DEVELOPMENT OF METHODOLOGY TO QUANTIFY INSTALLATION DAMAGE ... Yi Ting_Cheah_Thesis.pdf · Development of methodology to quantify installation ... values are derived from mechanical

DEVELOPMENT OF METHODOLOGY TO QUANTIFY INSTALLATION DAMAGE ON

GEOTEXTILE FOR COASTAL APPLICATION

Charmaine Yi Ting Cheah BEng(Hons)

Submitted in fulfilment of the requirements for the degree of

Doctor of Philosophy

School of Civil Engineering and Built Environmental

Science and Engineering Faculty

Queensland University of Technology

2017

Development of methodology to quantify installation damage on geotextile for coastal application i

Keywords

Apparent Opening Size, CBR Puncture Resistant, Coastal Protection Structure, Drop

Rock Test, Filtration properties, Geosynthetics, Geotextiles, Hydraulic Efficiency,

Impact Resistance, Installation Damage, Pore Size, Retained Strength, Robustness,

Subgrade Moisture Content, Unsaturated Subgrade

ii Development of methodology to quantify installation damage on geotextile for coastal application

Publications

Published Peer Reviewed Journal Paper

Cheah, C., Gallage, C., Dawes, L., Kendall, P., 2016. Impact resistance and evaluation of retained strength on geotextiles. Geotextiles and Geomembranes 44, 549-556.

Cheah, C., Gallage, C., Dawes, L., Kendall, P., 2017. Measuring hydraulic properties

of geotextiles after installation damage. Geotextiles and Geomembranes 45, 462-470.

Journal Papers under Review

Cheah, C., Gallage, C., Dawes, L., Kendall, P., 2017. Investigating installation robustness of geotextile in relation to subgrade moisture condition. Geotextiles and Geomembranes (under review).

Published Peer Reviewed International Conference Papers

1. Kendall , P., Austin, R.A., Cheah , C., Lacey , M., 2014b. Large Scale Controlled Testing of Geotextile Puncture Resistance for Rock Impact, 10th International Conference on Geosynthetics. Deutsche Gesellschaft für Geotechnik e.V., Berlin Germany.

2. Kendall, P., Austin, R.A., Cheah, C., 2014a. Installation Durability Testing of Revetment Geotextiles, 7th International Congress on Environmental Geotechnics, Melbourne, Australia.

3. Cheah, C., Gallage, C., Dawes, L., Kendall, P., 2015. Effect of Simulated Rock Dumping on Geotextile, 12th Australia New Zealand Conference on Geomechanics, Wellington, New Zealand.

Development of methodology to quantify installation damage on geotextile for coastal application iii

Abstract

Since geotextiles have been progressively incorporated into coastal protection

structures, the influence of construction stress on geotextile has been the primary

concern. During the construction of coastal structures with geotextiles, rock dumping

(installation method) imparts high mechanical stress on the laid geotextiles. These

stresses can degrade geotextiles’ mechanical strength and filtration properties. This

puts the stability and performance of geotextiles in coastal structures at risk. Research

studies point out that the heavy construction stresses imparted onto the material usually

by far exceeds the service stresses. This suggests that the critical period of geotextile

is during installation/ construction process. Therefore, it is essential to assess and

estimate any change in mechanical and filtration properties of geotextiles upon

installation where construction stress (dropping rocks) is involved. Filtration

properties of geotextiles herein refer to as hydraulic performance of geotextiles’ in

terms of permittivity and pore size. Long term observational studies and case studies

are the core of the current design guideline. These approaches are highly accurate in

assessing the extent of damage (mechanical and filtration properties) on geotextiles

after installation, but it requires heavy logistics (man power and time).

To minimise damage during installation, designers and engineers depend on

specifying minimum geotextile properties and on-site performance testing. These

values are derived from mechanical index tests such as California Bearing Ratio (CBR)

test, Wide Strip Tensile Test (WSST), Trapezoidal Tear Test etc. Conventional

laboratory approaches are highly conservative in accounting for the stress state of the

geotextile material under real installation conditions due to an inadequate ability to

replicate the construction stress (rock dumping). On-site tests are accurate in reflecting

iv Development of methodology to quantify installation damage on geotextile for coastal application

the performances of geotextiles under real installation conditions but there are many

installation parameters that cannot be fully controlled to produce repeatable results.

Besides, each installation site has its own unique field conditions which make

comparison of results difficult. Drawbacks of both index and field trials in minimising

installation damage highlight a requirement for a new testing methodology.

Towards this aim, a new Drop Rock Test (DRT) method is developed in this

research to produce similar construction stress imparted in the field. Referring to the

method developed the performance such as retained strength and filtration properties

of geotextiles can be evaluated in conditions more closely aligned with what happens

in practise. A series of tests with different non-woven geotextiles, drop height, weight

of armour rocks and subgrade moisture content with this newly developed apparatus

showed that the index derived values do not primarily govern the robustness of

geotextile during installation. This research study showed that geotextiles with better

mechanical properties did not outperform lower mechanical strength geotextiles. This

further supports the correlation between index tests and field performance is empirical

in nature. This research analysed the retained strength and filtration properties of

geotextile on dry subgrade and found that there is a strength reduction, up to 50% upon

installation which is critical when considering coastal structures are designed to last

for 100 years.

Installation damages could affect the hydraulic efficiency of geotextiles or in the

severest form of damage, punctured (hole), would limit the filtration function. The

properties investigated in this study include the permittivity and apparent opening size

(AOS) of geotextiles. This study showed that the greater the drop energy of an armour

unit (a function of drop height and weight of an armour unit) applied to geotextiles,

the greater the potential for damage. Findings show that the residual permittivity

Development of methodology to quantify installation damage on geotextile for coastal application v

increase significantly, approximately 45% upon installation. Using these results, this

research developed design charts predicting permittivity of geotextiles after

installation.

