Designing with Geosynthetics - 6Th Edition; Vol2

By Robert M. Koerner

Following the structure of previous editions, Volume 2 of this Sixth Edition proceeds through four individual chapters on geomembranes, geosynthetic clay liners, geofoam and geocomposites. The two volumes must accompany one another. Volume 1 contains geosynthetics, geotextiles, geogrids and geonets. The two volumes must accompany one another. All are polymeric materials used for myriad applications in geotechnical, geoenvironmental, transportation, hydraulic and private development applications. The technology has become a worldwide enterprise with approximate $5B material sales in the 35-years since first being introduced. In addition to describing and illustrating the various materials; the most important test methods and design examples are included as pertains to specific application areas. This latest edition differs from previous ones in that sustainability is addressed throughout, new material variations are presented, new applications are included and references are updated accordingly. Each chapter includes problems for which a solutions manual is available.

• Language: English
• Publisher: Xlibris US
• Release date: Jan 16, 2012
• ISBN: 9781465345264

Book preview

Designing with Geosynthetics

6th Edition

Volume 2

Robert M. Koerner

Emeritus Director, Geosynthetic Institute

Emeritus Professor, Drexel University

Copyright © 2012 by Robert M. Koerner.

Library of Congress Control Number:  2011913565

ISBN:  Hardcover   978-1-4653-4525-7

            Softcover     978-1-4653-4524-0

            eBook          978-1-4653-4526-4

All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the copyright owner.

