The Geosynthetic Institute (GSI) has a new standard test method for “Determination of the Polymer Loading in Geosynthetic Clay Liners (GCLs).” It was developed over several years without consensus of the membership. GSI forged ahead with it because we see many such products in the market and believe there is a need to start the standardization process to provide some means of quantitative assessment of these products.
This test method covers the measurement for loss of ignition (LOI) of GCL infills to determine the polymer loading. The method also covers the measurement of moisture content and ash content, which are required for the calculation of LOI by default.
It should be noted that the techniques described within the standard are only two of 13 methods that Schumacher (2002) suggests for the determination of total organic carbon (TOC) in soils. The methods described have been shown to accurately identify the amount of polymer added to a GCL infill regardless of formulation. Gustitus et al. (2020) and Scalia et al. (2014) have identified many different types of polymers currently used to enhance bentonite performance in GCLs. It is freely acknowledged that these methods do not address the nature of the polymer, consideration of polymer durability, effectiveness in reducing hydraulic conductivity long-term, polymer distribution within the GCL infill and the potential for washout. But it is a first step.
GCLs have demonstrated excellent performance as hydraulic barrier systems that have a very low hydraulic conductivity to prevent contamination of the surrounding groundwater, particularly when teamed with a geomembrane as a composite liner system. When leachates have aggressive characteristics (e.g., high total dissolved solids and cations, such as K+, Na+, Mg2+, Ca2+ and NH4+), they may alter the bentonite and lead to an unacceptable increase in hydraulic conductivity through ion exchange. To resist these high-ionic-strength leachates, polymer-modified GCLs containing a blend of sodium bentonite and various polymers have been introduced as an answer to this challenge.
Polymer-modified GCLs are typically used with marginal bentonites to enhance swell index and fluid loss properties. These polymers are proprietary, and their long-term performance is unknown. Bentonite in GCLs can be supplied as a natural sodium bentonite, sodium-activated “peptized” bentonite or calcium bentonite. Polymers may be added to improve several characteristics of a GCL; however, the nature and suitability of these polymers needs to be evaluated on a case-by-case basis. If they are used, the manufacturer should provide details about the polymers contained within their products and their long-term durability performance.
Polymer-modified GCLs are being used more frequently in our industry. This new test method will help us identify when and to what extent polymers are being used in concert with the typical clay infill.
Gustitus, S. A., Nguyen, D., Chen, J., and Benson, C. H. (2020). “Quantifying polymer loading in bentonite-polymer composites using loss of ignition and total carbon analyses.” ASCE Geotechnical Journal, In Press.
Scalia, J., Benson, C. H., Bohnhoff, G. L., Edil, T. B., and Shackelford, C. D. (2014). “Long-term hydraulic conductivity of a bentonite-polymer composite permeated with aggressive inorganic solutions.” Jour. of Geotechnical and Geoenvironmental Engineering, 140(3).
Schumacher, B. A., (2002). “Methods for the determination of total organic carbon (TOC) in soils and sediments.” NCEA-C-1282 EMASC-001, U.S. EPA Environmental Sciences Division National Exposure Research Laboratory, Las Vegas, Nev.