To date, little research is available on the influence of site characteristics on the

robustness of geotextile during installation procedures. Moisture content, density and

soil type of the subgrade have significant effects on the installation damage of

geotextiles. The proposed DRT method was revised to replicate different subgrade

moisture content to examine the robustness of geotextiles under different installation

conditions (dry, field or saturated subgrade). Findings suggest dumping rock on

geotextiles on dry subgrades results in severe damage compared to saturated subgrade.

Considering data and findings into mechanical, filtration properties and site

characteristics, it can be concluded that the newly developed semi-laboratory method,

DRT in this research replicates construction stress in-situ realistically. With the use of

the DRT, the influence of construction stress and subgrade moisture content on the

robustness, mechanical and filtration properties of geotextiles can be examined. This

results in development of design charts. Engineers and designers would be able to use

the design charts to specify the appropriate geotextile that have sufficient robustness

to resist damage during construction which reduces risk and is cost-saving.

vi Development of methodology to quantify installation damage on geotextile for coastal application

Table of Contents

Keywords .................................................................................................................................. i

Publications .............................................................................................................................. ii

Abstract ................................................................................................................................... iii

Table of Contents .................................................................................................................... vi

Statement of Original Authorship ........................................................................................... ix

Acknowledgements .................................................................................................................. x

Chapter 1: Introduction............................................................................................. 1

1.1 Background ........................................................................................................................ 1

1.2 Aims and Objectives .......................................................................................................... 3

1.3 Scope .................................................................................................................................. 4

1.4 Significance of research ..................................................................................................... 5

1.5 Research Design ................................................................................................................. 6

1.6 Thesis Outline .................................................................................................................... 8

1.7 Linkage of scientific papers ............................................................................................... 8

Chapter 2: Literature Review ................................................................................. 11

2.1 Introduction ...................................................................................................................... 11

2.2 Geotextiles ........................................................................................................................ 14 2.2.1 Non-woven Geotextiles ....................................................................................... 14 2.2.2 Design and construction challenges .................................................................... 15 2.2.3 Selection of Geotextiles ...................................................................................... 16

2.3 Types of damages ............................................................................................................. 16

2.4 Previous research .............................................................................................................. 18

2.5 Existing Test Methods ...................................................................................................... 23 2.5.1 Index Test methods ............................................................................................. 23 2.5.2 Field Trials .......................................................................................................... 29

2.6 Summary .......................................................................................................................... 31

Chapter 3: Research Design .................................................................................... 35

3.1 Methodology .................................................................................................................... 35

3.2 Drop Rock Test ................................................................................................................ 37

3.3 Visual assessment ............................................................................................................. 42

3.4 Static Puncture Test .......................................................................................................... 43

3.5 Constant Head Test .......................................................................................................... 43

3.6 Bubble Point Test ............................................................................................................. 43

Chapter 4: Impact Resistance and evaluation of retained strength .................... 45

Statement of Contributions of Joint Authorship ..................................................................... 45

Linkage of paper to Research Methodology and Development ............................................. 46

Development of methodology to quantify installation damage on geotextile for coastal application vii

Abstract ...................................................................................................................................48

4.1 Introduction .......................................................................................................................48

4.2 Materials and Experimental Description ...........................................................................52 4.2.1 Drop Rock Test ....................................................................................................52 4.2.2 Materials ..............................................................................................................55 4.2.3 Static Puncture (CBR) Test .................................................................................57

4.3 Results and Discussion .....................................................................................................57 4.3.1 Impact Resistance ................................................................................................57 4.3.2 Retained Strength.................................................................................................62

4.4 Conclusion ........................................................................................................................65

Acknowledgements .................................................................................................................66

References ...............................................................................................................................66

Chapter 5: Measuring hydraulic properties of geotextiles after installation ..... 69

Statement of Contributions of Joint Authorship .....................................................................69

Linkage of paper to Research Methodology and Development ..............................................70

Abstract ...................................................................................................................................72

5.1 Introduction .......................................................................................................................72

5.2 Test Program .....................................................................................................................78 5.2.1 Materials ..............................................................................................................78 5.2.2 Equipment and Methodology...............................................................................79

5.3 Results and Discussion .....................................................................................................83 5.3.1 Permittivity ..........................................................................................................83 5.3.2 Retention ..............................................................................................................91 5.3.3 Strained/Elongation .............................................................................................93 5.3.4 Abrasion Damage ................................................................................................94

5.4 Limitations ........................................................................................................................95

5.5 Conclusion ........................................................................................................................95

Acknowledgements .................................................................................................................96

Notations .................................................................................................................................96

References ...............................................................................................................................97

Chapter 6: Investigation of installation robustness of geotextiles in relation to subgrade moisture condition ................................................................................... 99

Statement of Contributions of Joint Authorship .....................................................................99

Linkage of paper to Research Methodology and Development ............................................100

Abstract .................................................................................................................................102

6.1 Introduction .....................................................................................................................102

6.2 Materials and Experimental Description .........................................................................105 6.2.1 Drop Rock Test ..................................................................................................110 6.2.2 Static Puncture (CBR) test .................................................................................112 6.2.3 Materials and test program ................................................................................112

6.3 Results and Discussion ...................................................................................................113 6.3.1 Impact survivability of non-woven geotextiles .................................................113 6.3.2 CBR Puncture Strength ......................................................................................116

viii Development of methodology to quantify installation damage on geotextile for coastal application

6.4 Limitations ..................................................................................................................... 118

