DESIGN METHODS & CONCEPTS
A properly designed geotextile filter is a prerequisite for the proper functioning of a drainage geocomposite. If a geotextile is too open – i.e., the opening size is too large compared to the soil being retained – it can let too many fines pass through and that may clog the drainage layer. At the other extreme, a geotextile with too small an opening size can unnecessarily prevent fluid from getting into the drainage layer. To get the opening size of the geotextile filter right, select a geotextile with an upper limit on the opening size that meets the retention criteria. The lower limit of the geotextile opening size will be governed by permeability requirements. However, since geotextile permeability is usually much higher than that of the surrounding soils, geotextile selection is typically based on maximum opening size. This design approach suits drainage geocomposites especially well, since a significant loss of fines through the geotextile can negatively impact transmissivity performance.
Almost all the equations used for retention design are empirical in nature and were derived from tests performed with idealized soils or glass beads. For simple and well-behaved soils, these equations work fairly well. However, for certain “difficult” soil types or “critical” design conditions, a compatibility test with the target geotextile should be performed to supplement the use of the published retention criteria. Examples include broadly-graded soils, gap-graded soils, fine soil, and coal combustion residuals (CCRs). Two laboratory tests - Gradient Ratio Test (GR) ASTM D5101 and Hydraulic Conductivity Ratio Test (HCR) ASTM D5567 - are available to check the filter compatibility between the geotextile and base material. For noncohesive fine-grain soils, the HCR test offers advantages over a GR test. The HCR test comprises of back pressure saturation, better stress control and the use of higher gradients.
More often designs compare the geotextile AOS to a specific particle size of the upstream soil. For example, Carrol  recommended the following relationship seen in equation 3.1, that is now widely used in designs.
O < (2 to 3)d
O , also referred to as Apparent Opening Size (AOS), represents approximately the largest soil particle size that will pass through the geotextile. Typical AOS data for nonwoven needle-punched geotextiles is presented on Solmax’s geotextile webpage.
Other procedures, including those for non-steady-state flow conditions (such as dynamic or reversible flow) and problematic soils (such as gap-graded or broadly-graded soils), can be found in Luettich et al. .
The permeability of commercially available geotextiles is usually much higher than that of most soils. Geotextile permeability is therefore rarely a governing design consideration. The factor of safety is calculated as follows:
FS = Factor of safety for permeability
k = Permeability of the filter geotextile
k = Permeability of the adjacent soil
The FS value depends on the application and also on how critical permeability is to the proper functioning of the drainage layer. Practically, a value of 10 to 100 is used in designs, with the larger value being used in more critical applications.
k = ψ × t
k = Permeability of the filter geotextile
ψ = permittivity (sec )
t = geotextile thickness (cm)
Geotextile permeability can also be calculated using this alternative equation.
GEOTEXTILE FILTER DESIGN
Input parameter values for permeability criteria
Permeability of the adjacent soil
Permittivity of the filter geotextile
Thickness of the filter geotextile
Permeability of the filter geotextile
k = Ψ x t =
FS = k / k =
Overall Factor of Safety
LONG-TERM EFFECTS, SOIL-GEOTEXTILE COMPATIBILITY & CLOGGING
While limited soil loss through the geotextile is acceptable, continued piping can decrease geocomposite transmissivity below an acceptable value and can lead to under-performance or even failure of the drainage system. Although undesirable, over-transmissivity - the opposite extreme of geotextile clogging[K1] - is considered to be less of a concern because of the large spatial areas over which drainage geocomposites are placed. The compatibility of the candidate geotextile with upstream soil can be evaluated according to the Gradient Ratio test (ASTM D5101). The time-dependent behavior of geotextile filters from a Gradient Ratio test can be hypothetically represented as in Figure 1. The three possible responses are as follows:
Piping: In this case there is an increase in soil-geotextile permeability over time, accompanied by soil loss through the geotextile. This indicates that the geotextile opening size is too large to retain the upstream soil for the flow and gradients involved.
Stable: The second curve shows that permeability decreases over time, but then becomes more or less constant. This is the type of behavior generally desired from a properly designed geotextile.
Clogging: The last curve (no.3) shows a continued decrease in permeability over time, possibly due to particulate clogging of the geotextile. Such geotextile behavior may unnecessarily restrict the flow and prevent it from reaching the composite core
While response no.2 is desirable, the designer should definitely select a geotextile such that response no.1 is prevented. This means erring on the side of a lower geotextile opening size.
FIGURE 2.1 – HYPOTHETICAL CURVES INDICATING POSSIBLE RESPONSE OF GEOTEXTILE PERMEABILITY WITH TIME
Carroll, R.G.,Jr., (1983 ) “Geotextile Filter Criteria” Engineering Fabrics in Transportation Construction, Transportation Research Record, Washington. DC,
Luettich, S.M., Giroud, J.P. and Bachus, R.C. (1992), “Geotextile Filter Design Guide”, Geotextiles and Geomembranes, Vol. 11, pp. 355-370