Implicaciones de la obra de Kelly en el proceso terapéutico

Geomembranes are nowadays extensively used in landfills as part of basal and sidewall/slope liner systems and/or in capping systems. Their main function is to limit

contaminant migration, to reduce water ingress into the landfill and to control biogas escape to the atmosphere. This document provides details on minimum standards for geomembrane liner material, thickness, quality, strength, durability, installation procedures and testing

requirements. It is aimed at assisting engineers, specifiers, designers, regulators, facility owners and operators in assessing geomembranes for landfill engineering purposes.

D.1

Introduction

Geomembranes are flexible polymeric sheets mainly employed as liquid and/or vapour/gas barriers. They are designed as relatively impermeable liners for use in a variety of civil engineering applications. They are frequently used as a component of basal and side slope liners, and in capping systems for landfills.

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

D.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: (a) control the leachate head acting on the underlying liner and (b) 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).The purpose of a composite liner is to combine the advantages of two materials, each

having different hydraulic, physical 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 water ingress into the landfill and to 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 where a geomembrane is used 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, so as to minimise the generation of leachate (waste liquid).

D2.1 Types of geomembranes

The large number of commercially available

geomembranes can make it challenging to select which geomembrane has the most appropriate combination of performance properties for a given application. Each type of geomembrane material has different characteristics that affect its installation, durability, lifespan and overall performance. It is therefore necessary to match the project performance criteria with the right combination of properties of a particular geomembrane.

Selection of a geomembrane liner should consider: • the hazard posed by the contained material and

leachate

• susceptibility of the liner material to chemical or environmental attack or deterioration while in service • tensile strength and elasticity

• thermal stability

• puncture, tear and shear resistance

• anticipated operational life required for effective containment

• local environmental conditions, including subsoil stability.

Geomembrane materials should be selected based on their overall performance with respect to issues such as chemical resistance, mechanical properties, temperature resistance, thermally induced stresses

(expansion/contraction), weathering resistance, product life expectancy, installation factors, cost effectiveness, and the type of application.

Due to the nature of the barrier system of which they will form part, geomembranes will often be subjected to

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coupled effects, whether they are used in bottom or side liners or in capping systems. For example, in bottom or side liners, the selected geomembrane might need to operate under the combined effects of mechanical stresses, leachate chemistry and increased temperature caused by the biological decomposition of the organic matter in municipal solid waste for short and long periods of time. Observed temperatures in different landfills reported in the literature range from 1 to 65 °C and at the liner from 7 to 60 °C (Yesiller et al. 2005, Rowe and Islam, 2009, Hanson et al., 2009, Bouazza et al., 2010).

On the other hand, in capping systems, the combined effect of the soil overburden pressure and differential settlements of the waste will tend to dominate and therefore will govern the selection of a given geomembrane.

Consequently, it is very important to recognise the difference between the lining applications and the conditions under which the geomembranes will operate for both short term and long term of applications. It follows from this that it is essential to select the geomembrane, based on scientific and technical data, such that it has the material properties required to meet the engineering requirements of the particular application.

D2.2 Selecting geomembranes

Since the engineering requirements vary depending on the specific application, the geomembrane property

requirements will differ from one application to another (in other words, requirements will be different for capping and bottom lining). Because of the different magnitude of the ‘stresses’ (tensile, interface, thermal, chemical and so on) that may be developed in different design

circumstances, a geomembrane suitable for one specific application may not be suitable for another even though, superficially, the two applications may look similar. The majority of the geomembranes used in landfills are thermoplastics (they can be remelted) which means that they are relatively easy to weld and repair.

Polyethylene is by far the polymer most widely used to manufacture geomembranes. It can be classified into several categories based on its density and branching: high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) are the geomembranes most used in landfills. It is important to emphasise that modern HDPE geomembranes are actually manufactured using a polyethylene resin with a density of 0.932–0.940 g/cm3_,

which falls into the medium-density polyethylene (MDPE) category as defined in ASTM D833.

It is the addition of carbon black and additives that result in a final density of the geomembrane between 0.941 and 0.950 g/cm3_{, which corresponds to an HDPE as defined in}

ASTM D833. The physical and mechanical properties of polyethylene are highly sensitive to resin density as shown schematically in Figure D1.

Figure D1: Generalised relationships between density of polyethylene and materials properties (from Hsuan et al. 2008).

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