6.5 Conclusions .................................................................................................................... 118

Acknowledgements .............................................................................................................. 119

References ............................................................................................................................ 119

Chapter 7: Discussion ............................................................................................ 121

7.1 Introduction .................................................................................................................... 121

7.2 Major Outcomes ............................................................................................................. 122 7.2.1 Drop Rock Test Methodology (Objectives 1 and 2) ......................................... 122 7.2.2 Geotextiles’ robustness against construction stress (rock dumping)

(Objective 4) .................................................................................................... 123 7.2.3 Damage assessment (permittivity) upon installation (Objectives 2 and 5) ....... 125 7.2.4 Geotextiles’ robustness against subgrade moisture condition (Objective 3

and 6) ............................................................................................................... 127 7.2.5 Research Impact for other forms of geosynthetics .......................................... 129

Chapter 8: Conclusion ........................................................................................... 130

8.1 Conclusions .................................................................................................................... 130

8.2 Recommendation for future studies................................................................................ 133

Bibliography ........................................................................................................... 135

Development of methodology to quantify installation damage on geotextile for coastal application ix

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To the best

of my knowledge and belief, the thesis contains no material previously published or

written by another person except where due reference is made.

Signature:

Date: 27 September 2017

QUT Verified Signature

x Development of methodology to quantify installation damage on geotextile for coastal application

Acknowledgements

Completion of this Doctoral research would have not been possible without the support

and assistance of numerous people throughout the research project. I would like to

express my appreciation to my Principal Supervisor, Dr Chaminda Gallage, Associate

Supervisor, Prof Les Dawes and External Supervisor, Mr Preston Kendall. Their

support, guidance and professional advice provided to me throughout the duration of

the research has been invaluable and I am very grateful for their assistance.

I wish to acknowledge QUT Australian Postgraduate Awards (APA) (2014-

2016) and QUT Research Training Program (RTP) (2017) for providing scholarship

for this research study. I would also like to acknowledge the financial support, test

materials and testing facilities provided by Geofabrics Centre of Excellence in Gold

Coast, Australia. The support given by technical team, Mr. Dan Gibbs, Mr. Warren

Mitchell, Ms. Dawn Smith is gratefully acknowledged.

I would also like to express my appreciation to Texas Research Institute (TRI)

Australasia for providing the testing facility for the study of filtration properties of

damaged geotextiles and thank the technical staffs, particularly Mr. Warren Hornsey

and Miss Patricia Voussem for sharing insight and expertise that greatly assisted the

research.

To my colleagues, Mr Biyanvilage Dareeju and Mr Glen Barnes, I appreciate

them for their assistances and advices in conducting experimental studies on the

permittivity of geotextiles. Finally, the support given by QUT technical staffs and QUT

undergraduate students (Mr. Gerard Vink, Mr. Matthew Lacey, Mr. Lawrence Chai,

Mr. Oliver Iacopi, and Mr. Luke Blacklock) are gratefully acknowledged.

Introduction 1

Chapter 1: Introduction

1.1 Background

Geotextiles were first introduced as construction expedients for low risk civil

structures like transport applications. Parallel with the development of man-made

fabric, the multifaceted geotextiles have been incorporated in coastal protection

structures such as revetments and bund walls. Geotextiles act as filters and separators

as well as providing drainage and reinforcement for structural stability. Geotextiles

has progressively been incorporated into geotechnical applications especially coastal

protection structures like rock revetments and armoured bank (Heerten, 1984; Koerner,

2016a; Koerner, 1984; Mannsbart and Christopher, 1997; Nielsen et al., 2013;

Pilarczyk, 2000; Pilarczyk, 2003).

Geotextiles are incorporated into revetment armours, gabions and riprap to

prevent soil erosion. For coastal protection application, geotextiles have been

extensively used as filters where the material acts a filter layer between the rocks laid

above and subgrade beneath. The material has gradually substituted traditional

granular material as filter material because of their comparable function and consistent

properties. The installation technique for geotextile is also quicker and easier and more

labour efficient compared to placing granular material in position.

However, the material often suffers damage due to high dynamic bulk loading

of rock placements (construction stress), which may compromise the material’s

mechanical and filtration properties (Hornsey, 2012; Wong et al., 2000b). The long-

term performance of geotextiles depends on the retained properties upon installation.

Therefore, it is essential to examine the influence of construction stress (rock dumping)

on the physical, mechanical and filtration properties of geotextiles. The influence of

2 Chapter 1: Introduction

construction stress on geotextiles’ properties is yet to be sufficiently evaluated due to

the lack of an appropriate method to replicate installation conditions. As a result,

designers and engineers rely on long term observational studies (Heerten, 1984;

Heerten, 2007; Hsuan et al., 2008; Loke et al., 1995; Mannsbart and Christopher, 1997;

Watn and Chew, 2002; Wong et al., 2000a) to understand the mechanical and filtration

properties after installation. Knowledge of mechanical and filtration properties can be

identified and measured but these studies require high operational costs and time

(mostly in years).

The common methods adopted by engineers and designers when selecting

geotextiles (type and grade) to ensure installation damage resistance involve

specifying minimum mechanical properties and field trials. This often leads designers

and engineers specifying thicker geotextiles (higher cost) or adopting construction

method that is uneconomical and not efficient. Minimum mechanical properties are

usually specified from index tests so that the material for the intended function can be

selected. Though the current index based classification system is able to predict the

performance behaviour of geotextiles, it fails to take account of the site characteristics.