Rev. date: 08/23/2016

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Contents

Chapter 5: Designing with Geomembranes

5.0 Introduction

5.1 Geomembrane Properties and Test Methods

5.1.1 Overview

5.1.2 Physical Properties

5.1.3 Mechanical Properties

5.1.4 Endurance Properties

5.1.5 Lifetime Prediction

5.1.6 Summary

5.2 Survivability Requirements

5.3 Liquid Containment (Pond) Liners

5.3.1 Geometric Considerations

5.3.2 Typical Cross-Sections

5.3.3 Geomembrane Material Selection

5.3.4 Thickness Considerations

5.3.5 Side-Slope Considerations

5.3.6 Runout and Anchor Trench Design

5.3.7 Summary

5.4 Covers for Reservoirs and Quasi-Solids

5.4.1 Overview

5.4.2 Fixed and Suspended Covers

5.4.3 Floating Covers

5.4.4 Quasi-Solids Covers

5.4.5 Complete Encapsulation

5.5 Water Conveyance (canal) Liners

5.5.1 Overview

5.5.2 Basic Considerations

5.5.3 Unique Features

5.5.4 Summary

5.6 Solid-Material (landfill) Liners

5.6.1 Overview

5.6.2 Siting Considerations and Geometry

5.6.3 Typical Cross Sections

5.6.4 Grading and Leachate Removal

5.6.5 Material Selection

5.6.6 Thickness

5.6.7 Puncture Protection

5.6.8 Runout and Anchor Trenches

5.6.9 Side Slope Subgrade Soil Stability

5.6.10 Multilined Side Slope Cover Soil Stability

5.6.11 Access Ramps

5.6.12 Stability of Solid-Waste Masses

5.6.13 Vertical Expansion (Piggyback) Landfills

5.6.14 Coal Combustion Residuals

5.6.15 Heap Leach Pads

5.6.16 Summary

5.7 Landfill Covers and Closures

5.7.1 Overview

5.7.2 Various Cross Sections

5.7.3 Gas Collection Layer

5.7.4 Barrier Layer

5.7.5 Infiltrating Water Drainage Layer

5.7.6 Protection (Cover Soil) Layer

5.7.7 Surface (Top Soil) Layer

5.7.8 Post Closure Beneficial Uses and Aesthetics

5.8 Wet (Or Bioreactor) Landfills

5.8.1 Background

5.8.2 Base Liner System

5.8.3 Leachate Collection System

5.8.4 Leachate Removal System

5.8.5 Filter and/or Operations Layer

5.8.6 Daily Cover Materials

5.8.7 Final Cover Issues

5.8.8 Exposed Geomembrane Covers

5.8.9 Waste Stability Concerns

5.8.10 Summary

5.9 Underground Storage Tanks

5.9.1 Overview

5.9.2 Low Volume Systems

5.9.3 High Volume Systems

5.9.4 Tank Farms

5.9.5 Spray-Applied Geomembranes

5.10 Hydraulic and Geotechnical Applications

5.10.1 Earth and Earth/Rock Dams

5.10.2 Concrete and Masonry Dams

5.10.3 Roller-Compacted Concrete Dams

5.10.4 Geomembrane Dams

5.10.5 Tunnels

5.10.6 Vertical Cutoff Walls

5.11 Geomembrane Seams

5.11.1 Seaming Methods

5.11.2 Destructive Seam Tests

5.11.3 Nondestructive Seam Tests

5.11.4 Seaming Commentary

5.12 Details and Miscellaneous Items

5.12.1 Connections

5.12.2 Appurtenances

5.12.3 Leak Location (After-Waste Placement) Techniques

5.12.4 Wind Uplift

5.12.5 Quality Control and Quality Assurance

5.13 Concluding Remarks

References

Problems

Chapter 6: Designing With Geosynthetic Clay Liners

6.0 Introduction

6.1 GCL Properties and Test Methods

6.1.1 Physical Properties

6.1.2 Hydraulic Properties

6.1.3 Mechanical Properties

6.1.4 Endurance Properties

6.2 Equivalency Issues

6.3 Designing with GCLs

6.3.1 GCLs as Single Liners

6.3.2 GCLs as Composite Liners

6.3.3 GCLs as Composite Covers

6.3.4 GCLs on Slopes

6.4 Design Critique

6.5 Construction Methods

References

Problems

Chapter 7: Designing with Geofoam

7.0 Introduction

7.1 Geofoam Properties and Test Methods

7.1.1 Physical Properties

7.1.2 Mechanical Properties

7.1.3 Thermal Properties

7.1.4 Endurance Properties

7.2 Design Applications

7.2.1 Lightweight Fill

7.2.2 Compressible Inclusion

7.2.3 Thermal Insulation

7.2.4 Drainage Applications

7.3 Design Critique

7.4 Construction Methods

References

Problems

Chapter 8: Designing with Geocomposites

8.0 Introduction

8.1 Geocomposites in Separation

8.1.1 Temporary Erosion and Revegetation Materials

8.1.2 Permanent Erosion and Revegetation Materials: Biotechnical-Related

8.1.3 Permanent Erosion and Revegetation Materials:

