZl

mm

(l

in.)

sieve

Fig.

1) .

Both

the sandy

gravel

and

he

drainage

ayer

were

compacted

o

a relatively high density.

In

all

tests,

the

water

was

caused

o

percolate

hrough

the

specimen

irst

for about

20-30

min

without

vibrating.

Then he

specimen

was

vibrated

by

pounding

the exterior

of

the

plastic

cylinder

with many

strong

blows

of a

heavy

rubber

hammer,

with the

flow

continuing.

Sometimes

his

cycle

was repeated

several

times.

At

intervals

over

the

entire

test

all

the

water

emerging

at

the bottom

wascaught

and

he

ratemeasured.

Also,

all sandloming

out

of

the bottom

during

the

entire

test

was

caught

and

measured.

At

the

end

of the

test

the

specimen

was

carefully

dismantled.

All

the sand

caught

in the

voids

of the

drainage

material

was separated

ut,

and

the total

weight

and

particl- size

distribution

measured.

n

two

of

the five

tests he

ihickness

of

the specimen

before

and

after

was determined.

n

one of

these,

he thicknessandgradationof the four individual

compacted

ayers

of

the

specimen

were

measured

before

and

after

the

test.

All

tests

gave

he

same

esults:

he

specimens

cted

as

wholly

stable

materials.

1.

Before

vibration

very

little sand

migrated

out

of the bottom

of the

specimen

nto

the

voids of

the

drainage

material.

2. Aa

the

result

of

the

vibration,

a

small

quantity of

sand

emerged

mmediately

from

the bottom

of the

cylinder

with the

flowing

water.

When

the

vibration

was

erminated

he

sand low

essentially

topped.

3.

The

amount

of

water coming

out

of

the bottom

of

the

cylinder

ranged

oughly

between

15 and

600

ml/s,

depending

on

the

head.

This

remained

essentially

constant

or

constant

head

applied

and

gave

essentially

he

same

omputed oefficient

of

permeability.

For

all

tests,

ft

was

approximately

0.02-

0.03 cm/s.

The

flow

through

the

specimen

was

not changed

significantly

by the

vibration

or sand

migration.

+. ttt.

total

quantity

of sand

hat

migrated

out

of

the

bottom

of

the

specimens

nto

the

voids

of

the

drainage

layer

varied

greatly with the

hydraulic

gradient and

velocity

of

flow,

gen-

erally.t

follo

Approximate

APProximate

otal

gradient

sand

migrating

g)

45

3

@ N {

\ :

O - O

S i e v c

o o o o

9 - d

a r l r

o

6

o

. O O

, t

I

L

o

i i

uo

c

o

o

o-

t n

0

500

25



420

CAN. GEOTECH.

.

VOL.

23.

1986

Since he

areaof the specimenwas about

500 cm2,

or the high-

gradient

tests the

quantity

of sand that migrated

out of the

bottom

of t he specimen

was

about 1.0

E/cm2.

This

value

of

l.\g/cmz

was

the necessary

penetration

of sand

from the

specimen nto the drainage ayer to

enable he drainage

ayer

o

act as a

filter and

prevent

urther

sandmigration.

Assuming

hat

the compacted sand formerly had

a dry

unit

weight

of

2.09/cm3,

then about 1.0/2.0

:

0.5cm

(5mm)

of the

specimen ength at the lower interfacepenetratedhe voids of

the

drainage-layer

gravel.

This decrease n

specimen

ength

(5

mm) is consistentwith

the measured hange

n

length of

the

specimenn the spring oad

est

(0.19

n.

:

5 mm).

5.

About 80 or 90Vo f the

sandmigration nto

the

voids

of the

drainage

ayer occurred

as the result of the vibration

imposed.

6. In the one est n

which

the

particle

size

distribution

of the

upper

three individual layers

of four

compacted

ayers n

the

specimen

was

measured

efore

and

after

the

test, there

was

no

measurabledifference,

showing again

that all

the sand

that

migrated nto t he voids

of the

drainage

ayer

came

from

the

bottom of the

specimen,

directly

adjacent

o

the

upper

face

of

the drainage ayer.

7.

In all cases

he

particle

size distribution

of the

small

quantity of sandthat migrated into the voids of the drainage

layer

was

approximately

sshown

n Fig.

l,

with

maximum

size

of about2.0mm.

