1. Geosynthetics for Filtration by Jean Lafleur
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.
Introduction
Among the various types of large man-made structures (with containment function), dams have surely the most important impact on the environment, due not only to the size of the structure itself, but also to the creation of water storage reservoirs, often of very large size.

2. Retain particles of the base soil to be filtered
Functions of a Filter
Retain particles of the base soil to be filtered
Avoid piping
Allow free flow of water
upstream of the filter
Avoid external clogging
(With unstable soils)
through the filter
Avoid internal clogging
Survive construction and environmental stresses
Function can be provided by either natural aggregates or by Geotxtiles

4. 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)

5. 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

6. Base - Filter Interaction

7. Filter Applications
Wrapping of trench drains (Koerner, 1998)

8. Filter Applications
Wall drains

9. Filter Applications
Erosion protection (Pilarczyk, 2000)

10. Filter Applications
Earth and rockfill dams (Lafleur & Paré, 1991)

11. Vertical consolidation drains
Filter Applications
Vertical consolidation drains
(Van Santvoort, 1994)

12. Filter Applications
Silt fences (Holtz, et al., 1997)

13. Filtration Flow Conditions
Dynamic vs static

14. Laboratory Filter Characterization
Opening size O90 by wet sieving

15. Filter Characterization
Constriction size vs filtration opening size

16. Filtration Behaviour
Theoretical opening size of the filter given fibre diameter df of non woven geotextile
(Giroud, 1996)

17. 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)

18. Gradation Curve of Base
di = indicative size of the base

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

20. Mechanisms Bridging : equilibrium is attained after limited
amount of washout

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

22. Mechanisms
Evaluation of internal stability
(Kenney & Lau, 1985, 1986)

23. Filter Selection
By index tests on base soils
(Lafleur, 1999)

24. Filter Selection By performance test (Fannin, et al., 1994)
Gradient Ratio
GR = (h2 – h1) / 25
(h3 – h2) / 50
Mass of Piped Particles
Mp = Mass
Sample Area

25. Filter Selection Performance test Gradient Ratio GR < 1
Mass of Piped Particles
Mp < 2500 g/m2

26. 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

27. 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
GIROUD, J.P. (1996). Granular filters and geotextile filters. Proceedings GEOFILTERS'96 Conference, Montréal, Canada, edited by Lafleur & Rollin. pp
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
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
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
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.