Geosynthetic clay liner (GCL) and Geomembranes are two popular materials used in the containment of hazardous waste and liquids. Both materials have their own unique features, benefits and drawbacks that make them suitable for different applications.
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Geosynthetic clay liners are a composite material consisting of a layer of sodium bentonite clay sandwiched between two layers of geotextile fabric. The bentonite clay swells when it comes into contact with liquid, forming a barrier with low permeability. Geosynthetic clay liners are ideal for applications where high strength and puncture resistance is required, such as landfills, lagoons and ponds. The geotextile layers provide added strength and stability to the material, which is important in areas where heavy loads are expected.
Geomembranes, on the other hand, are made of synthetic materials such as high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE). They are flexible, impermeable sheets that are used to prevent the migration of liquids and gases. Geomembranes are typically used in applications such as pond liners, canal liners, and landfill caps. They are ideal for applications where a smooth, exposed hydraulic barrier is required.
In terms of cost, Geosynthetic clay liners are generally more expensive than geomembranes. This is due to the cost of the bentonite clay, the geotextile layers and the added complexity of production. They are also a popular choice for applications where a liner is expected to be in service for an extended period of time and offer several advantages over other types of liners, such as geomembranes.
One of the primary advantages of geosynthetic clay liners is that they are often more robust and can withstand a greater amount of wear and tear. This is because they are made from a combination of high-grade clay (bentonite) and geosynthetic materials, which gives them greater strength and durability.
In addition to their strength, geosynthetic clay liners are also easier and cheaper to install than some other types of liners. Unlike geomembranes, which require welding to join sections together, geosynthetic clay liners can be simply overlapped together. This makes them a more cost-effective option for applications where a large area needs to be lined.
In conclusion, both Geosynthetic clay liners and geomembranes have their own unique features and benefits that make them suitable for different applications. When choosing between the two materials, it is important to consider the specific requirements of your project, including strength and puncture resistance, cost, and installation process. By carefully considering these factors, you can select the best liner material for your specific application and ensure that your project is a success.
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Introduction
Uncontrolled waste releases can cause severe environmental damage. Fortunately, modern waste management facilities are designed and constructed to prevent these releases by providing a high level of containment of waste materials. The most obvious means to accomplish containment is to install a bottom liner system in the facility. This discussion evaluates some available liner system alternatives and their performance under various cost, time, and construction constraints. It will be shown that a strong argument can be made for the use of geosynthetic clay linersas the primary barrier component of waste management facilities and other containment systems.
Liner Options
A few of the most popular lining options are summarized in Table 1 below. Also included are estimated leakage rates for the various liners, which are based on theoretical and actual performance data from landfills.
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Table 1: Various landfill liner options and their relative containment abilities
Notes:
Leakage rates in litres/hectare/day, calculated assuming a hydraulic head of 0.3 m is present on each liner.
Leakage rates calculated using the method of Bonaparte, et. al. ().
Actual number and size of holes depends on installation quality. See references for further information.
This value is one-hundredth the calculated leakage of 860 lphd for standard geotextile-based GCLs at 1 x 10-11 m/sec permeability. The 100x adjustment is for the geomembrane component of the GCL, which will reduce leakage by an even greater amount.
The information in Table 1 reveals some important concepts:
All of the liners will dramatically reduce leakage in comparison to the case where there is no liner. The presence of a functional liner system is clearly the most important part of a containment system design.
The amount of leakage through the liner is lowest when “composite” liners are used; in other words, those liners containing both a geomembrane component and a clay component offer the best performance available.
Similar to the previous statement, any single liner system—geomembrane alone, clay alone, etc.—will not perform as well as a composite system. The synergistic behaviour of a geomembrane and a clay liner is extremely valuable with respect to achieving the lowest possible leakage values.
Liner Selection Factors
Other factors in addition to leakage performance will affect the choice of a liner system. How is the most appropriate liner system selected? First, it is necessary to evaluate standard design issues such as slope stability, chemical compatibility, site preparation, and others. If this evaluation eliminates any previously considered options, then the final decision of which liner system to select should involve careful consideration of the degree of containment required relative to its ease of installation, quality control/quality assurance requirements, and cost. Why these three issues? It is because they have the greatest influence on overall performance as explained below.