This

is consistent

with

the results

of earrier

tests,which

show

that

the maximum particle

size

hat can

pass

through he

voids

of a uniform

filter is roughly

l\Vc

of

the D15

size

(Sherard

et al.

1984b).

In

this

case

he

D15

size

of the

drainage ayer

is roughly

20mm,

Fig. l,

so

that

it would

be

expected

hat he sandparticles

hat could

enter

he

voids

would

have

a maximum

size

of

about

Zmm.

Conclusions

l. The

authors' experimental

esults

showing

that

sandy

gravesl

A, As,

and D

are nternally

unstable

are

surprising.

2.

We have

subsequently

ade

similar

ests

on sandy

gravel

specimens-

f similar

gradation

that show the material

to

be

stable.

3. The

reason

for

the difference in

the two

groups

of test

results s not

known. More

experiments

are

needed

o reconcile

the difference.

4.

We

believe hat such sandy

gravels

are nternally

stable.

5.

This

is a relatively important

practical

point

from

the

standpointof the useof such materialsas ilters n embankment

dams.

Kovlcs,

G.

1981.

Seepageydraulics. lsevier

cientific ublishing

Company,

msterdam, ew York, 730

p.

Snrnenp,

J.

L. 1979.

Sinkholesn dams

of coarse, roadly

graded

soils.

3th

ICOLD

Congress,ndia,

Vol.

II,

pp.

25-35.

1985.The

upstream

one n the concrete

ace ockfill dam.

Proceedings,

SCESymposium

n

Concrete

aceRockfill

Dams.

Detroit,

MI .

SuEneRp,

. L., and DUNNIGAN,

. P. 1985.Filters

nd eakage

control

n embankmentams.Proceedings,

SCE

Symposium n

Seepage

nd

*akage

from Damsand mpoundments,

enver,

CO,

pp.

1-30.

SnrRnnp,

.

L., DuNNtcnN,

. P., andTlraor,

J. R. 1984c. ilters

for claysandsilts.ASCEJournal f theGeotechnicalngineering

Divis ion,

10,

p.

701-718.

1984b.Basic

properties

f

sandand

gravel

ilters.

ASCE

Journal

f the Geotechnical

ngineering ivision, l0,

pp.

684-

700.

WWirtueNN,

. 1978.Phenomena

nd

parameters

f

two-component

soil.

Symposium

n the

Effectsof Flow

throughPorousMedia.

International

ssociation

f HydraulicResearch,

ommittee n

Flow

hrough

Porous

Media,Salonika,

Greece,

p.

68-80.

I

Internal

stability

of

granular

filters:r

Reply

T.

C. KENNey

eNo

D. Lnu

Department

of

Civil Engineering,

Universin' f Toronto,

Toronto,

Ont.,

CanadaMSS

1A4

Received

ay

12, 1986

Accepted

May 12,

1986

Can.

eotech.

.23,420-423

1986)

We

wish

to thank

Messrs.

Mill igan, Ripley,

Sherard,

nd

Dunnigan or spendingimeandeffor-tn studying ur paperand

preparing

hallenging

discussions.

s is

so often

he

case,

he

value

of the discussions

s

at least

equal o

that

of the

onginal

paper,

and

we

are

grateful.

The

first tem

concerns

emantics

nd he mprecision

f the

term

"unstable

grading."

The picture was presented

hat

a

granular

material

onsisted

f

a oad-bearing

abric

of

particles,

or

primary

abric,

and oose

particles

ocated

n

the

void

spaces

of

the

primary

fabric.

If the

loose

particles

were

sufficiently

rDiscussions

by

V.

Mi l t igan

t986).

C.

F. Ripley

t986).

andJ. L.

Sherard nd

L. P.

Dunnigan

1986).

Canadian

Geotechnical

ournal.

23.

this ssue. p.

255-258.

his ssue.

small

o

pass

hrough

he

void

networkof

the

primary

abric hey

could be removed by the actionsof gravity, waterflow, and

vibration,

and such soils

were

described

as having unstable

gradings.

f

the

primary

fabric and its

void

spaces

were

to

remain

unchanged nder he

actionsof

gravity, water

low. and

vibration, t follows hat here s

a maximum imit to hechange

of

gradation hatcanoccur-particles

canonly be ost f they

arc

smaller than

the

void

network-and

increasesof seepage

velocity

and

vibrationwill

not ead o steadilyncreasedosses.

