Bentonite Swell Index
(ASTM D 5890, USP NF XVII, GRI GCL-1)

Significance - Sodium Bentonite clay is widely known for it’s high swelling characteristics. A typical sodium bentonite clay has the ability to absorb 4-5 times it’s own weight in water and can swell 5 - 15 times its dry volume at full-unconfined saturation.

The Swell Index or Free Swell test procedure is used to determine the general swelling characteristics of sodium bentonite clay. For Geosynthetic Clay Liners (GCLs), the Swell Index test has not been demonstrated to have a proportional correlation to hydraulic properties. Although there are correlative limitations, (especially in contaminated environments), a high swell is considered by most to be a good indicator of bentonite quality. Regardless, this test parameter can be used as a simple qualitative indicator of the base clay.

Test Methods - Several test methods have been developed over the years to analyze the bentonite in GCL products. With the exception of the GRI GCL-1 procedure, all of the tests are generally founded upon the same procedure outlined in the United States Pharmacopoeia and National Formulary, 17th Edition (USP NF XVII). This was a compendium of analytical standards maintained by the U.S. Pharmacopeial Convention (USPC), used for drug formulation and dosage.

The GRI GCL-1 test method involves placing a 100g sample in a six-inch diameter cylinder equipped with a porous plate/dial gauge indicator. The apparatus is placed in a pan of water for 24 hours, at which time the dial gauge is read for the swell increase. Results of this index test are product specific and much more difficult to reproduce than the ASTM or USPC Swell Index tests. GRI GCL-1 was intended as an interim method until the ASTM procedure could be developed. As ASTM adopted a test procedure specific to GCLs, the GRI GCL-1 procedure was discontinued by GRI and is typically only employed on projects where preexisting specifications still call for its use.

The procedures most commonly used are USP NF XVII and the newly adopted ASTM D 5890, an adaptation of the USP NF XVII method.

To perform these tests, a 2g sample of dried and finely ground bentonite clay is dispersed into a 100 ml graduated cylinder in 0.1g increments. A minimum of 10 minutes must pass between additions to allow for full hydration and settlement of the clay to the bottom of the cylinder.

Test Methods Continued:
These steps are followed until the entire 2g sample has been added to the cylinder. The sample is then covered and protected from disturbances for a period of 16 - 24 hours, at which time the level of the settled and swollen clay is recorded to the nearest 0.5 ml.

QC Test Frequency -
The new ASTM Guideline for GCL Quality Control (ASTM 5889-95 Standard Practice for Quality Control of Geosynthetic Clay Liners), suggests a frequency of one test per 50 tons, which has been adopted for the Bentofix&¨; Manufacturing Quality Control program.

Bentofix®; specifies a minimum of one (1) Swell Index test for every 50 short tons (100,000 pounds) of bentonite supplied to the plant, which equates to two tests per production lot of Bentofix® (approximately 2 for every 18,000 m_). As this is a quality assurance test on the base bentonite, not the finished GCL, it is most appropriate to refer to this parameter based on a frequency per square meter or square foot.

Bentofix® Specified Value
While the sodium bentonite clay utilized in Bentofix® GCLs typically meets a Swell Index in the range of 24 ml – 36 ml, the standard certified value is 24 ml / 2 g minimum - a value that will ensure the specified performance of a GCL product is achieved.


The information and data contained herein are, to the best of our knowledge, true and accurate. All suggestions and recommendations are offered without guarantee. Final determination of suitability for use based on any information provided is the sole responsibility of the user. There is no implied warranty of merchantability or fitness of the product for the contemplated use. Bentofix® is a registered trademark of NAUE GmbH & Co. KG GmbH (status 11/01)