How can textured geomembranes be used in final solid-waste landfill-closure designs?
Su Jong Hao and Douglas Sutherland
In solid-waste containment applications, ensuring the stability of the waste mass, the lining system and subgrade are critical to the success and longevity of the solution. Liners with a textured surface can improve the friction strength between the liner and the subgrade, or between liners in multilayer systems, thereby improving slope stability. But which liners offer the best outcomes?
The World Bank conservatively estimates current global waste generation at 1.3 billion tons per year for municipal solid waste alone. This is expected to increase to approximately 2.2 billion tons by 2025. With steep side slopes increasingly standard in landfills – as owners strive to increase capacity and revenues – a number of liner characteristics become important.
Landfill cell liners and closure solutions are massive, critical projects. While the geosynthetic-liner solution represents a small fraction of the overall cost, it is the greatest defense in a landfill, forming a powerful barrier between pollutants and the environment. It also contributes significantly to the stability of landfill structures.
For geomembrane, liner thickness, asperity height (the maximum variation in height between peaks and valleys on a given surface), interface-shear strength, seam integrity, and several other considerations all play a role in containment design. Decisions on the most appropriate lining system will also depend on factors such as the need for filtration, surface runoff and gas drainage, seepage prevention, and leak detection. In addition, the ability to accommodate differential settlement and gas venting – along with resistance to heat and corrosive, flammable and poisonous materials – need to be taken into account.
Geosynthetics are recognized for their ability to create safe and cost-effective barrier solutions. Geosynthetic clay liners (GCLs) act as a secondary or tertiary barrier, while geomembranes are used as primary and secondary barriers. However, in terms of stability (i.e. stability of the lining system on slopes), research1 has shown that a higher texture density provides the best performance in terms of achieving higher frictional resistance.
At a slope of approximately eight degrees the friction angle becomes critical, and your system could begin to slide. Textured membranes can give a friction angle of up to 35 degrees, improving the opportunity to design much steeper slopes.
ADVANTAGES OF DIFFERENT TYPES OF TEXTURED LINER
Three common types of geomembrane texturing are co-extrusion, spray on, and embossing.
Extrusion in three layers (cover, core, and under layer) on round-die or blown-film extrusion equipment allows for control of the formulation of the two surface layers. Nitrogen gas is injected into the molten polymer surface layers and allowed to escape after exiting the die, leaving a roughened and irregular surface. Other additives, such as white pigments or a higher concentration of carbon black (for leak-location liners), can also be added at this stage. While the core layer is not affected by this procedure, an aggressive surface-asperity profile (generally greater than 0.4 mm high) is formed. This improves frictional resistance against soils and geotextiles, with a robust Velcro-like effect under the typical low-to-normal load conditions found in landfill closures.
Spray-on geomembrane texture:
This is a two-step process that involves spraying molten polyethylene mixed with hot air onto a previously manufactured round-die or flat-cast smooth geomembrane base. The resulting engineering properties include the Velcro-like effect and improved friction resistance when in contact with non-woven geotextiles and geosynthetic clay liners. Greater friction resistance can be achieved by adjusting the weight per unit area of the material sprayed.
Patterned or embossed geomembrane:
This is done as part of a single-step process. As the polyethylene sheet emerges from the flat die in a viscous state, embossing rollers imprint a structure on its surface. Advantages include a consistent core thickness and texture, which has little effect on the tensile and elongation properties of the geomembrane.2
Is one textured geosynthetic manufacturing method better than another?
The advantage of the co-extruded textured geomembrane is that the material is monolithically formed from a one-step process, which can improve the material’s long-term performance.
Another benefit is that the asperity height can be altered on-line. The geomembrane can be applied to a wide range of slope designs by changing the textured surface. Also, the Velcro-like effect from the texturing process reduces geomembrane wrinkling with better adhesion to the non-woven geotextile.
The main advantage of a spray-on texture is that the physical properties of the membrane are not compromised. In addition, the edge strip on the geomembrane remains smooth and can be covered with a protective tape – this makes for easier welding than on other textured materials and is highly beneficial for installation in muddy or wet conditions. However, adhesion of the texture particles in the spray-on process needs to be thoroughly tested in production.
And what about embossed geomembrane? The value of embossed membrane is that it provides a good friction angle (depending on pattern structure), especially at high loads. While increasing the asperity height can be beneficial, high-texture density should also be retained.
WHICH LINER WHEN?
All three texturing varieties have benefits and limitations. Performance testing is crucial for optimal design. The properties of each surface structure, along with the application demands (for example, multiaxial elongation will be vitally important in areas of differential settlement), need to be considered.
At Solmax, our ability to manufacture liners to meet the specific needs of each project ensures we can meet our clients’ needs in the toughest conditions. All our liners, including our specialized solutions – e.g. our GSE LLX with its multi-axial break resistance and improved aging and UV resistance for applications where higher settlements and uneven surfaces demand additional support and greater flexibility – can be manufactured in various color finishes, with a conductive under layer, and as single or double-textured geomembranes.
For more on drainage and solution design, click through to Solmax’s Drainage Design Tool: https://www.drainagedesigntool.com