Hard Armor Related

8.1.4 Design Considerations

8.1.5 Summary

8.2 Geocomposites in Reinforcement

8.2.1 Reinforced Geotextile Composites

8.2.2 Reinforced Geomembrane Composites

8.2.3 Reinforced Soil Composites

8.2.4 Reinforced Concrete Composites

8.2.5 Reinforced Bitumen Composites

8.3 Geocomposites in Filtration

8.4 Geocomposites in Drainage

8.4.1 Wick (Prefabricated Vertical) Drains

8.4.2 Sheet Drains

8.4.3 Highway Edge Drains

8.5 Geocomposites in Containment (Liquid/Vapor Barriers)

8.6 Conclusion

References

Problems

Chapter 5

Designing with Geomembranes

5.0 Introduction

5.1 Geomembrane Properties and Test Methods

5.1.1 Overview

5.1.2 Physical Properties

5.1.3 Mechanical Properties

5.1.4 Endurance Properties

5.1.5 Lifetime Prediction

5.1.6 Summary

5.2 Survivability Requirements

5.3 Liquid Containment (Pond) Liners

5.3.1 Geometric Considerations

5.3.2 Typical Cross-Sections

5.3.3 Geomembrane Material Selection

5.3.4 Thickness Considerations

5.3.5 Side-Slope Considerations

5.3.6 Runout and Anchor Trench Design

5.3.7 Summary

5.4 Covers for Reservoirs and Quasi-Solids

5.4.1 Overview

5.4.2 Fixed and Suspended Covers

5.4.3 Floating Covers

5.4.4 Quasi-Solids Covers

5.4.5 Complete Encapsulation

5.5 Water Conveyance (canal) Liners

5.5.1 Overview

5.5.2 Basic Considerations

5.5.3 Unique Features

5.5.4 Summary

5.6 Solid-Material (landfill) Liners

5.6.1 Overview

5.6.2 Siting Considerations and Geometry

5.6.3 Typical Cross Sections

5.6.4 Grading and Leachate Removal

5.6.5 Material Selection

5.6.6 Thickness

5.6.7 Puncture Protection

5.6.8 Runout and Anchor Trenches

5.6.9 Side Slope Subgrade Soil Stability

5.6.10 Multilined Side Slope Cover Soil Stability

5.6.11 Access Ramps

5.6.12 Stability of Solid-Waste Masses

5.6.13 Vertical Expansion (Piggyback) Landfills

5.6.14 Coal Combustion Residuals

5.6.15 Heap Leach Pads

5.6.16 Summary

5.7 Landfill Covers and Closures

5.7.1 Overview

5.7.2 Various Cross Sections

5.7.3 Gas Collection Layer

5.7.4 Barrier Layer

5.7.5 Infiltrating Water Drainage Layer

5.7.6 Protection (Cover Soil) Layer

5.7.7 Surface (Top Soil) Layer

5.7.8 Post Closure Beneficial Uses and Aesthetics

5.8 Wet (Or Bioreactor) Landfills

5.8.1 Background

5.8.2 Base Liner System

5.8.3 Leachate Collection System

5.8.4 Leachate Removal System

5.8.5 Filter and/or Operations Layer

5.8.6 Daily Cover Materials

5.8.7 Final Cover Issues

5.8.8 Exposed Geomembrane Covers

5.8.9 Waste Stability Concerns

5.8.10 Summary

5.9 Underground Storage Tanks

5.9.1 Overview

5.9.2 Low Volume Systems

5.9.3 High Volume Systems

5.9.4 Tank Farms

5.9.5 Spray-Applied Geomembranes

5.10 Hydraulic and Geotechnical Applications

5.10.1 Earth and Earth/Rock Dams

5.10.2 Concrete and Masonry Dams

5.10.3 Roller-Compacted Concrete Dams

5.10.4 Geomembrane Dams

5.10.5 Tunnels

5.10.6 Vertical Cutoff Walls

5.11 Geomembrane Seams

5.11.1 Seaming Methods

5.11.2 Destructive Seam Tests

5.11.3 Nondestructive Seam Tests

5.11.4 Seaming Commentary

5.12 Details and Miscellaneous Items

5.12.1 Connections

5.12.2 Appurtenances

5.12.3 Leak Location (After-Waste Placement) Techniques

5.12.4 Wind Uplift

5.12.5 Quality Control and Quality Assurance

5.13 Concluding Remarks

References

Problems

5.0 Introduction

According to ASTM D4439, a geomembrane is defined as follows:

Geomembrane: A very low permeability synthetic membrane liner or barrier used with any geotechnical engineering related material so as to control fluid (or gas) migration in a human-made project, structure, or system.

Geomembranes are made from relatively thin continuous polymeric sheets, but they can also be made from the impregnation of geotextiles with asphalt, elastomer or polymer sprays, or as multilayered bitumen geocomposites. In this chapter, we will focus on continuous polymeric sheet geomembranes since they are, by far, the most common.

