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8/18/2019 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
8/18/2019 Kenney Lau Discussion Reply
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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
^"^i
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 -^;^,,- -i^i-- 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.
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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
,upu^Uitity
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
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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.
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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
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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
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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
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O
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S H E R A R D
T
A L .
I 9 8 . {
ry
/ -4^
*/ _z /./
- ,--,/ -7
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'
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./
8/18/2019 Kenney Lau Discussion Reply
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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
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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
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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
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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
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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$9
tl*ff^":i^::_":.:-::f:
:omple'te.
nize
hat our nterpretatiol
of
this test
was ncorrect
and hat
he
We^are
rateful o Sherard
ndDunnigan
or
questioningur unchanged
rading
n
the upper
Pan
of the specimen
as
esults
o the
extent
hat hey
p€rformed
ndependent
ests,
evidence
hat
gradingAs is stable
nd hat
he ower,coarser
hey
eported
hat heir
esults
were nconsistent
ith hose zone
wasa transition
one.
i"p;r
lntum.
we
repeated
he Sherard
ndDunnigan
.
Fig.2 is arevisionof
Fig. 12 n
our
paper.ln
he eft-hand
several
f our earlier
ests.
There
werevictories nd &awing,
grading
As has
been emoved
nd
A' hasbeen dded.
, '
l,l"l,cll-139..9Tty^'.fY_*:.._"".t"':lti9:.j::":jf":1":::
r".utt,
"t consistent;
he
badnewsls
hatour nterpretation
the
boundary
etween table
nd
unstable
radings
avebeen
reston
material
As must
be changed
nd he- oundary
plotted.TheseareCu
12,
U,
As, anda."j:ll31"l"j:lryf:
,,uUte
and
unstable
gradingsmrist be modified. the
enerF standing
or
Fuller
curve.
It
wasMilligan
who made
Strerara
na
Dunnigan
periormed estson a
grading
hat
was
the
observation
hat
the conclusions
n our
paperwould ead-to
riJaf.
,-s.
oT
A,'Ar, and
D, and
we ill reier o
it as
condemning
heFuller
gradation sbeing
unstable, nd
o him
U. thev lound hismaterialo be stable nd oncluded this did not seemcorrcct.Although
grading
F is
a slight
UE""ur.
gradingU appeared
o be
similar o
gradings
and modification-of
.Fuller
urv-e
particles.smallerthanand ize
dweriilsostable,incontrastioourfindings
arenot ncluded)he esttesults
rcsenled
n Fig. I
for
grading
nd
P",ere
unstable..we
epeatedur ests^on
(new
19l:,lh: ry1* 3::-d l YitliqT-9i1.1,"-I*g.i$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
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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.
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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
Recommended