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GCL effects on Landfill and strip footing resting on a layer of GCL
Reza Hassanvand
1, Mahdi Khodaparast
21- University of Qom, Iran ([email protected])
2- Department of civil engineering, University of Qom, Iran ([email protected])
Abstract
A geosynthetic Clay Liner (GCL) is a thin layer of clay (or bentonite) that bonded to a geomembrane or fixed between two sheets of geotextile. GCLs accepted as a barrier system for sealing and Landfills application. This paper debated about the sealing purpose of GCLs. A comprehensive review is provided to identify key considerations for the general sealing use of GCLs, design and construction and compile the latest research. In this paper, identifies areas where more research is needed and provides recommendations for future research and development.
Keywords: Geosyntethic Clay Liner (GCL), sealing, Landfill, seepage
1. INTRODUCTION
Geosynthetic clay liners (GCLs) are most typically comprised of a thin layer of sodium (calcium) bentonite contained between two layers of geotextile with the components being held together by needle-punching or stitch bonding. The swelling and hydraulic performance of these “mixed occupation” sodium bentonites are improved by activating with soda-ash (sodium carbonate) to replace the other cations with sodium cations (John Buckley, Will P. Gates, Daniel T. Gibbs, 2011). They are widely used in lining systems of modern waste containment facilities to minimize migration of contaminants and gases (Bouazza, 2002; Rowe, 2012, 2014). GCLs have several advantages over sealing application including ease of installation and often much better performance in terms of reducing leakage when used in conjunction with a geomembrane in a composite liner. Manufacturers' installation guidelines typically recommend that following installation of the GCL, either alone or as part of a composite liner, the liner be covered by at least 0.3 m (and sometimes 0.5 m) of drainage layer, soil cover, or ballast layer shortly after installation (Rowe et al., 2004).
GCL technology provides some unique advantages over conventional bottom liners and covers. GCLs, for example, are fast and easy to install, have low hydraulic conductivity (i.e., low permeability), and have the ability to self-repair any rips or holes caused by the swelling properties of the bentonite ( sodium or calcium) from which they are made.
GCLs are expensive in regions where clay is not readily available. A GCL liner system is not as thick as a liner system involving the use of compacted clay, enabling engineers to construct landfills that maximize capacity while protecting area ground water.
While there are many potential applications for prefabricated bentonite clay liners, their major use to date has been beneath a geomembrane, thereby forming a composite liner. For landfills, surface impoundments and waste piles this has been in the formation of the primary composite liner and/or the secondary composite liner (depending on the site-specific design requirements and regulations).
The primary function of the bentonite component is to limit the migration of fluids (Rowe, 2001; Bouazza et al., 2006). The geotextile component is essentially the carrier/reinforcement network which allows the
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placement of a uniform barrier layer of processed bentonite (generally 5-8 mm thick) and reinforcement of the bentonite layer to restrict free swelling and improve hydrated shear strength characteristics.
The connection between the three components is usually achieved by needle punching from the nonwoven geotextile to the woven one, extracting fibers (reinforcing fibers) from the nonwoven that cross the bentonite reaching the woven geotextile. Next, the thermal treatment is mainly used to improve the bonding strength of the connection between the woven geotextile and the reinforcing fibers.
During the last 20 years, various tests have been carried out to characterize the shear behavior of the GCLs, obtaining the following main mechanical strengths:
•Shear strength of internal GCL and GCL/Geosynthetic interfaces.
• Bonding strength characterized by the peel strength and the shear tensile strength of the GCLs (Belén M.
Bacas, Elena Blanco-Fernandez, Jorge Cañizal, 2013).
Fig. 1 GCL types (United States Environmental Protection Agency, EPA530-F-97-002, December 2001) Typical liner applications include landfill (municipal and hazardous), water reservoirs, tailings facilities and mine waste residue impoundments (Gates et al., 2009; Hornsey et al., 2010). The performance parameters (e.g., swell index, hydraulic conductivity, shear strength) for these applications are generally highly variable across GCLs, due primarily to differences in manufacturing and material sourcing, and these need to be thoroughly understood in order to accurately predict field behavior.
GCL field hydraulic performance can be affected by a variety of factors, such as compatibility with overlying soils (cation exchange), presence of roots from vegetation, humidity of the environment (GCL moisture content) and age of the installation (e.g Dobras and Elzea, 1993; James et al., 1997; Meer and Benson, 2007; Benson et al., 2007).
This paper includes a thorough summary of technical literature on GCL, aiming at identify key consideration for use of GCL as sealing and ground stabilization measure against high ground water table. The intent is to offer insight for design and construction, and more importantly to identify source of useful information. In the next four sections, the relevant fallowing aspects are discussed based on four literature review: a) GCL materials, b) Hydraulic conductivity of GCL, C) design and construction, and d) final conclusion. The paper offers a comprehensive areas where more research is needed and provides recommendations for future research and development.
