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Characterizing the soil particle retention ability of geotextiles through pore size distribution
This technical note focuses on the various past and present test methods that have been used to characterize the soil particle retention ability of geotextiles. It provides an overview of these methods, discusses their strengths and weaknesses, and highlights the advantages of using ASTM D6767 - Test Method for Pore Size Characteristics of Geotextiles by Capillary Flow Test. This test method provides geotextile properties used to select the correct geotextile to meet the filtration requirements relative to the project soil properties using porometer testing. An authoritative examination of the test methods used to determine the filtration capacity of geotextiles and the accuracy of their test data is presented in “White Paper #31 from the Geosynthetic Institute” (Koerner, 2014).
Introduction
Geotextiles have long been used for filtration and drainage applications in civil engineering projects. Their ability to retain soil particles while allowing water to pass through is key to their performance in these applications. Therefore, accurately characterizing the opening size and pore structure of geotextiles is critical for predicting their behavior in the field and ensuring their long-term success.
Historically, the opening size of geotextiles was measured by the Equivalent Opening Size (EOS) method (Test Method CW02215-77) until 1988 in Canada and 1993 in the U.S.A. The EOS of a geotextile is determined by dry sieving uniform particles (glass beads or sand) of a known standard sieve size through the geotextile. Successively finer sizes are tested to find the smallest size of particles that have 5% or less by mass passing through the geotextile. The value obtained is expressed either as a US Sieve # and/or mm. Assuming that geotextiles and screen mesh (sieves) have comparable retention ability, the EOS was a rational means of correlating fabric pore structure to an equivalent mesh size. The EOS method is no longer used by the geosynthetics industry and has been replaced with the Apparent Opening Size (AOS) method.
The U.S.A adopted the AOS method ASTM D4751 in 1993. This test method (current standard ASTM D4751-21a), although similar to the EOS method, is used to indicate the AOS in a geotextile that reflects the approximate largest opening dimension available for soil to pass through.
The AOS is also determined by dry sieving uniform sized glass beads of a known standard sieve size through the geotextile until the weight of beads passing through the geotextile is 5% or less. Its value is expressed as a US Sieve # and/or in millimeters.
The Ministry of Transportation of Ontario, together with several eastern provinces, adopted the Filtration Opening Size (FOS) method (CAN CGSB148.1 No.10) in 1990. The FOS of a geotextile is determined by wet sieving a well-graded mixture of glass beads through a geotextile. The glass beads are forced through the geotextile under hydrodynamic forces rather than by a shaking action. The geotextile specimens are alternately plunged in and out of a water tank for exactly 1000 cycles. At the end of the test, the water in which the specimens have been immersed is decanted and the glass beads retained on the geotextile sample are collected. The glass beads collected are then graded to obtain a particle size distribution. The d95 of the soil is determined and is expressed in microns.
For comparison purposes EOS and AOS values can be interchanged. However, this cannot be said for the relationship between EOS/AOS and FOS. The values obtained from FOS testing are different from those obtained from the EOS/AOS test method. For example, MIRAFI® 1100N has an AOS of 0.150 mm (150 microns) but has an FOS of .070 mm (70 microns). In general, the thicker the geotextile, the lower the FOS, i.e. MIRAFI 1100N will always have a FOS value lower than MIRAFI 160N.
Please note that AOS, FOS, and EOS do have some significant shortcomings. While these tests are not completely destructive, setting the fabric in the sieve can lead to inaccurate results due to distortion of the sample from overhandling. These tests do not have the ability to distinguish between material defects, such as a singular larger hole that does not reflect the typical properties or product characteristics of the material. Other problems with these tests include: electrostatic effects; testing beads sticking together; damaged/fractured beads; and beads becoming trapped in the material through friction.
The most obvious issue encountered in the AOS, FOS, and EOS tests is that these test methods provide only a single opening size value. Most geotextiles typically have a range of different opening sizes, but the distribution of these various opening sizes across the surface of the geotextile cannot be characterized from these test methods. An alternative test method, ASTM D6767 - Pore Size Characteristics of Geotextiles by Capillary Flow Test, has been developed to address the above concerns regarding overhandling, glass bead problems, and it provides a range of various geotextile opening sizes (pore size distribution) that a specific geotextile offers.
ASTM D6767 - Test Method for Pore Size Characteristics of Geotextiles by Capillary Flow Test, was first adopted as an ASTM Standard in 2002. The standard has gone through revision and updates in 2008, 2011, 2014, and most recently in 2016 (ASTM D6767-21). This test method is a significant improvement over the AOS, FOS, and EOS methods for several reasons.
