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Geotextile Filter



The function of a geotextile filter is to retain the soil while allowing the liquid to flow as freely as possible. In order to achieve this objective, a geotextile filter needs to meet: (1) Retention criterion: the filter opening size must be sufficiently small to retain soil particles. (2) Permeability criterion: the filter must be sufficiently permeable to ensure that the liquid flow is as free as possible, and (3) Porosity criterion: the filter should remain a high porosity so the probability for clogging is small.

Giroud’s filter criteria is used in the calculation. It is recommended that AASHTO M288-96 minimum hydraulic requirements as shown in the table below be also considered in the selection of a geotextiles filter.

Table 1 - Geotextile Criteria for Subsurface Drainage
(after AASHTO M288-96)
Filter Criteria

PercentSoil Passing No. 200 (0.075mm) Sieve
<15 15 - 50 >50
Minimum Permittivity, ASTM D-4491 0.5 sec-1 0.2 sec-1 0.1 sec-1
Maximum AOS, ASTM D-4751 0.43 mm 0.25 mm 0.22 mm

Retention Criterion

Giroud (2000) uses a linearization of the particle distribution curve that, when plotted with the classical log scale horizontal axis, is as close as possible to the actual particle distribution curve (Figure 1). A least variance approach was used to determine the best linearization of the central portion (Equation 1 & 2).  It should be noted in Figure 1 that there is greater uncertainty on the two extremities (d0 and d100) of the actual particle size distribution.  This justifies the use of the linear particle size distribution curve. The result obtained using Giroud's retention criterion is not affected by the truncation of the particle size distribution curve.  A coefficient of determination (R2) is calculated to indicate the effectiveness of the linearization (Equation 3).

Eq. 1 - Slope determination by the method of the least squares Eq. 2 - Intercept determination by the method of the least squares Eq. 3 - The r-squared value can be interpreted as the proportion of the variance in y attributable, to the variance in x 


Figure 1 - Linearization of Particle Size Distribution Curve (after Giroud, 2000)

Table 2 - Retention Criterion for the Hyperstable Case
(C'cu = 3) expressed using d'85S
Soil Density Density Index (Relative Density) ID Relative Compaction (RC)

Linear coefficient of uniformity of the soil, C'u
1 ≤ C'u ≤ 3 C'u ≥ 3
loose ID ≤ 35% RC ≤ 86% OF ≤ (C'u)0.3 d'85S OF ≤ (9/C'u1.7) d'85S
medium dense 35% < ID ≤ 65%   86% < RC ≤ 92% OF ≤ 1.5 (C'u)0.3 d'85S OF ≤ (13.5/C'u1.7) d'85S
dense ID > 65% RC > 92% OF ≤ 2 (C'u)0.3 d'85S OF ≤ (18/C'u1.7) d'85S

Table 3 - Retention Criterion for the Hyperstable Case
(C'cu = 3) expressed using d'50S
Soil Density Density Index (Relative Density) ID Relative Compaction (RC)

Linear coefficient of uniformity of the soil, C'u
1 ≤ C'u ≤ 3 C'u ≥ 3
loose ID ≤ 35% RC ≤ 86% OF ≤ (C'u) d'50S OF ≤ (9/C'u) d'50S
medium dense 35% < ID ≤ 65%  86% < RC ≤ 92% OF ≤ 1.5 (C'u) d'50S OF ≤ (13.5/C'u) d'50S
dense ID > 65% RC > 92% OF ≤ 2 (C'u) d'50S OF ≤ (18/C'u) d'50S

where:

C'u linear coefficient of uniformity of the soil  = d'60S / d'10S
ID relative density or density index of the soil
d'ms the particle size such that m% (on the linear particle size distribution curve) of the linear soil particles by mass are smaller than d'ms
RC relative compaction
OF maximum filter opening size

Permeability Criteria

kf ≥ ks * Is -> against excessive pore water pressure
kf ≥ ks -> against excessive reduction of flow rate

where:
    kf - Hydraulic conductivity of the geotextile filter
    ks - Hydraulic conductivity of the soil
    Is - Hydraulic gradient in the soil (typical values presented in Table 4)

Table 4 - Typical Hydraulic Gradients in the Soil in the Vicinity of the Filter
Application Typical hydraulic gradient
Ordinary dewatering trench 1
Vertical wall drain 1.5
Pavement edge drain 1
Landfill leachate collection/detection removal system 1
Landfill closure surface water collection removal system 1
Dam toe drains 2
Dam clay cores 3 to 10
Island channel protection 1
Shoreline protection 10
Liquid impoundment with clay liners >10

Porosity Criteria

NGTX > 0.3

where NGTX is the porosity of geotextile filter.

There are two mechanisms that are known to cause progressive clogging in a filter: (1) Chemical, biological and biochemical clogging.  (2) Accumulation of soil particles on or in the filter. 



Particle Size Distribution
d10 mm
d20 mm
d50 mm
d60 mm
d85 mm
w factor
w Factor
loose medium dense dense
1 1.5 2
 


Input data
Actual Particle Distribution Curve
d10 mm
d20 mm
d50 mm
d60 mm
d85 mm
                                             
Output Data
Values of the Linear Particle Distribution Curve
d'0 mm
d'10 mm
d'20 mm
d'50 mm
d'60 mm
d'85 mm
d'100 mm



Coefficient of Determination (R2)
   Indicates how acurate the linearization is

Coefficient of Uniformity
  If greater than 1 and smaller than 5, it is said to be uniformly graded.
  If greater than 20, it is said to be broadly graded.
Linear Coefficient of Uniformity

Maximum Filter Opening Size mm



 


Giroud, J.P., 2000, "Filter Criteria", in Jubilee Volume 75th Anniversary of K. Terzaghi's Erdbaumechanik (Soil Mechanics), Technical University, Vienna, Austria, Vol 5/2000, Brandl, H., editor.

Giroud, J. P., 1994, "Quantification of Geosynthetics Behavior", Special Lecture, Proceedings of the Fifth International Conference on Geotextiles, Geomembranes and Related Products, Singapore, September 1994, Vol. 4, pp. 1249-1273.

Giroud, J. P., 1988, "Review of Geotextile Filter Criteria", Proceedings of the First Indian Geotextiles Conference, Bombay, India, December 1998, pp. 1-6.

Giroud, J. P., 1982, "Filter Criteria for Geotextiles", Proceedings of the Second International Conference of Geotextiles, Vol. 1, Las Vegas, Nevada, USA, August 1982, pp. 103-108.


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