Measuring pore size distribution (PSD) with the use of capillary flow porometry

By Trent Smith and Melissa Medlin

An ASTM D35 Committee Workshop on Pore Size Distribution was conducted on Jan. 30, 2013, in Jacksonville, Fla. The current industry standard for measuring a geotextile’s particle retention characteristics—ASTM D4751, Test Method for Determining Apparent Opening Size of a Geotextile (AOS)—shows one attribute for the potential filtration capability in a geotextile but does not accurately characterize the pore spaces. The workshop demonstrated a superior method for determining the pore size of a geotextile: ASTM D6767, Test Method for Pore Size Characteristics of Geotextiles by Capillary Flow Test. This method can provide the data needed to select the correct geotextile to meet filtration and soil requirements.

The AOS test method is limited because it only indirectly measures the largest apparent pore diameter, as defined by O₉₅, in a geotextile. The method is performed by mechanically shaking a known amount of calibrated glass beads of known diameter to induce particle movement through a geotextile. The beads that pass through the geotextiles are weighed and the process is repeated with decreasing glass bead diameters until the AOS of the sample is determined.

This method provides only one opening size value; a pore size distribution cannot be obtained. An AOS test has several issues: the possibility of inaccurate results due to distortion of the sample from overhandling, electrostatic effects, and beads trapped in the material from friction.

The most obvious issue is the inability to predict the clogging potential because of the high probability of smaller sized test beads passing through larger sized openings in the fabric rather than passing through the openings that are the same size as the specific test bead diameter. With so many potential problems, an improved test is greatly needed. Capillary Flow Porometry provides more geotextile opening size information accurately and precisely.

ASTM D6767 is a test method where a wetting liquid saturates the pores of the geotextile specimen followed by a nonreacting gas displacing the liquid from the geotextile pores. At the start of the test, the porometer forces air through a dry specimen, incrementally taking measurements of pressure and flow rate. When the maximum predetermined pressure or flow rate is reached, the testing device resets, readying for the test’s second phase.

During the second phase, the sample is saturated with a wetting agent of known surface tension and subjected to the same series of pressure and flow rate measurements. Several attributes are determined from the data such as the minimum, maximum, and mean pore size; filter flow percentage (comparable to permeability, can be used to calculate retention values such as O₉₅), and pore size distribution. The porometer test method is not limited to one opening size and can measure opening values between O₀ and O₉₈.

Each test has a range of opening sizes that are reported as an average for a sample. The values obtained from the porometer testing can accurately predict the way the material will perform during both AOS and water flow testing (ASTM D4491). The porometer test method is more accurate and can be used to define a material’s drainage and filtration characteristics. The porometer records any pores larger than the O₉₅, such as O₉₈, while AOS testing may report the O₉₈ falsely as the O₉₅ value.

As seen in Figure 1, the benefits of knowing the pore size distribution was proven in a recent wave impact study to determine sand-loss within a geotextile tube. Textile A would appear to lose more sand because the AOS results indicate the geotextile to be more open (around 300 microns) compared to Textile B (around 150 microns). However, the porometer information shows the average pore size greater in Textile B, which resulted in lower retention ability and greater sand loss. Without the porometer data, the wave impact study would have appeared to be inaccurate because of the discrepancy in the AOS test results.

The porometer test method is far superior to the AOS test method—which suffers from the limited information and inherent testing problems—because it provides more in-depth information about a geotextile.

Melissa Medlin is quality manager and Trent Smith is porometer technician, both at TenCate’s Cornelia (Ga.) facility.

References

ASTM D4751-12, Standard Test Method for Determining Apparent Opening Size of a Geotextile.

ASTM D6767-11, Standard Test Method for Pore Size Characteristics of Geotextiles by Capillary Flow Test.

ASTM D4491-99a (2009), Standard Test Methods for Water Permeability of Geotextiles by Permittivity.