Polymeric geomembranes are not absolutely impermeable (actually nothing is), but they are relatively impermeable when compared to geotextiles or soils, even to clay soils. Typical values of geomembrane permeability as measured by water-vapor transmission tests are in the range 1 × 10−12 to 1 × 10-15 m/s, which is three to six orders of magnitude lower than the typical clay liner. Thus, the primary function is always as containment of, or barrier to, liquids or vapors. As noted in section 1.6.3, the current market for geomembranes is extremely strong. New applications are regularly being developed, and this is directly reflected in sales volume; geomembranes are currently the largest segment of geosynthetics as far as product sales are concerned.

Since the primary function of geomembranes is always containment, as a barrier to a liquid and/or gas, this chapter is organized on the basis of different application areas. Liquid containment is treated first, and then solid-waste containment, followed by numerous geotechnical applications.

In order to augment the design calculations to follow, the next section describes geomembrane properties and the test methods used to obtain these properties. This information allows the design of the major geomembrane-related systems that are currently in use. The specific designs then form the subsequent parts of the chapter.

5.1 Geomembrane Properties and Test Methods

To design-by-function (the theme of this book) is to make a conscious decision about the adequacy of the ratio of the allowable property to the required property—that is, the factor of safety value. This section on properties is devoted to providing the test methods that form the numerator of this ratio. That said, a comment on organization promulgating test methods is in order. The majority to be referenced are by the American Society of Testing and Materials (ASTM) due to their long history in this activity. More recently, are test methods developed by the International Organization for Standardization (ISO). They will be dual referenced or designated in the absence of an ASTM method. Lastly, the Geosynthetic Research Institute (GRI) has developed test methods that are only for test methods not addressed by ASTM or ISO. All three organizations have websites from which such appropriate standards can be obtained.

5.1.1 Overview

The vast majority of geomembranes are relatively thin sheets of flexible thermoplastic polymeric materials (recall figure 1.1). These sheets are manufactured in a factory and transported to the job site, where placement and field seaming are performed to complete the job. Table 5.1 lists the principal ones currently in use; these are the geomembranes that will be focused on here. Section 1.2 gives an overview of the various polymers listed and describes a number of chemical identification or fingerprinting tests used to quantify their composition and formulation. Emphasis here is on individual test methods that will be grouped into three large categories of the manufactured sheet: physical properties, mechanical properties, and endurance properties.

TABLE 5.1 GEOMEMBRANES IN CURRENT USE

173471.png

5.1.2 Physical Properties

Physical properties have to do with the geomembrane in an as-manufactured and/or as-received state. They are important for specifications, confirmation evaluation, and, in many cases, design as well.

Thickness. Depending on the type of geomembrane, there are three types of thickness to be considered: the thickness of smooth sheet, the core thickness of textured sheet, and the thickness (or height) of the asperities of textured sheet.

Smooth Sheet. The determination of the thickness of a smooth geomembrane is performed by a straightforward measurement. The test uses an enlarged-area micrometer under a specified pressure, resulting in the desired value. ASTM D5199 and ISO 09863 are the test methods generally used for measuring geomembrane thickness. The pressure exerted by the micrometer is specified at 20 kPa. A number of measurements are taken across the roll width and an average value is obtained. When measuring the thickness of a geomembrane, there is little ambiguity in the procedure. Nonreinforced geomembranes are made in thicknesses from 0.5 to 3 mm. When measuring materials with scrim reinforcement or aged membranes that have swelled, extreme care must be exercised, particularly in the preparation of the test specimen and in the application of pressure. Test conditions and applied pressures should always be given together with the actual values. Scrim-reinforced geomembranes are manufactured from multiple plies of materials that when laminated together result in geomembranes of thickness from 0.91 to 1.5 mm (recall figure 1.24).

