No. 4 of 19

Geosynthetics for Filtration

by

Jean Lafleur

Dept. of Civil, Geological and Mining Engineering

Ecole Polytechnique de Montréal

The information presented in this document has been reviewed by the Education Committee of the International Geosynthetics Society and is believed to fairly represent the current state of practice.

However, the International Geosynthetics Society does not accept any liability arising in any way from use of the information presented.

Retain particles of the base soil to be filteredAvoid piping

• Allow free flow of water- upstream of the filter

Avoid external clogging(With unstable soils)

- through the filterAvoid internal clogging

• Survive construction and environmental stresses

• Function can be provided by either natural aggregates or by Geotxtiles

Functions of a Filter

S IM IL A R IT IE S

- T h ick n ess

- R isk s o f in tern a l c lo g g in g b y

2 . a ero b ic b a c ter ia l a c tiv ity (o ch re c lo g g in g )3 . d e ic in g sa lt p rec ip ita tio n4 . ic e len s fo rm a tio n w ith in th e fro st p en e tra tio n zo n e

1 . f in er p a r t ic le s o f th e so ils to b e fi ltered

2 5 - 4 0 % 7 5 - 9 5 %- P o ro s ity

M ed iu m to h ig h

F a cto ry -co n tro lled m a ss p eru n it a rea a n d th ick n ess

A ltered b y u ltra v io le t ra y s

Va r ia b le

L o w to h ig h

- C a p illa ry r ise hc

N o n e

N o n e

- Ten sile stren g th

N eg lig ib le- C o m p ress ib ility

In v a ria b le

Va r ia b le g ra d a tio n a s p er b o rro w p it

C o m p le te ly in ert

M u st n o t b e co n ta m in ed b yth e su rro u n d in g so il.

C o m p a ctio n n eed ed

S u b jec t to p u n ctu re a n dtea rin g

M u st b e in sta lled in in tim a teco n ta c t w ith th e so il to b e fi lteredIn sta lla t io n ea sed b y sea m in go f th e jo in ts

- Tra n sm iss iv ity u n d er co n fin in g s tress

- U n ifo rm ity

- D u ra b il ity

- In sta lla t io n

- R isk o f d a m a g e

D IF F E R E N C E S

A G G R E G AT E S G E O S Y N T H E T IC S

H ig h ( 1 5 0 m m ) L o w ( 3 0 m m )

L o w to n o n e ( 5 0 m m )hc Im p o rta n t ( 5 0 0 m m )hc

Geotextile = Catalyst

• Promotes equilibrium of particles after limited washout of finer particles by inducing a self-filtration zone (bridging) at the interface (Lafleur et al., 1989)

d 0 O F d100 d0100

log d

0

O R IG IN A LG R A D ATIO N

R R 2 R R m - 1 R R

d p 1 d p 1 d p 1

p m

p 3

p 2

p 1

LAYER m

LAYER

3 LA

YE

R

2

LAY

ER

1

FILTRATIO N OPENING SIZE O F

FILTRATIO N OPENING SIZE O F

(100 - )p 1

d 1 0 0

dp1

100

y

PERCENT IN M ASSCO ARSER THAN O F

H SF

INTERFACEFILTRATIO NOPENING SIZE O F

FLOW

m

3

2

1

NO T AFFECTED

BYSELF-

FILTRATIO N

H

y ml

y 3l

y 2l

y 1l

y m

y 3

y 2

y 1

H

Filtration Behaviour

• Clogging: the voids of a medium are progressively filled by solid matter to the point that the passage of water is compromised

- Decrease in hydraulic conductivity

• Internal clogging

- By mineral particles- By precipitation and chemical deposition in the voids by water containing iron, de-icing salts- By biological growth encrustation in aerobic conditions

Base - Filter Interaction

in tern a l

In terfa ce

P ip ed p ar tic le s C on tin u o u s p a th s

ex tern a l

C logg in g p ar tic le s F ilam en ts / F ib res

t G T

Filter Applications

• Wrapping of trench drains (Koerner, 1998)

D

B

T

Soil subgrade

Stone base

Pavem ent Shoulder

GeotextileCrushed

stone/perforatedpipe underdrain

1 4 l Weephole

G eotextile

1 4 l Weephole

G eotextile

1 4 l Weephole

G eotextile

Filter Applications

• Wall drains

W eeph o le

G eo tex tile

H

Filter Applications

• Erosion protection (Pilarczyk, 2000)

RIVER

PO ND

BUILD ING

G EOTEXTILE

Filter Applications

• Earth and rockfill dams (Lafleur & Paré, 1991)