2. GCL MATERIALS
The GCL contained sodium (calcium) bentonite sandwiched between a nonwoven cover and carrier geotextile and the system was punched together or bonded to a geomembrane (Fig. 1). The measured (air dry) mass per unit area is typically 4500 g/m2 and the measured peel strength is usually 12 K N. The bentonite had a montmorillonite content of 54%, and cation exchange capacity is typically 100 meq/100 g. A geomembrane is a polymeric sheet material that is impervious to liquid as long as it maintains its integrity. A geotextile is a woven or nonwoven sheet material less impervious to liquid than a geomembrane, but more resistant to penetration
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damage. Although the overall configuration of GCL impress its performance characteristics, the primary performance factors are bentonite quality, mass of clay used per unit area and uniformity.
Bentonite is a powerful absorbent, granular clay formed from volcanic ash. The bentonite in a GCL consists of thin smectite particles that are negatively charged. Bentonite absorb positively charged water particles (Fig.2); thus, it hydrates when exposed to liquid, such as water or leachate. The negative charge is balanced by cations in a thin layer of bound water surrounding the clay particle (Rowe et al., 2004). Examining the transport of clay particles with deionized water through fissures in bedrock, (Moreno et al, 2011) As the clay hydrates it swells, giving it the ability to “self-sealling”.
Fig2. Water and leachate absorbtion (B.M.Das, 1983)
Bentonite is affixed to synthetic materials in several ways to form the GCL system (Fig1 & Fig3). In configurations using a geomembrane, the clay is affixed using a glue. In geotextile configurations, however, adhesives, stitch bonding, needle punching, or a combination of the three, are used. Although stitch bonding and needle punching make small holes in the geotextile, these holes are sealed when GCL’s clay layer hydrates (Fig.3).
When a GCL rests on a soil with significant cations in its pore water or comes into contact with municipal solid waste leachate, sodium bentonite may experience an exchange of the sodium ions in the bound water around the clay particles with other cations (predominantly calcium and magnesium) in the pore water or leachate. This cation exchange leads to both an increase in free water pore space (at a constant effective stress), with a consequent increase in hydraulic conductivity, and reduction in swelling capacity (Rowe et al., 2004).
Bentonite surface
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Fig3. GCL’s Cross section (Robert M.Koerner, 2005)
3. HYDRAULIC CONDUCTIVITY OF GCL
One of most important norm to select an effective landfill barrier system is hydraulic conductivity. Before select a barrier system, the landfill operator should test the technology under consideration to ensure that its hydraulic conductivity, as well as other characteristics, are appropriate for the particular landfill site. GCL can offers barrier systems with low permeability that is the rate at which a liquid passes through a material. The bentonite’s high swelling capacity and low permeability provide an effective hydraulic seal. Sodium Bentonite is a non-metallic clay composed mostly of the mineral montmorillonite (Fig.4). It is formed from volcanic ash.
Montmorillonite is a layered clay mineral with broad, flat platelets that are ideally formed to provide a hydraulic barrier. Sodium ions located between these platelets allow water to hydrate the bentonite in a remarkable fashion that results in the high swelling characteristic.
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Fig4. Montmorillonite structure (B.M.Das, 1983)
Because of its high swelling capability, bentonite can also seal around penetrations, giving geosynthetic clay liners their self sealing characteristic.
Several studies have shown the importance of GCL initial hydration with a non-chemically aggressive hydration fluid to improve its chemical compatibility to leachates or other more aggressive solutions (Vasko et al., 2001; Lee and Shackelford, 2005; Katsumi et al., 2008; Liu et al., 2015). In common practice, initial hydration of GCL could be achieved through a passive process in which water is transferred from the subgrade to the GCL if there is no intimate contact (Rouf et al., 2014) or through an active process where moisture can be taken from the subgrade in liquid form if there is an intimate contact between the subgrade and the GCL (Rayhani et al., 2011).
Fig5. Self sealing (Robert M.Koerner, 2005)
Sodium bentonite is the preferred clay in most all commercially available GCLs is that with sodium in the bentonite structure, hydraulic conductivity tends to be extremely low. Under low normal loads, if the sodium is exchanged by calcium, the hydraulic conductivity of bentonite will increase from one to three orders of magnitude. Significant cation exchange can occur in unprotected sodium bentonite in most soil environments
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within several months to several years (Egloffstein, 2002). Thus, a important factor that effect the hydraulic conductivity of GCLs is the type and valence of the cations within the bentonite platelet structure.
Laboratory tests show that the hydraulic conductivity of dry, unconfined sodium bentonite is approximately 2*10-8 cm/sec. When GCL saturated, the hydraulic conductivity of bentonite typically drops to less than 2*10- 10 cm/sec.