Therefore,

or anymaterial.

here s an upper imit to the nternal

losses hat are

possible;

hat

is, there s a maximum

imit to

which

a

material

anbe

nternallyunstable.

Some

granular

materials ave

gradings

hat are

potentially

unstable.

t is also rue hat

he authorshave

potential

or bcing



tscussroNs

42r

We would naturally

eny

suchallegations.n trurh,

results.

aking

irsr he estongrading

in

Fig' l, it isappar€nt

ur

.sponse

oulddepend

pon heconditions eing

that

no

change

f

grading

ccurred uring

he est n

aye$2-4

osed.The sime

is true

regarding he instability

of soil

and

below

this a transition

zone

developed hat became

Thus,

when a soil

grading

s described s

being progressively

oaner

owards

hedrainage

ayers. heseesults

as n Fig.

I of the

paper,

i

is

meant

hat t has

he

indicate

hat

Srading

U

is stable nd

we

agree

whh

Sherard

nd

or

losinl

panicles,-but

ne

cannot onclude

eces- Dunnigan's

onclusion

o hiseffect.

n

the

paper

weconcluded

that

he osJs

will occurbecause

ehaviour

s dependent that

gradingAs

was

unstable,

ased

n the results

n Fig. 5,

h.

severity f

$e imposed

onditions.

which ndicate

hat a

homogeneous

oarser

rading

eveloped

We have

ientified

hi

possibility

or

grading

nsrability

ut in ayers

-7. Above hese.layers

here

wasa zone

layers

and

have not given any'guidance on how to evaluate

this

3)

in

which there

was no change

ofgradiflg,

butwe discounted

or held

coniitilons,

nd

n that espect

urwork

s this sa.test

no."ly:

:: :*4i$9

tl*ff^":i^::_":.:-::f:

:omple'te.

nize

hat our nterpretatiol

of

this test

was ncorrect

and hat

he

Weâre

rateful o Sherard

ndDunnigan

or

questioningur unchanged

rading

n

the upper

Pan

of the specimen

as

esults

o the

extent

hat hey

p€rformed

ndependent

ests,

evidence

hat

gradingAs is stable

nd hat

he ower,coarser

hey

eported

hat heir

esults

were nconsistent

ith hose zone

wasa transition

one.

i"p;r

lntum.

we

repeated

he Sherard

ndDunnigan

.

Fig.2 is arevisionof

Fig. 12 n

our

paper.ln

he eft-hand

several

f our earlier

ests.

There

werevictories nd &awing,

grading

As has

been emoved

nd

A' hasbeen dded.

, '

l,l"l,cll-139..9Tty^'.fY_*:.._"".t"':lti9:.j::":jf":1":::

r".utt,

"t consistent;

he

badnewsls

hatour nterpretation

the

boundary

etween table

nd

unstable

radings

avebeen

reston

material

As must

be changed

nd he- oundary

plotted.TheseareCu

12,

U,

As, anda."j:ll31"l"j:lryf:

,,uUte

and

unstable

gradingsmrist be modified. the

enerF standing

or

Fuller

curve.

It

wasMilligan

who made

Strerara

na

Dunnigan

periormed estson a

grading

hat

was

the

observation

hat

the conclusions

n our

paperwould ead-to

riJaf.

,-s.

oT

A,'Ar, and

D, and

we ill reier o

it as

condemning

heFuller

gradation sbeing

unstable, nd

o him

U. thev lound hismaterialo be stable nd oncluded this did not seemcorrcct.Although

grading

F is

a slight

UE""ur.

gradingU appeared

o be

similar o

gradings

and modification-of

.Fuller

urv-e

particles.smallerthanand ize

dweriilsostable,incontrastioourfindings

arenot ncluded)he esttesults

rcsenled

n Fig. I

for

grading

nd

P",ere

unstable..we

epeatedur estsôn

(new

19l:,lh: ry1* 3::-d l YitliqT-9i1.1,"-I*g.i$i:i1:

')

andD and

performed

a test

on

Sherard

and Dunnigan's

shouldbestable. n

Fig. 2 thesuggested

oundary €tween

tsble

. Our

reiultsconfirmed

hat

gradingsA andD were and

unstable

radings

asbeen

evised

rom hat

n our

PaPer.

(jf,

=

0.12and0.10,

respectively,imilar

o resultsn

The boundary

we originally

suggested

as the

lrebotsjkov

'

'.le

t oi o*

paper),

and we found

grading

U to be stable,

n curve

and the

boundary

we now suggest

s the Fuller

curve.