Textured Sheet. The roughened surface of a textured geomembrane results in a significant increase in interface friction with the adjacent material versus the same geomembrane with a smooth surface. The thickness of such textured sheets is measured as the minimum core thickness between the roughened peaks or asperities. To measure the core thickness, a tapered-point micrometer is recommended. The tapered point dimensions per ASTM D5994 are a 60° angle with the extreme tip at 0.08 mm diameter. The normal load on the tapered point is 0.56 N. For a single-sided textured sheet, only one tapered point is needed, while double-sided textured sheet requires a micrometer with two opposing tapered tips. Within a limited area, the local minimum core thickness is obtained. Typically, ten measurements across the roll width are taken and an average core thickness value is calculated and compared to the specification value.

Asperity Height. For textured geomembranes, the height of the asperity is of interest insofar as it relates to mobilizing the desired amount of interface shear strength with the opposing surface. Optimized texturing is a daunting task and a topic of interest to both the manufacturing and user communities. Profilometry has been attempted (Dove and Frost [1]), as well as three-dimensional topography (Ramsey and Youngblood [2]). Less involved, but still useful as a quality control and quality assurance method, is to merely measure the height of the asperities per ASTM D7466. In so doing, a depth gage micrometer with a 1.3 mm diameter pointed stylus is recommended. The gage is zeroed in on a flat surface and then is placed on the peaks of the textured sheet with the stylus falling into the valley created by the texturing. The localized maximum depth is the asperity height. A number of measurements are taken across the roll width and an average asperity height is obtained and compared to the specification value.

Density. The density (or dimensionless specific gravity) of a geomembrane is dependent on the base material from which it is made. There are distinct differences, however, even in the same generic polymer. For example, polyethylene comes in several varieties: low density, linear low density, medium density, linear medium density, and high density. The range for all geomembrane polymers falls within the general limits of 0.85 to 1.5 g/cc. The relevant test methods for density are ASTM D792 and ISO R1183. These two equivalent methods are based on the fundamental Archimedes’ principle of the weight of the object in air divided by its weight in water.

A more accurate (but more tedious) method is ASTM D1505. Here a long glass column containing liquid varying from relatively high density at the bottom to low density at the top is used. For example, isopropanol with water is often used for measuring densities less than 1.0, while sodium bromide with water is used for densities greater than 1.0. Upon setup, spheres of known density are immersed in the column to generate a calibration curve. Small pieces of the polymer test specimen are then dropped into the column. Their equilibrium level within the column is used with the calibration curve to find the specimen’s density. Accuracy is very good, within 0.002 mg/l when proper care is taken.

A comment on the density of HDPE geomembranes should be made. The ASTM classification for HDPE resin requires a density ≥ 0.941 mg/l. However, commercially available HDPE geomembranes use polyethylene resin from 0.934 to 0.938 mg/l; the resin, itself, is actually in the medium-density range (MDPE). Only by adding carbon black and additives to the mixture is its formulated density raised to 0.941 mg/l or slightly higher. Thus, what is called HDPE by the geomembrane industry is actually MDPE resin to the resin producer. An appropriate relationship between formulated density and resin density is as follows:

ρf  =  ρr + 0.0044 (% CB + % AO)

(5.1)

where

      ρf  =  formulated density

      ρr  =  resin density

%CB  =  percent carbon black

% AO  =  percent antioxidants

Melt (Flow) Index. The melt-flow index or melt index (MI) test is used routinely by geomembrane manufacturers as a method of controlling polymer uniformity and processability. It relates to the flowability of the polymer in its molten state. It is used for both the incoming resin and the final formulated geomembrane sheet. The test method often used for geomembrane polymers is ASTM D1238. Here a given amount of the polymer is heated in a furnace until it melts. A constant weight forces the fluid mass through an orifice and out of the bottom of the test device. The MI value is the weight of extruded material in grams for a 10 min flow duration. The higher the value of melt-flow index, the lower the density of the polymer, all other things being equal. High MI values suggest a lower molecular weight, and vice versa, albeit by a relatively crude method in comparison some of the techniques discussed in section 1.2.2.

The test is also sometimes performed using two different weights forcing the molten polymer out of the orifice; for example, the standard test is performed at 2.16 kg, and then repeated at 21.6 kg. The resulting MI values are then made into a ratio as follows.