Over a range of normal loads, sodium bentonite of GCLs offer excellent hydraulic performance. Figure.6 (Daniel D.E, 2000), can be used to estimate the design hydraulic conductivity value of a typical sodium bentonite of GCL when exposed to compatible liquids under a given design normal load.
Fig6. Hydraulic conductivity (Daniel, D.E., 2000)
4. DESIGN AND CONSTRUCTION
GCLs are normally installed as part of a sealing system. The interaction of the various elements of a sealing system and the GCL is an implex problem that you must address at the design stage. You should consider the physical and chemical interactions between different elements of the system. The physical and chemical issues are mostly have close related with matters such as interface friction angles, normal and shear stresses, assessments of the material life in the predicted environment, anchor trench pullout strengths and reactions to heat and chemicals in the leachate. Your design have to take account of all of these issues and interactions.
To design GCLs for landfills we should consider 6 issue that mentioned below.
a. GCL anchor trenches
Anchor trenches should be design to hold the GCL in place during construction and the full design life site. As a guide, trenches are generally in the order of 300 - 500mm wide and 500mm to 1000mm deep. Backfill should well be compacted to minimize water ingress and to prevent GCL pullout. Where a slope exceeds 70 meters or the roll length, or where the forces created by the GCLs weight approach the internal shear strength of the
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material, intermediate anchor trenches will be required. This is to prevent the forces created by the weight of the GCL exceeding its tensile strength, causing it to stretch or tear.
b. Slope stability
Slopes shouldn’t exceed 1v:3h. A slope stability analysis which clarify there is a factor of safety of 1.5 or more.
c. GCL subgrade
The surface that the GCL is installed on, should be smooth and free of debris, roots, sticks, and sharp rocks/boulders larger than 50 mm. The subgrade shall be final-graded to fill in any remaining voids or desiccation cracks and proof-rolled to ensure that no sharp irregularities or abrupt elevation changes exist greater than 25 mm (Fig.7).
Fig 7. Surface subgrade (Mokarrar composite CO, 2014)
d. Chemical and landfill gas compatibility
The chemistry of the subgrade should be considered. Landfill gas, vapors and condensates can all adversely effect on GCL if they contain substances harmful to either the clay or geotextile layers.
Consequently, the GCL should be protected against the likely permeants in the hydration stage and also in the operational and restoration stages.
e. GCL installation
the GCL should not install during rough weather conditions such as snow or heavy continuous rainfall.
Fig 8. GCL overlap (Mokarrar composite CO, 2014)
During installation, Construction Quality Assurance (CQA) Inspector should ensure the GCL is placed in the anchor trenches in accordance with the drawings. The CQA inspector must also ensure the GCL is laid parallel to the direction of the slope, and is free from kinks and folds.
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Joints between panels are usually formed by overlapping the panels by a minimum of 300 mm and sealing, usually with bentonite powder or paste (Fig.8).
Fig 9. Junction with pipeline (Mokarrar composite CO, 2014)
In designing, should avoid penetrating the GCL with pipes or other structures. Where such penetrations are absolutely necessary, an agree manner that will install them should be consider before beginning (Fig.9).
f. Mineral protection layers
Only the amount of GCL that can be anchored, inspected, and covered should be installed the same day or prior to any rehydration. In cases where the GCL is the sole hydraulic barrier, the GCL should be covered with the specified thickness of cover soil ≥ 300 mm. Cover soil placed directly on the GCL should not damage the GCL.
It is recommended that the soil is well graded and particle sizes range between fines and 25 mm (SW or GW).
Calcareous materials such as limestone quarry fines in protection layers should not be use because the potential for cation exchange between calcium and sodium exist
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5. FINAL CONCLUSION
The purpose of using GCLs is to combine the advantages of the two materials, each having different hydraulic, physical and endurance properties so that contaminant migration can be limited to levels that will result in negligible impact.
GCL products of the type described in this paper provide to the landfill owner/designer many of the same advantages offered by natural soils. These are the following::
Factory quality-controlled materials with regard to their fabrication.
Generally lower cost per unit area of coverage.
Rapid deployment at the field site.
Relative ease in performing construction quality control and assurance.
Relatively large savings in air space.
These positive features, should be counterpointed against potential problems that may arise with any new material.
This emerging technology needs additional field and laboratory test to further assess its effectiveness as a landfill barrier system in terms of the key performance factors discussed below. Improved product design and installation standards should also be established.
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A study provides the basis for allaying some concerns about the bearing capacity of hydrated GCLs, but more research is required. The study demonstrated that an adequate layer of cover soil, placed on GCLs during installation, prevents a decrease in liner thickness with the application of a load. Without a sufficient soil layer, GCLs become compressed, raising their hydraulic conductivity (i.e., making them more permeable) and reducing their effectiveness as a barrier.
6. ACKNOWLEDGEMENTS
The authors thank university of Qom to support us in this study. We wish to thank the two anonymous reviewers who made insightful and constructive improvements to this manuscript.
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