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 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 Functions of a Filter

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)

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

Filter Applications Wrapping of trench drains (Koerner, 1998)

Filter Applications Wall drains

Filter Applications Erosion protection (Pilarczyk, 2000)

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

Filter Applications Vertical consolidation drains (Van Santvoort, 1994)

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

Filtration Flow Conditions Dynamic vs static

Laboratory Filter Characterization Opening size O 90 by wet sieving

Filter Characterization Constriction size vs filtration opening size

Filtration Behaviour Theoretical opening size of the filter given fibre diameter d f of non woven geotextile (Giroud, 1996)

Filter – Base Soil Interaction (Lafleur, 1999) Mechanisms and parameters R R = Retention Ratio R R = Opening Size of the Filter (O f ) Indicative Size of the Base (d i )

Gradation Curve of Base d i = indicative size of the base

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

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

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

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

Filter Selection By performance test (Fannin, et al., 1994) Gradient Ratio GR = (h 2 – h 1 ) / 25 (h 3 – h 2 ) / 50 Mass of Piped Particles M p = Mass Sample Area

Filter Selection Performance test - Gradient Ratio GR < 1 -Mass of Piped Particles M p < 2500 g/m 2

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 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 ‑ 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.