Writing geotextile specifications can be an excursion into unfamiliar territory for specifiers. The best intentions can result in a confusing specification or one for which there are no conforming products.
It’s important to note that geotextiles cannot be created ala carte. Specifiers must review existing geotextile products and choose the product with the combination of strength and hydraulic properties that best fit the project’s needs.
Click “Geotextile Used As Filters” for an example of a specification that appears to be the result of this ala carte approach to specifying. I want to discuss the issues that arise with this particular specification.
Woven or Nonwoven?
Section “2.1 Materials, A” calls out for a “woven pervious sheet”. However, the accompanying “Table – Geotextile Physical Properties” requires the geotextile to “equal or exceed” a grab tensile of 180 at 50% elongation. A woven geotextile will typically only meet a maximum elongation of 15-20%. However, nonwoven geotextiles will meet the 50% requirement.
The property table requires a minimum apparent opening size (AOS) of #100 US sieve. There are currently no woven geotextiles on the market with an AOS meeting or exceeding #100.
A 7 oz. nonwoven meets the grab tensile strength requirement, but has an AOS of #70 US sieve. A 10 oz. would be required to meet #100. But an increase in weight (and hence grab tensile strength) corresponds with a decrease in flow rate. This could be an issue for a geotextile being used as a filter.
Sieve Size (Bead or Number)?
Sieve size is confusing because the SMALLER the US Sieve number, the LARGER the bead size that will pass through the geotextile. As such, a #70 sieve fabric may actually meet the specification since it filters out a larger bead size (0.21 mm) than the #100 sieve (0.149 mm).
Perhaps the specifier understands this, but how do you know for sure? Listing only the bead size eliminates any doubt as to the intent of the specification. You can read more about Sieve Size here.
The table requires ASTM D-4833 pin puncture and ASTM D-4884 mullen burst. These two properties are no longer recognized by AASHTO and have been replaced by the more appropriate ASTM D-6241 CBR puncture. CBR puncture is more relevant since it simulates big stones pressed onto a geotextile laying on a relatively soft sub-base.
While ASTM D-3884 abrasion resistance is a valid ASTM test method, it has several issues. According to ASTM: “. . . caution is advised since information on the precision of the test is lacking.” They further note: “The resistance of abrasion is also greatly affected by the conditions of the tests, such as the nature of abradant, variable action of the abradant over the area of specimen abraded, the tension of the specimen, the pressure between the specimen and abradant.” They also state that “the dimensional changes in the specimens; and the resistance of geotextile materials to abrasion as measured on a testing machine in the laboratory is generally only one of several factors contributing to performance or durability as experienced in the actual use of the material.” The result is an index property that offers little practical value to the engineer.
As such, abrasion resistance is not performed as part of standard geotextile conformance testing. It may be difficult to get a value from the geotextile manufacturer, much less a certification.
What is a Specifier to Do?
Choose a product best suited to the job’s requirements from existing, published manufacturer’s technical data sheets. Do not make any changes or add any qualifiers to the description and table data on the data sheet.
Most geotextile manufacturers publish their current technical geotextile data sheets by product type on their websites. US Fabrics has a “Product Data Sheets” drop down menu box on most pages to locate current data sheets by product type/application.
We Can Help!
You can always call us at (800)518-2290 and we will be glad to point you in the right direction. We also offer a free account with access to detailed product information as well as drop in specifications for many of our products for various applications. Above any product data sheet you will see a green button that says “Detailed Product View” or “Drop In Specification”. Click on either and you will be directed to a sign up page.
You can also sign up to download our “Guide to Better Geotextile Specifications” to learn more.
Thanks for reading and happy specifying!