M ORAINE CORE GEO TEXTILE

ROC KFILL(CLOSU RE PHASE)

(R A IS IN G PH A SE )

DUM PED

FILL(D rain ing)

SETTLEMENT

d e

CO

MP

RE

SS

IBL

ES

OF

T C

LA

Y

DRAINING LAYER

H

Filter Applications

• Vertical consolidation drains (Van Santvoort, 1994)

Filter Applications

• Silt fences (Holtz, et al., 1997)

S ed im en ts

G eo tex tile

S ed im en t-carry in g

sh ee t ru no ff

W a ter su r fa ce

“ C lea r” w a ter

Tu r b id w a te r

Mai

n su

ppor

t po

sts

Clo

gged

fab

ric

Flo

w

H

X

h 1

h 2

Filtration Flow Conditions

• Dynamic vs static

C O N D IT IO N S M O R E S E V E R E

D Y N A M IC , P U L S AT IN G ,C Y C L IC

S TAT IC , U N ID IR E C T IO N A L

E R O S IO N P R O T E C T IO N (WAV E S , C U R R E N T )

W A L L D R A IN S

S ILT F E N C E S

E A RT H & R O C K F IL L D A M S

V E RT IC A L C O N S O L ID AT IO N D R A IN S

R O A D D R A IN A G E (T R A F IC S T R E S S E S )

R E D U C E F ILT E R O P E N IN G S IZ E

Laboratory Filter Characterization

• Opening size O90 by wet sieving

FEEDIN G WATER SPRINKLER

SOIL PARTIC LES

GEOTEXTILE SA MPLE

COLLECTING TROUGH

SH AKER

FILTER PAPER

Filter Characterization

• Constriction size vs filtration opening size

PLANAR CONSTRICTION

FIBRE

PARTICLE

t G T

P a r tic leF ib erP a th s

FILTRATION OPENING SIZE OF =SIZE OF THE LARGEST CONSTRICTION OF THE LARGEST CONTINUOUS PATH

Filtration Behaviour

• Theoretical opening size of the filter given fibre

diameter df of non woven geotextile

(Giroud, 1996)

Geotextile thickness / Fiber diam eter, t GT / d f

200

0.95

0.90

0.80 n = 0.70

160120804000

2

4

6

8

10G

eo

tex

tile

op

en

ing

siz

e /

Fib

er

dia

me

ter,

O10

0 /

df

Filter – Base Soil Interaction (Lafleur, 1999)

• Mechanisms and parameters

RR = Retention Ratio

RR = Opening Size of the Filter (Of)

Indicative Size of the Base (di)

Gradation Curve of Base

di = indicative size of the base

10 0

90

70

50

30

10

0

Particles size

Pe

rce

nt

pa

ssi

ng

(%

)

di = d 8 5

uniform

di = d 3 0

concave upward

d i = d 5 0

rectilinear

d i = d G

gap-graded

Mechanisms

• Piping: extensive washout of base particles leads to clogging of drainage system downstream by washed out particles

Piping

R R 1

Local

Initial

O FO F

K B

K F

K B

t

Piping

R R 1

Local

Initial

OFO F

K B

KF

K B

t

Base gradation curve

po

sit

ion

tim e K B o

= hydraulic conductivity K = baseB = filterF = initialo = finalf

K B f

K F

K B

PIPING

Initial

Local

Blinding

R R 1

tK BO FOF

BRIDGING

R R 1

Base gradation curve

tim e K B o

po

sit

ion

KF

K B

= hydraulic conductivity K = baseB

In itial

Local

= filterF = initialo = finalf

K B f

Mechanisms

• Bridging : equilibrium is attained after limited amount of washout

Mechanisms

• Blinding or external clogging with suffosive soils: migrated particles in the base soil form a cake at the filter interface

BLINDING

R R 1

KF

K B

O F

O Ftim e

po

siti

on

K B o

K B f

= hydraulic conductivity K = baseB

In itial

Local

= filterF = initialo = finalf

Base gradation curve

Mechanisms

• Evaluation of internal stability(Kenney & Lau, 1985, 1986)

10 0 10 0

10 00 0

0d 0 d

H 2

H 1

log d

F (

%)

SOIL No. 2 ( STA BLE )

SOIL No. 1 ( UNSTABLE )

GRADATION CURVE SHAPE CURVE

H = F4d - Fd

H (% )

S T A B L EB O U N D A R Y (

= 1 . 0 )

H

F

U N S T A B L E

H + F = 100

F (

%)