Consequently, the incorrect selection of material leads to poor on-site performance

(Giroud, 2000, 2005; Heerten, 2007; Koerner and Koerner, 2015). Authors presented

sixty-nine (69) case studies related to inadequate performance of geotextile filter after

installation due to inadequate design, atypical soils, unusual permeants and improper

installation (Koerner and Koerner, 2015). Although Lawson established a strong

correlation between index tests and performance on site, the relationship is still

empirical in nature (Lawson, 1992). It is difficult and risky to rely on minimum

specified index values as installation sites have various parameters influencing the

materials’ resistance to damage. Hence, many experimental studies and research

Introduction 3

constantly review and revise existing design guidelines and standards (Briaud, 2013;

Christopher and Fischer, 1992; Koerner, 2016b; Loke et al., 1995; Luettich et al., 1992;

Pilarczyk, 2003; Rawal et al., 2010b).

Field trials provide accurate results, but currently, there isn’t a standard

procedure for on-site testing that is adopted by the industry. This is because procedures

in conducting performance test on-site varies in many ways and is influenced by many

factors including geotextile characteristics, height and weight of released armour rock

and characteristics of soil base. Given that each installation/construction site has

distinct environmental features, makes it even harder to develop a standard procedure

that is adoptable for different construction sites. These field/ on-site performance tests

are subjected to high operation costs, undergo difficulties in controlling site

characteristics and comparing results. These limitations highlight a requirement for a

new experimental approach to evaluate the influence of construction stress on the

behaviour of physical, mechanical and filtration of installed geotextiles. Development

of a new methodology to evaluate the robustness of geotextiles during installation is

therefore promising to the designers and engineers from both designing, construction

and maintenance perspectives, in providing a stable coastal protection structure.

Robustness herein refers to geotextiles’ or ability to survive (without puncturing)

under impact loads that are applied by dropping rocks onto geotextiles laid on the

subgrades or resist damage during installation (no perforation holes).

1.2 Aims and Objectives

The main aim of this research is to develop a new experimental methodology to

examine the influence of construction stress (rock dumping) on geotextiles’

robustness, mechanical strength and filtration properties in coastal protection

structures. The study also examines the influence of subgrade characteristics on


geotextiles’ robustness. The aims of the research are achieved through the following

objectives:

1. Develop new experimental method to replicate construction stress (rock

dumping) on geotextiles for coastal application

2. Examine the influence of construction stress on geotextiles properties

(robustness, mechanical strength, physical and filtration)

3. Examine the influence of subgrade characteristics (i.e. moisture condition) on

geotextiles’ robustness during installation

4. Develop design chart to predict robustness of geotextile during installation

5. Develop design chart to predict the hydraulic/ filtration performance

(permittivity) after installation

6. Develop chart for robustness of geotextile against subgrade moisture condition

1.3 Scope

The research undertaken was specifically focused around installation damage

associated with construction stress (rock dumping) on geotextiles. The influence of

construction stress and site characteristics on the robustness, mechanical and filtration

properties of geotextiles upon installation were investigated. The study was confined

to non-woven geotextiles (staple fibre and continuous filament) as they are commonly

used with coastal protection structures. Note that non-woven geotextiles are just one

part of a geosynthetics group that are often constructed to support various civil

applications. Geosynthetic material such as woven geotextile, geogrids,

geomembranes, geosynthetic clay liner and geo-composites are not considered in this

study.

The focus of this research was to examine the influence of construction stress on

the short term performance (robustness, retained strength, permittivity and pore size)

Introduction 5

of geotextiles. Long term performance related to the resistance to degradation of

geotextiles installed (clogging, thermal sensitivity, chemical and oxidation resistance,

weathering) were not investigated. The testing method, Drop Rock Test (DRT) was

developed to replicate construction stress, i.e. dumping rock on geotextiles laid on

subgrade during construction of coastal protection structures. The outcomes are

applicable to comparable drop heights and weight of armour rocks in similar field

settings.

Construction of coastal revetment is commonly conducted on natural sand that

could either be dry, semi-dry/wet (field) or saturated condition. An investigation of

robustness in relation to subgrade moisture condition provides relevant information

for site conditions which can have varying levels of moisture depending on the tidal

influence and the climate.

1.4 Significance of research

The lack of methodology to simulate construction stress for coastal application works

has led to the need for developing a new experimental method that would provide

similar nature and extent of damage on geotextiles as the construction process. The

benefit of using the developed method to examine the influence of construction stress

on geotextiles would reduce the underestimation or overestimation of the performance

of geotextiles. This will give engineers and designers greater confidence in selecting

the appropriate geotextile for a given function. The outcomes would be developed

charts to predict the robustness, retained strength and the filtration properties of

geotextiles during installation (rock dumping) process.

The introduced method provides adequate information to help engineers to choose

appropriate geotextile that have sufficient robustness to resist damage during


construction and is cost effective. Designers and engineers would save cost as the

appropriate geotextile and construction method are selected (i.e weight of armour

rocks and height of released stone). The stability of the finished coastal structure will

be maximised as the extent of damage on geotextile during construction is minimised.

1.5 Research Design

Figure 1 illustrates the research process in the form of a flow chart. The initial phase

of the study focuses on the research literature and identifying knowledge gaps to

establish the research problem. The outcomes from experimental investigations

provide a scientific basis for assessing the short term performance (robustness, retained

strength, and filtration properties) of geotextile. As a result of detailed data analysis,

identification of threshold drop energy, analysis of retained strength, residual

permittivity and pore size of geotextile, robustness and damage assessment for

permittivity charts for non-woven geotextiles were developed.