FRR = MI21.6 / MI 2.16

(5.2)

where

FRR  =  flow-rate ratio,

MI21.6  =  melt flow index under 21.6 kg weight, and

MI2.16  =  melt flow index under 2.16 kg weight.

High values of FRR indicate broad molecular weight distributions and various empirical relationships have been proposed. Both MI and FRR tests are very important in the quality control and quality assurance of polyethylene resins and geomembranes.

Mass per Unit Area (Weight). The weight of a geomembrane (actually its mass per unit area but invariably called simply weight) can be determined using a carefully measured area of a representative specimen and accurately measuring its mass. It is measured in units of g/m². The test is straightforward to perform and usually follows ASTM D1910 procedures.

Water-Vapor Transmission. Since nothing is absolutely impermeable from a diffusion perspective, the assessment of the relative impermeability of geomembranes is an often-discussed issue. This discussion is placed along with physical properties for want of a better location. The test itself could use an adapted form of a geotechnical engineering test using water as the permeant, and this is the approach taken in European Standard designated as NF-EN 14150. In this case, a 200 mm diameter specimen is subjected to a 100 kPa differential water pressure against its surfaces. Monitoring is very sophisticated since extremely small values are obtained even over a long time period.

Preferred by the author, water vapor transmission can be readily monitored as in ASTM E96 wherein the mechanism is clearly diffusion. In this water-vapor transmission (WVT) test, a test specimen is sealed over an aluminum cup with either water or a desiccant in it and a controlled relative humidity difference is maintained across the geomembrane test specimen. With water in the cup (i.e., 100% relative humidity) and a lower-relative humidity outside of it, a weight loss over time can be monitored (see figure 5.1). With a desiccant in the cup (i.e., 0% relative humidity) and a higher-relative humidity outside of it, a weight gain over time can be seen and appropriately monitored. The required test time varies, but it is usually from 3 to 40 days. Water vapor transmission, permeance, and (diffusion) permeability are then calculated, as shown in example 5.1.

PDF1.jpg

Figure 5.1 A water-vapor transmission test setup and resulting data for a 0.75 mm thick PVC geomembrane.

Example 5.1

Calculate the WVT, permeance, and (diffusion) permeability of a 0.75 mm thick PVC geomembrane of area 0.0030 m², which produced the test data in figure 5.1 at a 80% relative-humidity difference while being maintained at a temperature of 30°C.

Solution: Calculations proceed in stages using the slope of the curve in figure 5.1.

(a) Find the water-vapor transmission;

116832.png

(5.3)

where

g  =  weight change (g),

t  =  time interval (h), and

a  =  area of specimen (m²).

62833.png

(b) The permeance is given as

62898.png

where

∆P  =  vapor pressure difference across membrane (mm Hg),

S  =  saturation vapor pressure at test temperature (mm Hg),

R1  =  relative humidity within cup, and

R2  =  relative humidity outside cup (in environmental chamber).

25736.png

(c)  (Diffusion) permeability = permeance × thickness

= (0.0703)(0.75) = 0.0527 metric perm-mm

Note that this is a vapor-diffusion permeability following Fickian diffusion, not the customary Darcian permeability (see the following example).

Example 5.2

Using the information and data from example 5.1, calculate an equivalent hydraulic permeability (i.e., a Darcian permeability, or hydraulic conductivity) of the geomembrane as is customarily measured in a geotechnical engineering test on clay soils.

Solution: The parallel theories are Darcy’s formula for hydraulic permeability, q = kiA,

Img59.psd

and the WVT test for Fickian diffusion permeability,

Untitled-2.psd

Thus, we must now modify the data used in example 5.1 into the proper units:

Img58.psdUntitled-1-1.psdImg57.psd

In terms of water pressure,

63722.png

Now using the density of water,

63840.png

and canceling the units out, we get a comparable Darcian k value for the geomembrane of

63950.png

_________________________

The WVT values for a number of common geomembranes of different thicknesses are given in table 5.2. It should be mentioned, however, that the above-described test method is statistically sensitive for thick geomembranes and particularly for HDPE since the WVT values are so low. The least amount of leakage around the test specimen-to-container seal will greatly distort the resulting test results. That said, the test results are indicative of the extremely low permeability of all factory manufactured geomembranes and a relative ranking is possible.