4 d d 10 0

LINEARLYGRADEDdl = d85 dl = d50

COHESIONLESSYES

YES YESNO

NO

END

GAPGRADED CONCAVEUPWARDINTERNALLYSTABLE INTERNALLYSTABLE

BASE SOIL GRADATION CURVE dl, d15HYDRAULIC CONDUCTIVITY kBPLASTICITY SUSCEPTIBILITY TO CRACKINGCOHESIVEO90 40 m UNIFORMCu 6

PERMEABILITYkF 20kB RETENTIONdl O90 3dlRETENTIONO90 dl

Minimum gapsizeRisk of Piping ofFiner ParticlesLEGEND :( )( )

dl = dG ( ) dl = d30 ( ) dl = d30 ( )

Filter Selection

• By index tests on base soils (Lafleur, 1999)

LIN EARLYG RADE D

d l = d85 d l = d50

CO HE SIO NLE SS

YE S

YE S YE S

NO

NO

END

G APG RADE D

CO NCAVEUPWARD

INTERNALLYSTABLE

INTERNALLYSTABLE

BAS E SO IL G RADATIO N CURV E ,dl d 15

HYD RAULIC C O NDUCTIVITY k B

PLASTIC ITY S USCE PTIB IL ITY TO CR ACKING

CO HE SIVE 40 mO 90

UNIFO RM 6Cu

PE RM EABIL ITY 20 k F k B

RETEN TIO Nd l O90 3d l

RETEN TIO NO90 d l

M inim um gapsize

R isk of P ip ing ofF iner P artic les

LEGEND :

( )

( )

dl = dG ( ) dl = d 30 ( ) dl = d 30 ( )

Filter Selection

• By performance test (Fannin, et al., 1994)

M anom eterports

Adj

ust

able

Sta

tiona

ry

Geotextile

Piped particles

PERM EAM ETER CO NSTANT HEAD DEVICES

OutflowPort

F low

Flow

Soil50 m m

4

3

2

1

25 m m

Gradient Ratio

GR = (h2 – h1) / 25

(h3 – h2) / 50

Mass of Piped Particles

Mp = Mass

Sample Area

Filter Selection

• Performance test

- Gradient Ratio

GR < 1

- Mass of Piped Particles

Mp < 2500 g/m2

Construction Requirements

• Intimate contact with protected soil• Minimum overlap : 300 mm or seamed joints• Avoid punching by construction equipment,• Angular stones • For silt fences :

- spacing is function of geotextile tensile strength- Posts have a minimum height of 1 m plus burial depth- Posts placed a minimum of 500 mm into the ground

(increased to 600 mm on a 3h:1v slope or steeper)- Cautions with slopes greater than 1:1

List Of References

• FANNIN, R.J., VAID, Y.P. & SHI, Y.C. (1994) Filtration behaviour of nonwoven geotextiles. Canadian Geotechnical Journal. Vol. 31, No. 4, pp. 555-563.

• GIROUD, J.P. (1996). Granular filters and geotextile filters. Proceedings GEOFILTERS'96 Conference, Montréal, Canada, edited by Lafleur & Rollin. pp. 565-680.

• HOLTZ, R.D., CHRISTOPHER, B.R. & BERG, R.R. (1997). Geosynthetic Engineering. BiTech Publishers Ltd. 452 p.

• KENNEY, T.C. & LAU, D. (1985) "Internal stability of granular filters”. Canadian Geotechnical Journal, Vol. 22, No. 2, pp. 215‑225.

• KENNEY, T.C. & LAU, D. (1986) “ Internal stability of granular filters : Reply ”. Canadian Geotechnical Journal, Vol. 23, pp. 420-423.

• KOERNER, R.M. (1998) "Designing with geosynthetics" 4th Edition Prentice‑Hall, 761 p.• LAFLEUR, J. (1999). Selection of geotextiles to filter broadly graded cohesionless soils.

Geotextiles and Geomembranes. Vol. 17, Nos. 5 & 6, pp. 299-312.• LAFLEUR J., MLYNAREK J. & ROLLIN A.L. (1989). Filtration of Broadly Graded

Cohesionless Soils. Journal of Geotechnical Engineering, ASCE, Vol.115, No.12, pp.1747‑1768.

• LAFLEUR, J. & PARE, J.J. (1991). Use of geotextiles in the James Bay hydro-electric project. Geotextiles and Geomembranes. Vol. 10 No. 1, pp. 35-53.

• PILARCZYK, K.W. (2000). Geosynthetics and Geosystems in Hydraulic and Coastal Engineering. Balkema, 913 p.

• Van SANTVOORT G.P.T.M. (1994). Geotextiles and geomembranes in civil engineering. Balkema, 595 p.