Introduction 7

Figure 1-1: Flow chart of research process (Refer specific objectives in Chapter 1.2)

Investigate coastal protection structure with geotextiles

Selection of geotextiles for study

Evaluate existing methods that evaluates geotextiles' robustness against construction (rock dumping)

Examine the influence of construction stress (rock dumping) on mechanical and filtration properties

Development of testing methodology

Quantify and assess the influence of construction stress on geotextiles' robustness and retained strength

Analysis of construction stress on the filtration properties of geotextiles

Examine the influence of site characteristics on geotextiles' robustness and retained strength

Develop design chart for robustness of geotextiles against construction stress using Drop and CBR energy

Develop design chart to predict the permittivity performance of geotextile upon instllation

Develop charts to predict robustness of geotextiles against subgrade moisture condition

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1.6 Thesis Outline

This thesis consists of eight chapters. Chapter 1 presents the research background,

development of research problem, aim and objectives to address the research problem,

significance and scope of this study. Chapter 1 also illustrates the research design

process and presents the linkage of scientific papers. The relevant research literature

and knowledge gaps are summarised in Chapter 2. It highlights the types of non-woven

geotextiles, the challenges faced when designing geotextiles with coastal protection

structures and the types of damages geotextiles are subjected during installation

process. Chapter 3 describes the testing methodology developed, index tests that were

used in this study and mechanical properties of non-woven geotextiles tested. Chapter

4 and 5 in the form of peer reviewed papers examines the influence of construction

stress on geotextiles’ robustness, mechanical and filtration properties. Likewise

Chapter 6 describes the influence of subgrade moisture conditions on the robustness

of geotextile. The major findings from Chapter 4, 5 and 6 are discussed in Chapter 7,

along with the development of methodology, design charts and guidelines for

geotextiles’ robustness and permittivity. Chapter 8 finally concludes the research and

significant findings along with recommendations for future research.

1.7 Linkage of scientific papers

The central focus regarding the design of coastal protection structure with geotextiles

is the robustness (survivability) of the material during installation/construction

process. The effects of construction stress (installation method-dumping rock on

geotextiles) on geotextiles play a vital role in determine the short-term performance

(retained strength and hydraulic properties) of geotextiles in the structure. Currently

there is no index test method that could account for field conditions (drop height,

weight of rock and subgrade condition) that determines the survivability (robustness)

Introduction 9

of the material and any assessment study that focus directly on the influence of

construction stress on the mechanical and hydraulic performance of geotextile.

Chapter 4, Paper 1 (Impact resistance and evaluation of retained strength)

investigates and identifies the advantages and disadvantages of the current methods

(index tests and field trials) to assess the survivability of the geotextile during

installation/construction process. The paper also describes the use of a new

methodology, Drop Rock Test (DRT) to replicate construction stress on geotextile and

assesses the influence of rock dumping on the mechanical performance of geotextile

with static puncture CBR test.

Chapter 5, Paper 2 (Measuring hydraulic properties of geotextile after

installation) investigates the influence of construction stresses (rock dumping) on the

filtration properties of geotextiles. Non-woven geotextiles were submitted to damage

–through DRT test and further assessed with laboratory tests (Constant Head test and

Bubble Point Test). Findings suggest the filtration properties of severely damaged

specimen still meet the permittivity and retention criterion.

Both papers (1 and 2) discuss the mechanical strength and hydraulic properties

after induced damage test (Drop Rock Test), but the influence of site characteristics

on the robustness (survivability) of geotextile during installation have not been

investigated. Chapter 6, Paper 3 (Methodology to investigate installation robustness

of geotextile in relation to subgrade condition) contributes to a better understanding

of how subgrade moisture condition (dry, field, and saturated) can affect the robustness

of geotextile during installation process. The study suggests that damage found on

installed geotextile is more severe in the case of dry subgrade than in the case of field

and saturated subgrade.


Together Papers 1, 2 and 3 examined the mechanical and hydraulic influence of rock

drop installation on geotextile and identified the threshold drop energy during

installation for the relevant grade of geotextiles. The combined outcomes from Papers

1, 2 and 3 provided a scientific basis for assessing the short term performance of

installed geotextile via DRT. As a result of detailed evaluation of the laboratory testing

and data analysis, identification of threshold drop energy, analysis of retained strength,

residual permittivity and pore size of geotextile, damage assessment methods and

design charts were developed. Findings confirmed that Drop Rock Test can

realistically replicate installation conditions which include construction stress of an

armour rock dumped onto geotextiles and moisture condition of subgrade. DRT can

replicate construction stress by varying the drop energies of released armour rock

(concrete block). The varying moisture condition of subgrade can be achieved by

adding different moisture content into the subgrade. DRT is a realistic and practical

methodology to assess the robustness of geotextile during installation. Index tests

(CBR puncture, Constant Head, Bubble Point) were used to further assess the influence

of installation conditions (release of armour rocks and subgrade moisture condition)

on the mechanical and hydraulic performance of geotextile. The combined used of

DRT and index tests provided a more reliable approach to predict the short-term

behaviour of geotextile during installation. The outcomes of this research includes

the development of design charts for geotextiles’ robustness against construction stress

for non-woven geotextiles, design charts for geotextiles’ damage assessment for

permittivity behaviour upon installation and charts of robustness against subgrade

moisture condition for non-woven geotextiles.

Literature Review 11

Chapter 2: Literature Review

2.1 Introduction

Coastal erosion is one of the common problems in coastal and marine developments.