Ideally, the liner system should be easy to install, such that unskilled or semi-skilled labourers can deploy the liner with little or no previous training and with little chance for error. GCLs are clearly the easiest liners to install. Compacted clay liners require highly trained labour, special equipment, and rigorous monitoring of the materials and the materials placement process. Constant control over soil moisture conditions is also critical. Geomembranes require highly skilled labour for proper installation, which is expensive and time consuming if the liner system is deployed in small phases.
The amount of construction quality control and quality assurance (CQC/CQA) depends on the complexity of the liner system and the number of variables that must be controlled in order for the liner to perform at design standards. GCLs are factory-manufactured and therefore have little or no variation compared to natural soils. Geomembranes do not function unless individual panels are properly welded, a highly labour-intensive skill that even today is seldom mastered. Geomembranes are also frequently accidentally punctured, resulting in high leakage rates if these punctures are not detected by a rigorous CQA program. GCLs are self-seaming and self-healing, resulting in far fewer CQA-related problems in a typical application.
Finally, the cost of the liner system must be consistent with the performance it provides. This is where membranelaminated GCLs offer outstanding value. By combining the impermeability of a stand-alone geomembrane with the leak-sealing properties of a GCL, a product such as Claymat CL provides the state-of-the-art performance of a composite liner system at the cost of a single GCL. No other lining technology offers this price/performance ratio.
Quantifying the Selection Process
The selection of the “best” liner for a particular site involves consideration of the above factors in addition to its leakage performance. Table 2 provides a numerical comparison of these factors, resulting in an overall “composite performance factor” which summarizes the overall desirability of a liner system. In this analysis, performance and cost are the greatest importance, while installation difficulty and QA/ QC are given lesser importance.
Table 2: Liner system evaluation, with scores from 1 (worst/most difficult) to 5 (best/least difficult)
Notes:
Based on data presented in Table 1.
Composite Performance Factors are weighted averages based on the following components: Containment Ability = 40%; Installation = 15%; QA/QC = 15%; and Cost = 30%.
Cost varies with the availability of clay soils. Also, the materials cost of geosynthetic liners may be higher than clay, but resulting increased air space adds value to the landfill. Airspace issues are not considered in this analysis.
Geomembrane assumed to be 60-mil (1.5 mm) HDPE.
Table 2 is only a broad summary of some of the key issues involved in liner system selection and is not intended to be used as design guidance. However, the table does present some interesting results when performance is evaluated in consideration of installation issues and cost. In this analysis, the GCL and the membrane-laminated GCL have the highest performance factors because of their higher scores for some of the additional areas of consideration. This systematic analysis can be repeated using project-specific weighting factors if, for example, cost has a different level of importance than assumed herein.
Conclusions
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Selecting the “best” liner for a containment project involves more than just considerations of containment ability. Issues such as ease of installation, QA/QC, and cost must all be considered before such decisions are made. A numerical analysis as presented herein may be helpful in this regard. In the analysis performed above, it is clear that the GCL-based alternatives offer excellent performance and value.
Based on the comprehensive comparison between Geosynthetic Clay Liners (GCL) and Geomembranes, it's evident that both materials offer unique advantages for different containment applications. However, when considering factors such as strength, durability, ease of installation, and cost-effectiveness, GCL-based alternatives stand out as excellent choices. With our expertise and commitment to providing high-quality geosynthetic solutions, EcoGeoX is poised to meet your project's specific needs with innovative and reliable products. Contact us today to explore how we can help you achieve optimal containment performance and value. Or view more of our wide application projects!
Geogrids consist of a regular open network of integrally connected, tensile elements (ribs), which may be linked by extrusion, bonding or interlacing. The apertures between the ribs are larger than the constituents. The ribs are made of polymeric materials such as high-density polyethylene (HDPE), polypropylene, or other durable polymers. The manufacturing process may involve stretching of the polymer material to orient the molecular structure, increasing strength and stiffness of the ribs.
The stiff ribs and strong junctions of a geogrid enable a high degree of interaction between the geogrid and the surrounding soil. Soil particles are able to partially penetrate into the apertures and become restrained by the ribs or confined within the apertures. Geogrids are ideal for stabilization or reinforcement of soils, with applications such as construction over weak soils, road foundations and earth retaining structures – as such, they are one of the most commonly used geosynthetics.
Tensar produces four types of geogrids: uniaxial geogrids, biaxial geogrids, multi-axial geogrids (TriAx®), and the more complex Tensar InterAx® geogrids. Visit Tensar’s geogrids page to learn more about them.
