Applications of GCL

Applications of GCL

GCL is an alternative to traditional compacted clay liner with superior hydraulic performance. Its ease of installation and resilience to changing weather conditions make it a popular choice in the industry.

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Hydraulic Characteristics

GCL is often used as a geosynthetic liner to inhibit the seepage of liquids or gasses from landfill leachate, contaminated water and other chemical solutions. In this application, the GCL must be resistant to tensile stresses and have a low hydraulic gradient.

Depending on the chemical attack factors, solution categories, concentrations and soak times, GCL permeability can vary significantly. Therefore, a laboratory test method has been developed to systematically study the variation law of GCL permeability with these factors under specified environmental conditions.

For this purpose, a GCL specimen is first soaked and then permeated with a simulated test solution. The sample is then subjected to a defined set of test GCL parameters including confining pressure, up and down back pressures, as well as percolation and permeation flow rates. The test results are then analyzed to determine the hydraulic conductivity (also known as permeability) of the GCL.

GCL is often used as a liner in ponds, canals and secondary containment facilities. In these applications, the GCL must be resistant to a large range of shear stresses and have a low hydraulic gradient. In addition, the GCL must also be able to withstand the hydraulic head applied by the lining system as well as any overburden confinement pressure. With its high granular bentonite content, CETCO GCLs are designed to perform in these demanding conditions. The needlepunch reinforcement provides exceptional shear strengths and allows the bentonite to migrate out and self-seam at the overlap, which minimizes installation time and costs.

Chemical Resistance

GCL is able to resist the infiltration of water, gas and other contaminants due to its ability to hydrate and swell to form a dense layer. This allows it to be used in place of much thicker soil layers in composite liner systems. Additionally, the sodium ions that are released during this process reduce the overall permeability of the barrier by blocking the pores. This results in an extremely low hydraulic conductivity (of the order of 10-9 m/s) and therefore a very stable barrier.

To assess the chemical resistance of GCL, it was tested against a range of ARDs with different pH levels and metal concentrations. Tests were conducted until a chemical and hydraulic equilibrium was reached. The results showed that the ion exchange mechanism acted by replacing the Na in the bentonite with divalent and trivalent metals from the ARD. In addition, a precipitation mechanism was observed for Fe and other metals.

The tests showed that the hydraulic conductivity of GCL was not as low as predicted based on its swell index. This result may be explained by the fact that even after a chemical equilibrium was achieved, changes in EC, pH and metal concentrations continued to occur. This shows that the GCL is a viable barrier material for rock containment applications with ARD generation potential.

Moisture Resistance

GCLs are often deployed in landfills as barriers that limit the migration of liquids and gases. These barrier systems must be capable of withstanding the daily thermal cycles that are experienced during construction and operation of the landfill (Ruhl and Daniel 1997;Meer and Benson 2007;Bradshaw et al. 2013;Rowe 2020). During this process, cation exchange reactions with the leachate that is permeating the GCL can alter the relative abundance of monovalent and polyvalent ions in the exchange complex of the montmorillonite clay and reduce its ability to swell and its hydraulic conductivity.

Aggressive leachates typically contain high levels of calcium and magnesium ions that can inhibit the osmotic swelling of bentonite, leading to decreased porosity and reduced GCL hydraulic conductivity (Jo et al., 2001;Kolstad et al., 2004b). However, the GCL ionic strength is often lower than that of the aggressive leachate. This can allow the GCL to hydrate from relatively clean moisture in the adjacent soil long before it comes into contact with aggressive site leachate (TR-222).

CETCO GCLs are manufactured using premium sodium bentonite and hydrated with clean water prior to installation. This allows the bentonite to hydrate to its optimum state while maintaining a low hydraulic conductivity. During installation, the bentonite is held in a confined state by the needlepunch nonwoven fabric which helps to ensure consistent hydraulic performance. The high needlepunch density also provides better internal shear resistance when installed on slopes and reduces the need for supplemental bentonite.

Thermal Resistance

The RESISTEX GCL series is designed to withstand medium and high strength leachates, gabion mattress suppliers including coal ash, low pH mining leachates and other aggressive groundwater chemistries. This is achieved through a controlled ion exchange process and the inclusion of high levels of polymers in the material. The polymers reduce the rate of ion exchange and lower the hydraulic conductivity of the GCL.

The ion exchange process is controlled by the inclusion of hindered amine light stabilisers. This combination allows the GCL to maintain its physical integrity during long-term exposure to extreme conditions in landfills, such as cyclic loading. The stability of the GCL was demonstrated in a recent project that involved cyclic testing of a RESISTEX U60-filled GCL (shown below). No damage to the bentonite or binders was observed after 6000 h of immersion.

Laboratory shear tests are conducted by placing the GCL between two perforated stainless steel nail plates. Unlike field conditions, the nails do not separate and therefore shear displacements are restricted to a small area in the middle of the specimen. The results of these shear tests are used to calculate the internal shear strength of the GCL.

Internal shear test results are commonly presented in the form of t-d relationships plotting shear strength versus shearing normal stress (sn,s). The t-d relationships for static shear tests at the standard displacement rate of R = 0.1 mm/min demonstrate that peak and residual strengths increase with increasing sn,s. However, there is a lack of published information regarding temperature effects on shear strength of hydrated GCLs and GCL interfaces. Therefore, short-term laboratory tests are expected to remain the primary means of obtaining shear strength data for a GM/GCL interface.

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