It is often caused by sea level changes (rise/fall), extreme weather events (severe

storms, tides and winds), human activities and natural evolutionary phenomena. Shore

or bank protection structures use either armour rocks, rip raps or gabions to dissipate

water and tidal currents that would naturally induce erosion (Abromeit and Heibaum,

1996). To prevent coastal erosion, granular layers are placed beneath revetments,

overlaid with armour rocks/stone. The main objective is to protect and prevent the base

soil from eroding by layering granular filter materials beneath the primary armour

units. Besides achieving the main objective, the structure needs to be hydraulically

stable which requires the granular filters to have particle sizes that are both small

enough to prevent erosion of the base soil and large enough to prevent erosion of the

granular material itself through the m ore porous layer above. However due to the cost

and construction difficulties (construction below water level), revetments with

granular layers often fail to be constructed in a systematic manner (featuring four

protection layers) refer to Figure 2-1 (Lawson, 1992). Consequently, such revetments

often failed to perform as required.

12 Chapter 2: Literature Review

Figure 2-1: Revetment consisting wholly of granular materials (Lawson, 1992)

Figure 2-2: Revetment containing geotextile filter(Lawson, 1992)

Parallel with man-made development, geotextiles have gradually incorporated into

coastal protection structures, such as rip-rap revetments and armoured banks, refer to

Figure 2-2. Like granular filters, geotextiles act as filter and separator. They have

quick installation technique and less labour requirements compared to placing granular

filters in position. Their comparable function and consistent material properties have

naturally replaced the conventional granular material as filter beneath armour units.

Over the last 40 years, geotextiles have served successfully as separators and filters in


coastal protection structures(Loke et al., 1995). Despite the large number of successful

applications, there are examples where geotextiles fail to functions as required due to

damage during installation. Most geotextiles-related failures were reported to either

construction or design –related and are generally caused by (Kumar Shukla and Yin,

2006):

• Loss of strength due to UV exposure

• Lack of proper overlay

• High installation stresses.

The installation of geotextiles involves the bulk loading of primary armours above

geotextiles. This construction method maybe fast, but brings with it a high risk of

puncturing the geotextile if an inappropriate geotextile was selected and/or improper

selection weight of armour rock to release from too high a height. This mechanical

action imparts high installation/construction stresses onto the continuous sheet

material. Damage from the mechanical impact on the geotextile filters could easily

degrade their mechanical strength and filtration properties. Proper construction and

installation techniques are crucial in order to ensure that the intended functions of the

geotextiles are fulfilled. Hence, the installation of geotextile is thus the most important

step in governing the success or failure in the performance of coastal structure with

geotextiles.

This chapter describes the commonly used geotextiles for coastal erosion

protection structures and the types of damage mechanisms related to mechanical

impact and filtration properties during installation in the construction process. The key

challenges and the relevant testing methods dealing with geotextiles’ robustness,


including past investigations and studies evaluating the mechanical and filtration

properties of geotextiles after installation are reviewed

2.2 Geotextiles

Over the last 40 years, geotextiles have been extensively incorporated into these

revetments because of their cost-effectiveness, ease of installation, consistent material

properties (Giroud, 1984; Lawson, 1982; Palmeira et al., 2008). This material could

be manufactured either in woven or non-woven form. Woven geotextiles are made of

fibres manufactured in two perpendicular directions, one over the other. Non-woven

geotextiles are manufactured in random distribution manner (Bhatia and Smith, 1996).

These geotextiles are generally made from synthetic fibres such as polypropylene (PP),

polyester (PET), polyamide (nylon) or polyethylene (PE). Woven geotextiles have

higher strengths in machine direction and lower elongation properties. On the contrary,

non-woven geotextiles have lower mechanical strength with better elongation

properties than woven geotextiles (Raumann, 1982). Though woven geotextiles have

higher mechanical strength, its weak elongation properties hinder its’ ability to

conform to uneven shapes of armour units and elongate when forced is applied.

Therefore, non-woven geotextiles will only be considered for this research work.

2.2.1 Non-woven Geotextiles

There are two types on non-woven geotextiles, namely continuous filaments, and

staple fibres (Fig. 2-2). Both nonwovens are made from continuous extruded circular

cross section filament (Fig. 2-3) and the only distinct trait between them is the length

of the fibres. Continuous filaments are comparatively longer in length in contrast to

the short staple fibres which length ranges from 25-100mm(Bhatia and Smith, 1996).


(a)

(b)

Figure 2-3: (a) Continuous Filament (b) Staple Fibre (Bhatia and Smith, 1996)

(a)

(b)

Figure 2-4: Extruded cross section (a) Continuous Filament (b) Staple Fibre (Bhatia and Smith, 1996)

2.2.2 Design and construction challenges

In the process of constructing these coastal protection structures, the geotextiles are

subjected to high mechanical stress. Thus, the first challenge faced during construction

of coastal incorporated geotextiles structures is the geotextiles’ robustness to resist

construction stresses during bulk loading of armour rocks and throughout the design

life of the structure. Given that one of the purposes of geotextiles in coastal structures

is filtration function, it is important that the material does not experience excessive

strain (elongation) and abrasive damage before final service state. The enlarge

openings of pore size or perforations (punctures/holes) can alter its’ filtration

efficiency. Therefore, the second challenge is the installed geotextile must achieved

the required hydraulic properties to perform its filter function (Lawson, 1992). Thirdly,

geotextiles must exhibit adequate durability during its serviceability state, so that both


mechanical and filtration properties of geotextiles are sustained. Though geotextiles

are made of polymers that are fairly stable (do not degrade in the soil in a coastal

environment), to ensure the durability of geotextiles, they must be protected from the

long term exposure of UV light and abrasion during construction and installation phase

(Lawson, 1983). This research work does not refer to the durability criteria

specifically, it should be noted that it is implicit to every geotextile application.

2.2.3 Selection of Geotextiles

Proper selections of non-woven geotextiles are crucial for the success of any coastal

protection project. The selection criteria are generally depended on the intended

purpose of the material. For example, non-woven geotextiles are often selected for

filtration and separation functions because of their higher permittivity, transmissivity

and elongation properties compared to woven geotextiles. Selected geotextiles would

have a certain minimum mass per unit area, thickness and strength that will be

sufficient to survive the effects of placement of primary armours during installation.

