This is an expansion of an article contributed by Assoc. Prof. Will Gates for the publication “History of The Clay Minerals Society”, under the “waste management and geosynthetic clay liners” theme. The publication’s release will coincide with the International Clay Conference in Istanbul, which will run from 25-29 July.
The purposeful use of clay as a barrier to liquids is arguably nearly as old as civilisation itself.
Through trial and error and without the benefit of modern science, our ancestors probably used simple air-dried clay pots and bowls to hold liquids long before one fell into a fire, perhaps by accident, with the resulting consequences easily envisioned.
Modern compacted clay barriers are now extensively used in civil engineering, the simplest being clay liners made up of clayey soils, or by using bentonite mixed with sand and compacted at an optimum moisture content. Compacted clay liners remain an important part of both primary and secondary liners for municipal waste containment in most parts of the world.
Beginning in the late 1980s, researchers brought together the latest aspects of geotechnical engineering, soil physics and clay mineralogy to improve compacted clay liners, and began focused study on the performance of the then newly developed reinforced geosynthetic clay liners (GCLs).
The evolution of GCLs
All GCLs probably have conceptual origins in the use of needle-punched non-woven geotextiles in construction applications (soil stabilisation, erosion control), and by observing that some geotextiles were better able to limit clay movement (Shan and Daniel, 1991).
The widespread use of geotextiles in civil engineering, particularly since the 1980s, and developments in their manufacture enabled a rapid switch to an inexpensively manufactured engineered material with consistent properties and predictable behaviour (for example, Heerten, 2002).
Reinforced GCLs were developed in 1980s to mitigate the uncontrolled swelling, bentonite loss and slip-failure that layers of unreinforced geotextiles and bentonite were found to undergo when used in capping landfills.
Widely used since the 1990s, the modern needle-punched and thermal-locked GCL originally developed by G. Heerten for Naue, uses a core layer of sodium bentonite, through which a non-woven geotextile (generally a polypropylene) is needled through the bentonite layer (see figure 1) and then heat-bonded to a woven geotextile (also a polypropylene).
Numerous iterations of GCLs exist, with either non-woven carrier with woven cover geotextiles, or vice-versa, being the dominant versions, but scrim-reinforced versions also are important where slip-strain loads may be of concern. In these latter GCLs a non-woven geotextile is needle-punch bonded to a woven geotextile and then that is used as either the carrier or the cover geotextile in a GCL. GCLs have been quickly taken up by numerous manufacturers worldwide, including Naue, Cetco, Geofabrics, Kaytech and many others. During the earlier years of GCL development other types of bonding were also explored, including adhesives (for example, Gundel Environmental) in which bentonite was glued to a propylene geotextile or a high-density polypropylene geomembrane, but most manufactures have now settled on both needle-punching and thermal-locking as the method for bonding geotextiles and bentonites to produce the wide range of reinforced GCLs available today.
How GCLs are used today
GCLs are now the state of the art for secondary liner components in modern composite liner systems for municipal landfill as well as primary liners for numerous industrial activities including evaporation ponds, process water storage/recirculation, heap-leach pads and irrigation canals.
Given the obvious liability repercussions associated with waste containment liner failure, much of the key early clay minerals research focused on GCL specifications with an eye to forecasting performance expectations, and evaluating potential performance attributes in both controlled laboratory and field studies.
A sodium bentonite is widely recognised as applicable in most situations, having good water uptake and retention, swelling and low saturated and unsaturated hydraulic conductivity. There exist two ‘camps’ on the question whether the chemical form of bentonite should be natural sodium bentonite or if sodium activated bentonites provide equitable performance, but this argument may stem largely from (lack of) access to natural sodium bentonite.
The literature over the past decade or so has been also strongly divided on whether the physical form of bentonite should be powdered or granulated, with ‘camps’ posing strong evidence for and against their preferred form.
Despite these application- and specification-derived debates, it is a simple truth that GCL research was largely driven by geotechnical aspects of the geotextiles, with little regard to the bentonite component. Only recently has the bentonite, and specifically the sodium form of montmorillonite within it, been largely recognised in the engineering community as imparting nearly all favourable hydraulic characteristics to a GCL.
In that regard the Clay Minerals Society, and the Source Clays Repository, have provided base-line materials, data and expertise that has lifted the general appreciation among environmental and geotechnical engineers that GCLs cannot be fully studied nor successfully deployed without knowledge of the mineralogy and physical chemical characteristics of the bentonite component.