TABLE 5.2 WATER VAPOR TRANSMISSION (WVT) VALUES

173462.png

Of particular interest is the conversion of 1.0 g/m² day, approximately equal to 10 l/ha-day, which is the leakage sometimes associated with a flawlessly placed geomembrane. It has been referred to in various regulations as de minimus leakage (see footnote in section 4.2.2). Note that if such a low value is used, it automatically eliminates many geomembrane materials, even without a single leak! It also suggests that materials with extremely low WVT values are the best for all liquid containments. But as we will see in the next section, this is not necessarily the case.

Solvent-Vapor Transmission. When containing liquids other than water, the concept of permselectivity must be considered. Here the molecular size and solubility of the liquid vis-a-vis the polymeric liner material might result in very different vapor diffusion values than when using water. Organic solvents are in this category.

The test itself is a parallel to E96, the water vapor transmission test, except now the solvent of interest is placed within the cup. Obviously, care and proper laboratory procedures must be exercised when using hazardous or sometimes radioactive materials. As with the WVT test, proper sealing of the test specimen to the cup is essential and often extremely difficult to achieve. That said, some solvent-vapor transmission data is available and is reproduced in table 5.3. Notice the tremendous variation depending on the type of solvent being evaluated. Clearly, if solvents are to be contained by a geomembrane, the site-specific solvent-vapor transmission test should be used to assess the geomembrane’s containment capability in this regard.

The possibility of solvent vapor transmission has been investigated by Rowe et al. [5] who used synthesized leachates to measure diffusion through different thicknesses of HDPE geomembranes. A number of organic compounds are evaluated to obtain their diffusion coefficients. It is shown that the geomembrane provides an excellent barrier to acetic acid and chloride. Conversely, organic compounds (such as dichloromethane, 1,1-dichloroethane, 1,2-dichloroethane, and 2-butanone [MEK]) can diffuse though much more rapidly. Several preventative options in this regard are available: (i) using relatively thick geomembranes; (ii) treating the geomembrane’s surface, by fluorination [6,7]; (iii) to coextrude a thin film of EVOH (a random copolymer of polyethylene, polyvinyl alcohol and ethylene vinyl alcohol) within a standard polyethylene sheets, or (iv) other strategies. These include a back up of the geomembrane with a clay liner (CCL or GCL) for attenuation, thereby creating a composite liner; or using double-liner systems with an intermediate drainage layer.

TABLE 5.3 SOLVENT VAPOR TRANSMISSION (SVT) VALUES [4]

173451.png

5.1.3 Mechanical Properties

There are a number of mechanical tests that have been developed to determine the strength of polymeric sheet materials. Many have been adopted for use in evaluating geomembranes. This section attempts to sort out those having applicability to quality control or to design, i.e., index versus performance tests.

Tensile Behavior. Many tensile tests performed on geomembrane specimens are quite small in size and are used routinely for quality control and quality assurance (conformance) of the manufactured geomembranes. They are essentially index tests. The test procedures generally used are covered in ASTM D6693 or ISO 527-3 as well as ASTM D6392, D882, D751, and D413. Table 5.4 gives the currently recommended tests for commonly used geomembranes.