ASTM D6767 is a test method in which a wetting fluid is used to saturate the pores of the geotextile test sample, followed by a non-reacting gas that displaces the fluid from the geotextile pores. To perform the test, a geotextile sample is cut and placed into the chamber of the test device. The porometer forces air through the dry sample, taking measurements at different valve position increments (cc/min or l/min) and at varying pressures (psi, kPa). When the maximum predetermined pressure or flow rate is reached, the testing device resets, readying for the second phase of the test. Next, the geotextile sample is saturated with a wetting fluid with a known surface tension and subjected to another series of pressure and flow rate measurements.
The raw data from these measurements are run through a series of calculations to determine the minimum, maximum, and mean pore size, filter flow percentage (comparable to permeability, and can be used to calculate retention values such as O95), and pore size distribution.
What does Porometer testing provide?
The Porometer test method is not limited to a single opening size and will measure opening values between O5 and O98. Each test has a range of opening sizes which are reported as an average for a sample. One would have to run countless AOS tests to provide the same amount of information provided from one porometer test, but the test comparison is impractical because the AOS test only measures O95. Using the values obtained for the Porometer testing of the geotextile, one can accurately predict the way the material will perform during both AOS testing and water flow testing (ASTM D4491).
Porometer test method defines a material’s drainage and filtration characteristics (information not provided by AOS method). The Porometer test method is also more accurate.
To highlight the improved information that the porometer testing can provide, we have compared the data of several geotextiles (in Table 1, below). Utilizing porometer test data generated for MIRAFI HP370, and MIRAFI RS580i, the AOS and Water Flow for these products could be predicted. Table 1 indicates that MIRAFI RS580i has larger pore size openings and that there are fewer large pore openings compared to MIRAFI HP370. MIRAFI RS580i also has a more uniform pore distribution within the 30μm–250μm range which attributes to MIRAFI RS580i having a higher hydraulic flow rate.
The ability for the Porometer test to predict a geotextile’stest results from AOS and Water Flow testing is an indication of the Porometer test’s ability to predict a geotextile’s soil retention ability and permeability requirements in the field. While the AOS does have an application for use in geotextile manufacturing QC/QA testing, Porometer testing provides much more in-depth information about a geotextile without suffering from the limited information and testing problems encountered using the AOS type test.
A graphical representation of the data obtained from both Porometer testing and AOS testing on Mirafi® RS580i is shown in Figure 1. The typical pore size distribution curve for RS580i can be related to its filtration capacity for a candidate soil much more accurately than a single AOS value. Solmax Geosynthetics has developed a filter design guide for soil filtration guidance ( Solmax Filtration Design Guide Utilizing Pore Size Distribution) that utilizes pore size distribution to select the best performing geotextile for a particular filtration application/soil type. This guidance is based on the filter design method outlined in “Geotextile Filter Design Using Pore Size Distribution” (Sack, 2023).
Accurately characterizing the opening size and pore structure of geotextiles is crucial for predicting their performance in the field and ensuring their long-term success in filtration and drainage applications. While several test methods have been used over the years, ASTM D6767 - Test Method for Pore Size Characteristics of Geotextiles by Capillary Flow Test offers significant advantages over previous methods like EOS, AOS, and FOS. By providing a comprehensive pore size distribution, the Porometer test can also accurately predict a geotextile's behavior in terms of soil retention and permeability.
As an original manufacturer of geosynthetic products, Solmax takes quality control very seriously. That's why we have adopted ASTM D6767 for determining the opening size of our geotextiles. This test method provides the most in-depth information about a geotextile's properties, allowing us to select the correct product to meet the specific filtration requirements of each project. By continually using the most efficient, accurate, and precise test methods, we ensure the long-term success of our geosynthetic solutions in the field.
References
1. ASTM D6767 (2016), “Standard Test Method for Pore Size Characteristics of Geotextiles by Capillary Flow Testing,” ASTM, West Conshohocken, PA.
2. ASTM D4751 (2016), “Standard Test Method for Determining Apparent Opening Size of a Geotextile,” ASTM, West Conshohocken, PA.
3. Koerner, R. M., Koerner, G. R, “On the Need for a Better Test Method Than Dry or Wet Sieving to Obtain the Characteristic Opening Size for Geotextile Filter Design Purposes,” Geosynthetic Institute’s White Paper #31, September 18, 2014.
4. Sack, R. Sprague, J., Kuhn, J., “Geotextile Filter Design Using Pore Size Distribution,” Geocongess, Los Angeles, CA, April 2023.