Hydraulic design of geosynthetics

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!!!