TABLE 5.4 RECOMMENDED TEST METHOD DETAILS FOR GEOMEMBRANES AND GEOMEMBRANE SEAMS IN SHEAR AND IN PEEL

173435.png

The results for several of these geomembranes are given in figure 5.2. The scrim-reinforced geomembrane fPP-R resulted in the highest strength but failed abruptly when the fabric scrim broke. The response, however, does not drop to zero because the geomembrane plies on both sides of the scrim remained intact until ultimate failure occurred. This is typical of all fabric-reinforced geomembranes. The HDPE geomembrane responded in a characteristic fashion by showing a pronounced yield point, at 10 to 15% strain, dropping slightly, and then extending in strain to approximately 1000% when failure actually occurred. The PVC geomembrane gave a relatively smooth response, gradually increasing in strength until its failure at about 480% strain. The LLDPE geomembrane also gave a relatively smooth, but lower, response until it failed at approximately 700% strain.

PDF2.jpg

Figure 5.2 Index tensile test results of common geomembranes using criteria given in Table 5.4

The curves were generated using the specimen’s original width and thickness to calculate stress and the original gage length to calculate strain. Thus, the axes are engineering stress and strain, rather than true stress and strain. Quantitative data gained from these curves are focused around the following:

Maximum stress (at ultimate for PVC and LLDPE, at scrim break for fPP-R, and at yield for HDPE)

Maximum strain (usually called elongation in the geomembrane literature)

Modulus (the slope of the initial portion of the stress-strain curve)

Ultimate stress at failure (or strength)

Ultimate strain (or elongation) at complete failure

Table 5.5a gives these values for the four materials shown in figure 5.2. While all the listed values of strength are significant, attention is often focused on the maximum stress. It must be recognized, however, that polymers are viscoelastic materials and strain invariably plays an important role.

TABLE 5.5 TENSILE BEHAVIOR PROPERTIES OF VARIOUS GEOMEMBRANES

(a) Index Test Results (Figure 5.2)

Nominal thicknesses are: HDPE 1.5 mm, LLDPE 1.0 mm, PVC 0.75 mm, CSPE-R 0.91mm Abbreviations: + = did not fail

(b) Wide Width Test Results (Figure 5.3)

Nominal thicknesses are: HDPE 1.5 mm, LLDPE 1.0 mm, PVC 0.75 mm, CSPE-R 0.91mm Abbreviations: + = did not fail

(c) Axi-Symmetric Test Results (Figure 5.5)

Nominal thicknesses are: HDPE 1.5 mm, LLDPE 1.0 mm, PVC 0.75 mm, fPP-R 0.91mm Abbreviations: - = values felt to be high

Tensile Behavior (Wide Width). A major criticism of the previously described index test specimens is their contraction within the central region, giving a one-dimensional behavior not experienced with wide sheets in field situations. Thus, uniform width and wider test specimens are desirable. Just how wide is a matter of debate. A width of 200 mm has been used for testing geotextiles and has been adopted for testing geomembranes (see ASTM D4885). The strain rate for testing geomembranes is, however, different from geotextiles. D4885 recommends using 1.0 mm/min. For a 100 mm long specimen with 200% strain at failure, the test would require 3.3 hours to complete. For a geomembrane with 1000% strain at failure, the test would require 16.7 hours. Clearly, such tests are not of the index or quality-control variety and should be considered to be performance-oriented.

Figure 5.3 presents tensile stress-versus-strain curves on the same four geomembranes that are shown in figure 5.2, but now for a uniform 200 mm width. While the general shape of each material is the same, the results of the various properties of interest are quite different. These results are tabulated in table 5.5b. It is felt that the use of a 200 mm width test specimen results in a much more design-oriented value than do test results from dumbbell or narrow-width specimens. This is particularly the case when plane-strain conditions are assumed in the design process (e.g., in side-slope stability calculations).

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Figure 5.3 Tensile test results on 200 mm wide width specimens of common geomembranes using ASTM D4885 test method

Tensile Behavior (Axi-Symmetric). There are situations that call for a geomembrane’s tensile behavior when it is mobilized by out-of-plane stresses. Localized deformation beneath a geomembrane is such a case. This type of behavior could well be anticipated for a geomembrane used in a landfill cover placed over differentially subsiding solid-waste material. The situation