.e-ent

*ittr-

i,e

hnding.s

f

Sherid

ani Dunnigan.

ow-

it*""g

rh3

"lft

_dTt-JI -ti:^:TPf}: :.:.i":i J::

gradingU is essentiilly

he same s

grading

As,

which, inclination

hould

be slightly

cduced.

o

include uwesU

and

in our-paper]

we concluded

was unstable.

-This

inconsistency

As),

its advantage

s that

the inclination

in a

l1-F diagram

s

will bead&essed

n the ollowing

paragraph

nd

we are ndebted

l:1, a relationship

easily remembered.

to Sherard

nd Dunnigan

or thiii carlfui

investigation,

which

In summary

of Fig. 2, and

in rcply

to the

discussion

by

has ed to affirmation

of the

test results, mprovement

of inter- Sherard

and Dunnigan,

(i)

gradingsA and D are

unstable,

ii)

pretationfthe est esults, ndmodification f ourconclusions. gradingAs is stable, nd iii) on thebasis f new estdata he

Some

fthe

results

four teston

grading

U are

presented

n boundary

between

stable

and unstable

gradingshas been

the ight-hand

ortion f Fig. l,

andbecause

radings andAs revised.

l1eesientially

dentical thJresults

or

gradin

U

canbe directly One

of many

ntercsting

points raised, y Milligan

concerned

compated

wiih the resutts

or

grading

As in

Fig. 5 of our

papei.

what

he observed

n his

Figs.

I

a1d as

an absence f

any

In both

tests he

specimensizes

were dentical,

the nitial

dry

relationship

between

rading

or

hydraulic

stability and

grading-

Oinsiti"s

t"." Z. i/m3,

the otal

luxeswereabout

00ml/s,

for optimuh

density.

It

is difficult

o

acceptbatmaterials

f

and he

reductions

n sample

hickness

were less han 3

mm. gradation

so appropriate

or optimum

densityare ap?arcntly

o

These

aluescompare

los;ly

to those eported

by Sherard

nd

suspect

n

terms of

hydraulic

internal

stability."

In fact, the

Dunniganor

their estson

grading

U. The

outwashrom

our

gradation

hat

gives

heminimum

ompacted

ensity

s hatof a

rcst aiparatus

was

600-760

g

;d

in addition there

was

perfectlyuniform.material,

andsuch

a

gradationhas he

highest

upp.oii11ut"ty

an equalmass

oi foreign

particlescaught

n the degree f

hydraulic

stability.

The density

of sucha material

an

dilinage

ayers,

giving

total

massoi

trinsported

anicles

of beincreased

y

addingines

utunlessthese

ines rcof suitable

aUout

-tOUj-

tiOb

g,

'about

t*i." the

qdntity

reported

by

gradation

to resist

movement

hrough

the

void chamels he

Sherardand Dunnigan. The mass of o:ansporied 'anicles s combined radationwill beunstable.-lnhe exteme, if thevoids

affected

y

heduratlon

fvibration,and

t is notsurprising

o us ofthe

unifbrm

material

were

packed ull ofsmall

pafticlesn a

that we ciused

greater ossessimply

because

we liand-tapped

dense

arrangement,

he

density

would

be maximum and

the

our

apparatus

6r immoderate

eigitrs of

time. The

material

mixture

would be hydraulically

stable

btcause

ofthe absence

f

.ntering

our drainage

ayers

was

-significantly

more coarse-

loose

panicles.For any

panicular.$adation

ydraulic tability

graineithan

hat epineaby

StrerarOrd

Dunnigan, espite

he increases

s

densityncreases,

ut,by

tselfdenrrty

s not

a valid

iact

hat

we useda'finer-griined

rainage

ayer; he

eason

or criterion

y which ojudge

hydmulic

tability

f a

granular.soil.

is almost

cenainly

-because

we uled ionger

periodsof

Eachof

the contributors

as

Primarily

concemed

ith the

_.ration.

n summary,

t appears

hatour tests

n

giadings s application

o

practice f some f

theconclusions

f th€

paper'

U

ur" .onsistent

ith

the

ests y Sherard nd

dunnigin

on

Ripley

*as corcemed

bout

-the

use

n dams f

widely

graded

grading

U.

