Judíos y visigodos

Dr WP Gates, Research Fellow, Dept. Civil Engineering, Monash University, wrote section E2.1.1, ‘Bentonite used in GCLs’. His contribution is gratefully acknowledged.

E.1

Introduction

A geosynthetic clay liner (GCL) is a thin (typically 5 to 10 mm) factory-manufactured hydraulic or gas barrier comprised of a layer of bentonite supported by geotextiles (on either side) and/or geomembranes, mechanically held together by needling, stitching, or chemical adhesives. GCLs are also known as Geosynthetic Barriers-Clay (GBR-C) by the International Standards Organization. Their primary function is to act as hydraulic and/or gas barrier in a diverse range of civil engineering applications. They are frequently incorporated with other geosynthetics and soil components in basal and side slope liners, and in capping systems for landfills because of their very low hydraulic conductivity.

This document covers the use of geosynthetic clay liners in landfills. It provides details on minimum standards for liner material, quality, strength, durability, installation procedures and testing requirements. It is aimed at assisting engineers, specifiers, designers, regulators, facility owners and operators in designing GCLs for landfill engineering purposes.

E.2

Background

Modern municipal solid waste (MSW) facilities are typically designed with a bottom-barrier system intended to limit contaminant migration to levels that will result in negligible impact.

The system includes a leachate collection system (LCS), which is intended to: control the leachate head acting on the underlying liner, and collect and remove leachate. The leachate collection system typically incorporates a

geotextile filter, a granular drainage layer or geocomposite, and perforated collection pipes.

The liner may range from a thick, natural clay deposit to engineered liner systems involving one or more

geomembrane (GM) and/or compacted clay liner (CCL) or geosynthetic clay liner (GCL), and/or a compacted clay liner augmented with a GCL.The purpose of a composite liner is to combine the advantages of two or three

materials (depending in part on their availability and cost), each having different performance characteristics as hydraulic or physical barriers and endurance properties. Covers over municipal solid waste landfills are typically multicomponent systems that are constructed directly on top of the waste shortly after the site, or portion of a site, has been filled to capacity.

The aim of the cover system is to reduce the ingress of water into the landfill and control biogas escape to the atmosphere. Its design is usually driven by the landfill management approach put in place for a given site. One approach, in which a GCL is used either by itself or in combination with a geomembrane as part of the capping system, is referred to as the passive approach. In this case, the aim is to provide a cover system as impermeable as possible and as soon as possible after the landfill has ceased operating, to minimise the generation of leachate (waste liquid) and gas escape into the atmosphere.

E2.1 Types of geosynthetic clay liners

Geosynthetic clay liners (GCLs) have evolved into multicomponent systems with the incorporation of a thin layer of bentonite bonded by adhesive, needling or stitching to a layer or layers of geosynthetics. GCLs come in a variety of thicknesses, have a range of bentonite mass per unit area, different geotextiles confining the bentonite, different manufacturing details and different roll sizes.

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Originally the primary purpose of bonding bentonite to geosynthetics was to protect the bentonite during transport and installation and to provide a uniform layer of high swelling, low-permeability bentonite as a

component of the lining system. However, research has indicated that the confining of the bentonite during initial hydration that is achieved with some methods of

manufacture may improve the performance of the GCL, especially in applications where hydration may occur under low stress or where the GCL may be subject to wetting and drying cycles.

Figure E1 shows cross-sections of currently available GCLs. They offer a compromise between the hydraulic conductivity and shear strength requirements of containment projects. These products can be broadly categorised into unreinforced and reinforced GCLs. Unreinforced GCLs typically consist of a layer of sodium bentonite that may be mixed with an adhesive and then affixed to geotextile or geomembrane backing

components with additional adhesives (Bouazza 2002). For the geomembrane-supported GCL, the bentonite is bonded to the geomembrane using a non-polluting adhesive, and a thin, open-weave, spun-bound geotextile is adhered to the bentonite for protection purposes during installation.

Reinforced GCLs are geotextile-supported GCLs bonded by either needle-punching or stitch-bonding, with the bentonite contained by the geotextiles on both sides (Bouazza 2002).

Stitch-bonded GCLs consist of a layer of bentonite between two carrier geotextiles, sewn together with continuous fibres in parallel rows.

Needle-punched GCLs consist of a layer of bentonite between two carrier geotextiles, reinforced by pulling fibres from the top geotextile through the bentonite and into the bottom geotextile using a needling board. The fibres that are punched through the bottom geotextile rely on natural entanglement and friction to keep the GCL together.

However, some needle-punched GCL products are thermally treated to minimise fibre pullout. Thermal treatment involves heating the GCL surface to induce bonding between individual reinforcing fibres, as well as between the fibres and the carrier geotextiles (Lake and Rowe 2000). Thermally treated needle-punched GCLs are typically referred to as ‘thermal-lockedTM_{’ GCLs.}

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