Porous Flow Filters Porous Flow • Flow through granular medium (sand, pebbles, stones) porous flow in a dike flow under a caisson Examples of filters Function of filters: • Preventing erosion under a top layer • Preventing pressure build-up • Combination Types: • Granular material: one or more layer of grain of varying diameter • Geotextiles Filters against erosion: Prevent the washing away of underlying materials Needed in case of large gradients on interface soil-water Drainage filters: Prevent high exess presures under impervious revetment • When designing filters, stability and permeability are important aspects Influence of filter on flow under impervious revetment Gravel filter: 1m thick, 2 m wide Granular filters Values of k for various materials Material d 50 (< 63.10 -3 m) or d n50 (m) Permeability, k (m/s) Clay Silt Sand Gravel Small rock Large rock < 2.10 -6 2.10 -6 - 63.10 -6 63.10 -6 - 2.10 -3 2.10 -3 - 63.10 -3 63.10 -3 - 0.4 0.4 - 1 10 -10 - 10 -8 10 -8 - 10 -6 10 -6 - 10 -3 10 -3 - 10 -1 10 -1 - 5.10 -1 5.10 -1 - 1 Possible design criteria Geometrically closed filters Filter rules 15 85 5 F B d d < 60 10 10 d d < 15 15 5 F B d d > Stability: Internal stability: Permeability: [...]... uf k Ρ = = Δh e P Δh e k permittivity parameter head difference geotextile thickness ‘normal’ permeability coefficient Type O90 (mm) P (1/s) Mesh net Tape fabric Mat Non-woven 0.1 - 1 0. 05 - 0.6 0.2 - 1 0.02 - 0.2 1 -5 0.1 - 1 0. 05 – 0 .5 0.01 – 2 Blocking of geotextile Overall stability of filter with geotextile ... Test no velocity gradient 1 16 mm/s 0. 05 2 22 mm/s 0.07 3 28 mm/s 0.10 4 32 mm/s 0.13 threshold 5 35 mm/s 0.13 threshold 6 40 mm/s 0. 15 7 42 mm/s 0.18 Filters in unstationary flow Tests with flow over bed protection with filter Geotextile • Foil and cloth-like synthetic materials • Used in hydraulic, transport, enviromental engineering • Two main types: woven, non-woven Constructing dike with geotextile . 2.10 -3 2.10 -3 - 63.10 -3 63.10 -3 - 0.4 0.4 - 1 10 -1 0 - 10 -8 10 -8 - 10 -6 10 -6 - 10 -3 10 -3 - 10 -1 10 -1 - 5. 10 -1 5. 10 -1 - 1 Possible design criteria Geometrically. d 50 (< 63.10 -3 m) or d n50 (m) Permeability, k (m/s) Clay Silt Sand Gravel Small rock Large rock < 2.10 -6 2.10 -6 - 63.10 -6 63.10 -6 - 2.10 -3 2.10 -3 - 63.10 -3 . Possible design criteria Geometrically closed filters Filter rules 15 85 5 F B d d < 60 10 10 d d < 15 15 5 F B d d > Stability: Internal stability: Permeability: Effect of blocking Geometrically