Geotextiles are the largest group of geosynthetics, as well as one of the earliest types to be created. They are permeable fabrics that consist of synthetic fibers such as polyester or polypropylene, and can be created as either woven, knitted or non-woven textiles. The non-woven types are manufactured from directionally or randomly oriented fibres/filaments mechanically or thermally/chemically bonded together. They can vary in strength and weight, from lightweight filter products to high strength reinforcement materials.
This category of geosynthetic, when used in association with soil, can provide a wide variety of functions including separation, filtration, drainage, protection and reinforcement. Although most commonly used as a separator before construction of roads, or as filter/separators in drainage applications, they can be used across a range of applications in engineering projects.
Geofoam, also known as EPS (Expanded Polystyrene), is an incredibly lightweight durable material that's can be used in numerous applications as an alternative to soil backfill. Geofoam blocks are created via polymeric expansion of the polystyrene, which produces many gas-filled, closed cells throughout the block. This design is what makes them so low in density.
The low density of geofoam makes it very useful on engineering projects as a fill material over soft or compressible foundation soils. Used as a lightweight core for an embankment, it will reduce settlements and may make it possible to avoid staged construction.
Geosynthetic clay liners (GCL) are built using two sheets of non-woven geotextile with a layer of sodium bentonite clay sandwiched between. The sheets are bonded together (using stitching or needle punching) to create structural integrity; they’re then heat treated to secure the layers in place.
GCL’s provide a faster, more convenient alternative to traditional clay lining of containment ponds. These materials have an added advantage in that the sodium bentonite layer has swelling properties. As such, clay liners offer a degree of self-sealing that reduces leakage. GCL liners benefit many geotechnical applications, including waste treatment and landfill.
Geocomposites combine of two or more of the geosynthetic types discussed above. Combining the features of each geosynthetic creates a product with more benefits than any individual product type, particularly useful in drainage and containment applications and some road foundation situations. For example, Tensar combines stabilization geogrids with separation/filtration geotextiles for use in road and rail foundations where fine soil migration may be an issue. Take a look at the Tensar FilterGrid product page for an example of a geocomposite.
Where geosynthetics are used to stabilize granular soils, this typically occurs via an interlocking mechanism. With geogrids, for example, the apertures between ribs allow aggregate to strike through and interlock, confining the aggregate material. Provided that the geogrid has strong junctions, and ribs that offer high stiffness at low strain, movement of the soil particles can be minimized, improving the mechanical behavior of the soil. This mechanical stabilization creates a composite layer that is stronger and more resistant to deformation.
Geogrid stabilization is common in roadway foundations and in working platforms that will endure heavy loads, as it increases bearing capacity and reduces deformation under load. You can learn more about the stabilizing power of interlock in this Tensar article.
The drainage function of geosynthetics allows groundwater or other fluids, to be collected and pass through less permeable soils. Drainage geosynthetics can be used to dissipate pore pressure below embankments, intercept groundwater in slopes or behind structures, and provide edge drainage to road pavements.
Drainage geosynthetics are usually geocomposites, typically combining a geonet drainage core with one or more layers of geotextile. They are able to pass water (and other liquids or gas) through their structure to a collector or open space.
Good drainage is essential for roadways as water under the surface can lead to softening of subgrade soils and eventual loss of strength in the road structure. Therefore, geosynthetics can commonly be found in roads and railways, behind retaining walls, as well as below embankments where less permeable soils exist.
Erosion control is the practice of limiting damage to land due to the action of wind or water. Once the top layer of land is eroded, re-growth takes a long time, and this is where erosion control geosynthetics come in to give nature a helping hand.
Erosion control geosynthetics, typically, in the form of multi-layered mats, reduce soil erosion caused by impact of water droplets and surface runoff. They are rolled onto a surface and pegged in place. Some products combine synthetics with natural materials to provide enhanced moisture retention to encourage vegetation growth.
In areas where land is exposed to water flow or rainfall, erosion control geosynthetics are ideal for protecting the top layer of soil, encouraging vegetation to grow and preventing soil loss in the future. This is particularly common around areas of water and embankment slopes.
Soil particles, particularly finer particles, can be transported by water passing through soils. Filtration geosynthetics, usually geotextiles, are designed to retain soil particles on the upstream side of the filter, while allowing water to pass freely through. Even fine soil particles can be retained due to the ‘bridging’ effect of larger particles on the upstream side of the filter. Filtration is therefore most effective with one-directional water flow.