Areas of active research on GCLs have been largely driven by both industry performance needs and environmental oversight responsibility. One key research theme has been understanding geotextile shrinkage post installation and improving overlap performance. Another has been on understanding and improving in-field hydration processes of GCLs, since it is well-known that the hydraulic performance of GCLs to liquids, gases and solutes requires them to be in a well-hydrated state, particularly when exposed to harsh leachates or environmental conditions (Bouazza and Gates, 2014).
Another theme has been addressing in-field cation exchange due to either interaction of base GCLs with incompatible subsoils, or exposure of un-constrained cover GCLs to wet-dry cycling. A fourth theme which has seen a resurgence of activity recently is understanding the causes and conditions in which erosion of bentonite from GCLs occurs and how best to mitigate this behaviour. All four of these subjects were recognised early in the 1980s as potential limitations for the use of GCLs and remain critical to future performance assessments (Bouazza, 2002, Rowe, 2020).
Polymer enhancement of the bentonite is the most recent trend aimed at improving GCL performance and currently researchers worldwide are determining which polymers are most effective in increasing the useful life of composite barriers (for example, Lau et al., 2022).
Another key area of intense study is the impact of ‘emerging’ pollutants, such as per- and poly-fluoroalkyl substances (PFAS). In this case, GCLs lend themselves to flexible and adaptable manufacturing, enabling layers of functional materials (for example, activated carbon or organoclays) to be incorporated directly into the product (for example, Gates et al., 2020).
Some interest exists in extending GCL use into being an integral part of low emissions and low energy ‘green’ critical minerals recovery (Figure 2).
It seems unlikely that GCLs will be used extensively in containment of radioactive wastes due largely to the low mass of clay per unit volume of product. Instead GCLs may prove useful as reactive components of engineered barriers to assist in keeping surface and ground water from penetrating into radioactive waste containment systems.
Collaboration is Key…
The practical aspects of safe and sustainable modern waste management owe much to continued, targeted and applied research conducted by clay mineralogists and environmental and geotechnical engineers over the past few decades and this is likely to be the pathway toward future iterations of modern landfill design.
Collaboration will continue to be key to the successful achievement of any aim, because barrier engineers may lack complete understanding of bentonite surface and structural chemistry that is required to explain observed behaviour. Likewise, clay mineralogists may be unaware of many practical realities of engineered barriers.
Associate Professor Will Gates is based at the Institute for Frontier Materials at the Deakin University Burwood-Melbourne campus. He is a nationally and internationally recognised research leader in geotechnical, industrial and environmental applications of industrial clay minerals. Since 1998 he has been a Committee Member or Chair for the Source Clays Committee of the US based Clay Minerals Society, which has a mission to provide clay materials of known provenance to researchers throughout the world. He has personally overseen the procurement for the Repository three important clay mineral resources from Australia. When not pursuing clay research, he leads the concrete group in Advanced Alloys and Infrastructure Materials, where his team conducts research and development for using large volumes of waste resources (soils, construction materials) in cements and concretes within a circular economy context, to the lower carbon footprints of built infrastructure and to reduce reliance on primary sources of materials.
Bouazza A. 2002 Geosynthetic clay liners. Geotextiles and Geomembranes, 20,3-17.
Bouazza A, Gates WP. 2014. Overview of performance compatibility issues of GCLs with respect to leachates of extreme chemistry. Geosynthetics International. 21: 151-167.
Gates WP, Dumadah, G, Bouazza A. 2018. Micro X-ray visualisation of the interaction of geosynthetic clay liner components after partial hydration. Geotextiles and Geomembranes, 46, 739-747.
Gates WP, MacLeod AJN, Fehervari A, Bouazza A, Gibbs D, Hackney R, Callahan DL, Watts M. 2021. Interactions of per and polyfluoralkyl substances (PFAS) with landfill liners. Advances in Environmental and Engineering Research, 1, 4 dpi:10.21926/aeer.2004007.
Heerten G. 2002. Geosynthetic clay liner performance in geotechnical applications. Pg 3-21 In Zanzinger H, Koerner RM, Gartung E, Eds.) Clay Geosynthetic Barriers. Taylor and Francis, 400 pp. ISBN 90 5809 390 8
Lau, Z. C., Bouazza, A., & Gates, W. P. 2022. Influence of polymer enhancement on water uptake, retention and barrier performance of geosynthetic clay liners. Geotextiles and Geomembranes, In Press Online, Accepted for publication 25 February 2022. Doi:10.1016/j.geotexmem.2022.02.006Rowe RK, 2020. Geosyntheic clay liners: perceptions and misconceptions. Geotextiles and Geomembranes, 48, 137-156.