Designers and engineers need to consider the robustness (survivability), mechanical

strength and filtration requirements of geotextiles when designing coastal structures

with geotextiles. Often these requirements are expressed in terms of tensile strength,

tear strength, burst strength, puncture strength, impact strength, permittivity, etc.

Designers and engineers depend on values of these survivability properties before

installation, but this should not be the case. Selection of geotextile should be decided

on the robustness and the expected damage (retained strength and filtration properties)

on geotextiles upon installation.

2.3 Types of damages

Damage from the mechanical impact on the geotextiles during the construction process

will be subjected to different forms of damage mechanisms (Bräu, 1996; Chew et al.,


1999; Watn and Chew, 2002; Wong et al., 2000a; Wong et al., 2000b). There are four

common damage mechanisms that typically occur during the installation phase, these

are tensile strained (elongation), abrasion, tearing and puncturing. Tensile strained is

the result of geotextiles elongating to conform to the uneven shape of armour rock

during installation and operational phase. The physical change in length could

significantly affect the pore size and permittivity of the material, consequently,

affecting its filtration efficiency.

Abrasive shearing is the result of sliding friction between geotextile and

subgrade caused by the falling of rock amour during installation. This type of damage

is noted on the subgrade-geotextile interface where the fibrous layers of the material

are sheared away during the sliding motion of the armour rocks which results in

thinning of the material. Research show that severe abrasion by sharp particles (quartz

sand, max diameter 200µm) leads up to 50% strength reduction (Watn and Chew,

2002) and possibly affecting its’ filtration properties. Tearing is typically a result of

harsh installation and construction procedure; the multiple armour rocks released (bulk

loading) at the same time leads to a tear propagation that could degrade both the

mechanical and hydraulic performance of the material.

Puncturing damage is likely to occur when sharp edged amour units are dumped

directly on the geotextile filter. These armour rocks will penetrate the geotextile and

could compromise the separation and filtration efficiency (Watn and Chew, 2002).

However, research work by( (Chew et al., 1999; Lawson, 1992) found that the

puncturing damage in geotextile filter did not result in failure of the coastal structure,

as the armour rocks will seal the punctured hole, thus preventing soil loss. While

puncture damage may not govern the failure mode of the structure, from the viewpoint

of geotextile design and specification, it is important to minimise, or prevent


puncturing of the geotextile. Authors (Christopher and Fischer, 1992; Heerten, 2007)

strongly imply that any geotextile application would be deemed pointless if the

material was damaged during construction phase. This justifies the need to examine

geotextiles’ extent of damage caused by the placement of armour rocks on geotextile

during construction process.

2.4 Previous research

Because geotextiles are prone to installation damage, it is essential to investigate the

extent of damage in terms of ability to fulfil its intended function in the structure after

installation, therefore, research have been ongoing since 1980s ((Bräu, 1996;

Christopher, 1983; Heerten, 1984; Heerten, 2007; Hornsey, 2012; Watn and Chew,

2002; Wong et al., 2000b). These studies suggest that the high dynamic stresses lead

to physical changes (strain/elongation) which affects the geotextiles’ mechanical and

filtration properties. Christopher (1983) evaluated the performance of woven

geotextile in a rip-rap revetment type seawall at the 70th Street Causeway Project in

Miami, Florida that was installed over a decade. Test results indicated a strength

reduction in the excavated geotextiles ranging from 5 to 40% reduction compared to

new manufactured material. The permeability of excavated geotextiles was 2 x 10-

3cm/s which is a 50% reduction in comparison with the new fabric. Despite the

mechanical damage, the installed material still managed to perform its intended

filtration function.

Mannsbart and Christopher (1997) evaluated the mechanical and filtration

properties of excavated geotextiles at sites across Europe and Malaysia that were

installed for 6-14 years. Their investigation showed a reduction in permeability and

mechanical strength compared to clean geotextile. The theoretical assumption is that

the permeability of the material would increase as it is subjected to tensile strain to


conform to the uneven shape of armour rocks. But permeability of the material was

found to be reduced. This could be correlated into two possible factors, either

environmental deposits or fibre re-orientation. Environmental deposits could obstruct

drainage path and hinder its filtration efficiency. The continuity of pores (width and

depth of pores) is affected by the re-alignment of fibres (tensile strain) during

installation. The discontinuity of pores (width and depth of pores) would greatly

influences the filtration efficiency of geotextiles (Rawal et al., 2010a) .

Research by Loke et al. (1995) presented the results of a field investigation of

non-woven geotextile filter in coastal protections works for marine clays that had been

in service for more than 5 years. Two coastal revetment structures were constructed

under two different environmental conditions: site A, geotextile was laid above the

marine clay and a layer of sand and at site B, geotextile was directly laid over the

marine clay. Excavated samples (unclean) at both sites had great reduction in

permeability, ranging from 40-70%. Interestingly, excavated samples (unclean and

clean) at site B have some contrasting results. The permeability of excavated-unclean

samples was reduced by 42% while permeability of excavated-clean samples increased

up to 60%.

This possibly suggests that the difference in permeability is caused by the

deposition of particles, sediments, organic matter as well as salt deposition, mineral

precipitation and bacterial growth in exhumed excavated-unclean samples. These

deposits obstruct and clog the drainage path and decrease the permeability of the

material. The study also recorded a significant reduction in puncture resistance,

approximately, 40% at site A. The study also noted the importance of tensile strain/

elongation of geotextiles as a key design requirement to ensure the ability of the

geotextile to conform to the armour rocks and achieve close contact between soil


profile and primary armours (Loke et al., 1995). It is essential to achieve tight

interfaces as this helps to minimise the loss of the subsoil beneath the geotextile which

may results in structure instability.

