hydraulic design of geosynthetics
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Hydraulic Properties of Geotextiles
Rakesh J. Pillai
January 31, 2015
OutlineHydraulic Properties
Apparent Opening Size (AOS)Cross Plane PermeabilityIn-Plane PermeabilityGradient Ratio TestLong Term Flow Test
Testing of Geosynthetics
Hydraulic Properties
• Apparent Opening Size (or Equivalent Opening Size)
• Cross-plane permeability
• In-plane permeability
• Gradient ratio
• Long term permeability
Rakesh J. Pillai | NIT Warangal — Civil Engineering Department 3/28
Testing of Geosynthetics
Apparent Opening Size (AOS)• Also known as Equivalent Opening Size (EOS); Specified code is ASTM
D4751
• In this test, uniform sized beads of known diameters is sieved through thegeotextile
• Sieving is done using beads of successively different diameters until theweight of the beads passing through the test specimen is 5%
• This defines O95-size of the geotextile’s opening in mm
• Relatively faster test
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Testing of Geosynthetics
Procedure of AOS - Dry Sieving Method
• Take 50 gm of smallest size glass beads (75µ) and sieve them for 10minutes and determine the percentage retained on the geotextile.
• Repeat with next higher size glass beads until the percentage of glass beadspassing through is x% or less
• A graph is drawn with glass bead size on the x-axis and the percentagepassing on the y-axis
• If y% of certain particle size is retained on a geotextile, Oy of geotextile isthe size of the particle in mm (usually y is taken as 90 or 95)
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Testing of Geosynthetics
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Testing of Geosynthetics
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Testing of Geosynthetics
Limitations - AOS by dry sieving
• The test is conducted dry, whereas filtration and drainage always involvefluids
• Thick non-woven geotextile may entrap the glass beads
• Yarns in the geotextiles may move (woven split type) during the test, thusaffecting the AOS value
• Glass beads may simply float instead of going through the geotextilebecause of their low mass
• Electrostatic forces may affect the results
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Testing of Geosynthetics
AOS - Hydrodynamic Test Method
• Also known as wet sieving method
• Procedure is similar to dry sieving
• Geosynthetic along with uniform size sand particles (glass beads) is repeat-edly dipped in water and taken out
• Pecentage of particles passing through the geotextile is determined andOy is reported
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Testing of Geosynthetics
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Testing of Geosynthetics
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Testing of Geosynthetics
Filter Design Criteria based on AOS value
• Fine particles should not be lost (piping limit)
– Piping limit: O90 ≤ D85 of soil for granular soil
• Pore opening size should be large enough for the water to flow freelythrough the geotextile (permeability test)
– Permeability limit: O90 ≥ D15 and O90 ≥ 0.05mm
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Testing of Geosynthetics
Cross Plane Permeability
Cross Plane Permeability Test (ASTM D4491)
• Constant head test
• 50mm head difference is maintained between the upper and lower surfacesof the geotextile
• Water allowed to flow through an opening of 25mm diameter
• Volume of flow (>1 litre) in a given time (>30 seconds) is measured
• Temperature correction is applied
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Testing of Geosynthetics
Cross Plane Permeability
q = kniA = kn∆htA
kn
t= ψ = q
(∆h)(A)
• In the above equations:
– kn = permeability (m/s)
– ∆h = head difference (m)
– A = area of flow (m2)
– ψ = permittivity (s−1)
– t = thickness of geotextile
– q = flow rate (m3/s)
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Testing of Geosynthetics
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Testing of Geosynthetics
Variable Head Permeability Test
kn
t= ψ = 2.3 a
(A)(∆t) log10h0
hf
• Here:
– kn = permeability (m/s)
– a = area of water columnabove geotextile (m)
– A = area of flow (m2) - 25mmdiameter
– ψ = permittivity (s−1)
– t = thickness of geotextile
– h0 = initial height of watercolumn = 80mm
– hf = final height of water col-umn = 20mm
– ∆t = time taken for the waterhead to fall from h0 to hf
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Testing of Geosynthetics
In-Plane Permeability
In-plane permeability (Transmissivity) - ASTM D4716
• Normal pressure is applied on the sample
• Minimum size of sample is 300mm X 300mm
• Geotextile is sandwiched between two thick rubber sheets to prevent anyleakage
• Test carried out with different gradients
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Testing of Geosynthetics
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Testing of Geosynthetics
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Testing of Geosynthetics
Constant Head Method
q = kpiA = kpi(w × t); i = ∆h/L
kpt = θ = q
iw
• Here:
– kp = in-plane permeabilitycoefficient(m/s)
– q = rate of flow (m3/s)
– i = gradient of flow
– θ = transmissivity (m2/s)
– t = thickness of geotextile (m)
– w = width of the sample (m)
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Testing of Geosynthetics
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Testing of Geosynthetics
Radial In-plane Flow Apparatus
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Testing of Geosynthetics
Radial In-plane Flow
q = kpiA = kpdh
dr(2πrt)
2π(kpt)∫dh = q
∫dr
r
(kpt) = θ = qln(r2/r1)2π∆h
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Testing of Geosynthetics
Gradient Ratio Test
• Flow through a soil underlain by a geotextile filter layer is analysed andthe compatibility between the two is established
• Pressure head at different points are measured
GR = (h2 − h1)/25(h3 − h2)/50
• For good compatibility between the geotextileand the soil, steady state GR value should beless than 3
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Testing of Geosynthetics
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Testing of Geosynthetics
Long Term Flow Test
• Long term permeability can be determined using the gradient ratio appara-tus
• Flow rates can be determined after establishing steady state conditionsand long term permeability coefficient can be determined
• Usual permeability calculations
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Testing of Geosynthetics
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Thank You!!!