The filtration properties of geotextiles can be designed by varying the type and density of fibres, and the thickness and structure of the fabric. They are often combined with a drainage core in the form of a geocomposite. Suitably engineered products may be used to prevent soil migration into drainage aggregate layers or gravel filled drains, or for critical applications below riprap protection in river or coastal works.
To function as a separator, the geosynthetic must prevent soil with different particle size distributions from intermixing and causing the structural integrity to fail.
Separation is a required function in many applications, however it is vitally important to the layers of roads and pavements. Geotextile separators are routinely used below road and rail construction, in isolation or combined with a geogrid in the form of a geocomposite.
A geogrid can prevent expensive subbase material from punching into the soft subgrade. When a well-graded subbase is stabilized with a geogrid the geogrid/soil composite layer can prevent finer grained soil from migrating up into the subbase. When soil moisture levels are high, a geocomposite with geogrid and separator/filter properties may be used. Learn more in this blog article.
Geosynthetics are often applied in areas such as roadways, railways, airports and more.
For roads and runways, they’re primarily useful in stabilizing and separating unbound pavement layers. However, they can also be used to address issues with the underlying soil, or in providing side drainage.
Geogrids have been used to aid construction and enhance the performance of roads over soft ground since the s. More recent advancements have led to their increased use to enhance the service life of paved roads, reducing whole life costs. Visit this page on roads, pavements and surfaces to discover more on how Tensar products improve road construction projects.
Geosynthetics can be applied to solve a variety of problems below rail track. Stabilization geogrids are routinely used to increase the bearing capacity and stiffness of the trackbed over areas of weak soils. They can also be placed below the ballast layer to control lateral migration and deterioration of the ballast particles. Differential stiffness issues associated with transitions from rigid to flexible foundations can be addressed with geogrid stabilized transition zones. Visit this page on rail trackbed improvement to discover more on how Tensar products improve rail track maintenance and construction projects.
Geotextile separators and drainage geocomposites are used to control moisture related problems, while highly specialised sand filled geotextile mats can replace sand filters below trackbed.
The construction of earth embankments over weak soils presents challenges that can be addressed by the use of geosynthetics. Over-stressing of the foundation soil as construction proceeds can result in a deep rotational failure. The inclusion of geosynthetic reinforcement in the base of the embankment can maintain stability against this failure mechanism.
Three-dimensional cellular mattress systems, such as Stratum® Foundation Geocell, provide reinforcement at the base, but in addition, the inherent stiffness of the cellular mattress distributes load and influences the settlement profile. Geosynthetic wick drains, driven deep into the foundation soil can relieve excess pore pressure as the embankment rises, enabling more rapid construction.
Design and construction of a geogrid stabilized working platform for use with a ringer crane in the US was undertaken in and completed in . Problematic soil conditions were found at the crane pad area and consisted of fat clays and occasional sandy silt lenses and pockets. Stringent criteria for differential and total settlement needed to be met to ensure successful crane operation. Heavy seasonal rains were impacting construction.
A multi-axial geogrid-stabilized working platform was designed to improve allowable bearing capacity of the soil and to decrease potential settlement. The crane bearing pad was installed on time and on budget despite of numerous construction delays due to the heavy seasonal rains. Unlike the concrete option that was considered, the crushed stone could be easily removed and reused at other site locations. Estimated total cost savings of $3.1 Million when compared with original plans to construct a deep foundation system. Success of the geogrid stabilized platform was further demonstrated when it withstood Hurricane Harvey without damage and the crane was back in operation the day after the storm passed.
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The pavement section installation for the container storage area was underway. It was designed with two layers of a composite biaxial geogrid and geotextile fabric, with 12” ALDOT 825B aggregate placed on the bottom layer and 6” on the top layer. During construction, significant rutting and subgrade pumping occurred after installation of the first layer of geogrid and stone. There was significant risk in continuing to build the pavement section as designed due to the soft, unpredictable, and undocumented subgrade material.
Using Tensar+ design software, a Tensar representative designed a solution using NX850-FG to minimize cut and fill while also providing the separation benefits of a non-woven fabric. The design recommendation required a single 18” lift of coarse sand with low fines content to be placed on the NX850-FG. The original pavement section was then installed above the newly stabilized subgrade.
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