-

filiers,

particularly

ecause

f inevitable

ccurrence

f

segrega-

The issue

now boils

down to

the interpretation

f these

tion. His

discussion

resents

historical

nd

perspective

eview



422

SAND

GRAVEL

Fine lMed iuml Coarse

Fine lMedium Coarse

Grain

sizeD, mm

0 . 6

1

2

6 1 0

CAN. GEOTECH. .

VOL.

23.

I9E6

20 .06 0.1

0.2

0.8

0.2

SAND

GRAVEL

Fine

lMe diu m l Co ars e F in e lMe d ium Co a rs e

Grain

sizeD,

mm

0 . 6 1

2

6 1 0

.06

0. 1

0.2

1 . 0

20

0

60

c

E

o.B

o

E

E

0.6

a

c

o

8

0. 4

o

U)

(U

E

o.z

TL

SpecimenF

t-

E

E

o

C\

I

t

l--z+o

r

--l

t/

tnitiar

radineFl/

6

Layers

, 3 ,

4,

5, 6

$

i:

z7'

SpecimenU

ffi

f

3-----t----1

Fril

-r

E

E

@

| - . z + o

m J

///

rnitiat

ractine:-ZUf

.q

Layers

2,3

ct// '/ /

>

, 4 - - - + z u n

; '

r U ]

"gl

u I

1.0

0. 8

0. 6

0. 4

0.2

Flc. l. Results

f tests

on modified

Fuller

grading

F andSherard nd

Dunnigan's rading

U.

0.2 0. 4

F,

mass raction

maller

ha n

of filter design

or

dams

and n

a

polite

manner,

eflecting

his

own

personality,

e

chastises

he

profession

y

pointing

out

hat

Tany

of today'sproblems

ising

from

the

use

of

widely

graded

filters

are the

result

of

not

following

successfur

esign

and

construction practices

developed

decades

ago.

His

central

theme

s that narrowly

graded

materials

will

never

seriously

segregate,

hey

are nherently

table

or

conditions

f

seepage.

(b)

r t

STABLE

GR A

l l

) INGS

I

I

\.'

"il

)

,r

C u = 1 2

/-j

,i-l

'/'".;'rr

i

I

l-lr''

I I

.'l-----

Suggested

i

stable

nd

u

w c l

l

_=_ el

I

roundar

nstable

'y

between

gradings

i

4.:'

0. 2

0.4

0.6

and

any

additional

ost

or

screening

s money

well

invested

The

discussion s

presented

with

devastating

ogic

and

is

recommended

eading

or

dam

designers

nd

students

f the

geotechnical

ield.

Mill igan, Sherard

nd Dunnigan.

and J.-J.

Pard

in

privare

communication)

ecognized

he

seriousness

f

segregationn

widely graded

ilter

materials

ut were

not

asconcerned

bout

t

1 .0

o

$

o

F

o.€

o

c

o

o

3

o

-o

.E

0.4

o

(E

.:

o

q,

(

E

T

0.2

0.6

0. 4

odl0. 6

F,

mass

raction

maller

han

Ftc. 2.

Shape

curves

of selected

unstable

and stable

gradings

and the revised

boundarybetween

stable

and unstable

gradings.



Iter

material

hat would

be classified

s

unstable

n he

basis f

our

paper

and that

had been

subjected

o

prolonged

eriods

of

seepage

ithout

showing

signs

of

instability.

He

alsocorrectly

observed

hat stability

of a filter would

depend

n its

thickness,

among

other

factors.

The

general

eeling

of

the

discussers

as

that

our findingswere

oo conservative

o

ustify

using

directly

in design.

423

We share

the

same

reservations

as

the

discussers.

Our

findings elate

o

worsr

conditions.

Additional

work

is

pranned

in

which

only

seepage

s

used

o encourage

he

movement

of

loose

particles,

a condition

hat

is closer

o

most

geotechnical

field

conditions

han

the combined

low

and vibration

used n

our

tests.

Again,

we

are

ndebted

o the

discussers

f

our

paper

and o

people

who

have communicated

rivately.

It

is clear

hat

this

subject

is of interest

to

many

and that

there

remain

many

uncertainties.

DISCUSSIONS

as

Ripley.

Rather,

they were

more

concerned

hat filter

materials

hat hey

believedwere

competent

would

be

udged

as

rstable

y the

findings n

our

paper.

Par€

mentioned

estson

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