A detailed investigation on the performance of woven geotextiles in a reclaimed

project located in the south-western coast in Singapore that had been in service for 12

years at the time of excavation evaluated the degradation of mechanical and filtration

properties (Wong et al., 2000a). The observed permeability of excavated geotextiles

(dirty and cleaned) increased which is in contrast with the previous case studies. The

increased in permeability agrees with the theoretical assumption; it is assumed that the

permeability of geotextile filter would increase as pore size would increase since the

material is subjected to tensile strain to conform to the uneven rock armour rocks.

Thus, pore size of geotextile filters would be enlarged resulting in greater permeability.

The decrease in permeability found in excavated samples in previous studies appears

to be influenced by the salt deposition, mineral precipitation, organic deposits and

bacterial growth (Rollin and Lombard, 1988). This is agreement with previous

research (Loke et al., 1995) where these environmental deposits were found to likely

to obstruct and clog the drainage path, and decrease the permeability of excavated

samples.

Undoubtedly, the results gathered from these case studies give engineers and

designers better confidence to select the appropriate geotextile based on the degree of

damage of geotextiles that were investigated. But these research studies require large

setup, is time consuming (mostly requires time in years) and costly because specimens

need to be assessed after several installation years. Watn and Chew (2002) and

Diederich (2000) point out that the heavy construction stresses imparted onto the

material usually by far exceeds the service stresses. This suggests that the critical


period of geotextile is during installation/ construction process. Though past studies

accurately measure the extent of damage on geotextiles but these investigations are

conducted after several installation years. The additional damages imparted on

geotextiles by wave attacks, in-service stresses, and environmental deposits act in a

collective manner; the direct influence of construction stress on the extent of damage

on geotextiles cannot determined.

The role of site characteristics in geotextiles’ robustness during installation has

not received much attention as it is often difficult to characterize the variability

(subgrade type, density and moisture content) in detail. Research suggests that

subgrade characteristic and properties often play a critical and complex role in

geotextiles’ robustness on-site (Abu-Farsakh et al., 2007; Chew et al., 1999; Lopes et

al., 2001). Investigations led by Wong et al. (2000b) concluded that density of

subgrade is crucial in the occurrence of punctures and holes on the geotextile. Results

suggest that for the same geotextile and at the same drop height, severe damage is

found in the case of dense sand subgrade than in the case of loose sand subgrade.

Though Wong et al. (2000b) considered the type and density of subgrade, but they

have not considered subgrade moisture content as a parameter to measure geotextiles’

robustness.

Figure 2-5: Geotextile installation in coastal zone (Tessilbrenta, 2013)


Figure 2-5 illustrates an armour revetment in a coastal zone. Construction of revetment

structures could occur in fully dried, under water, or in partially saturated subgrade.

The question arises that under what circumstances the susceptibility of damage on

geotextile is lesser, saturated, unsaturated, or dry subgrade condition? To answer this

question, the study of geotextiles’ robustness against mechanical stresses should

consider the moisture condition. An evaluation of geotextiles’ robustness in relation to

subgrade moisture condition provides relevant information for site characteristics

which can have varying levels of moisture content depending on the tide and climate.

Summary key findings:

• Long term observational studies are conducted to determine the extent of

damage

• Both mechanical and filtration properties are affected after installation

• Case studies and research investigations are time consuming (mostly in

years), requires large setup and costly

• Difficult to isolate the influence of installation/construction stress on

geotextile from other damage factors

• Installation/ construction stresses (rock dumping) imparts the highest

mechanical impact

Identified knowledge gaps:

• Limited knowledge on the direct influence of rock dumping on geotextile

during installation

• Limited knowledge on the influence of subgrade moisture condition on

geotextiles’ robustness (survivability) during installation


• There is a need to develop a damage assessment chart to predict filtration

properties after installation. This would reduce the need to carry out large

scale and long term observational studies

2.5 Existing Test Methods

2.5.1 Index Test methods

The key factor ensuring geotextile filters continue to perform their intended function

after installation is geotextiles’ robustness to resist the perforation of armour rocks

during installation. To determine the robustness of geotextile for installation

procedures has led many attempts to develop appropriate laboratory test methods. The

geotextiles’ ability to resist these loads without damage must be proved. When placing

primary armour the geotextiles are subjected to dynamic perforation loads (rock

dumping influence by

- The shape and weight of stones

- Drop height of stones

- Strength of subgrade

- Placing in the dry or under water

To ensure installation damage resistance, minimum mechanical properties are usually

specified so that the material for the intended function can be selected; an example is

illustrated in Figure 2-6.


Figure 2-6: Specified mechanical properties for construction of coastal structure (NAUE, 2005a)

Properties such as tensile strength, tear strength, puncture resistance, etc. are derived

from index tests. Other than mechanical strengths, geotextiles are frequently specified

by mass per unit area and thickness. Table 2.1 summarises the common index tests

and appropriate standards that are carried out to determine geotextile specifications:

Table 2.1: Common index tests

No. Index Test Standard 1. Wide Strip Tensile Test AS 3706.2 2. Trapezoidal Tear Strength AS 3706.3 3. CBR Burst Strength AS 3706.4 4. Grab Tensile Strength AS 2001.2.3 5. Drop Cone Test AS 3706.5 6. Mass per unit area, Thickness AS 3706.1 7. Static Puncture Test ISO 12236: 2006 (E)

1. Wide Strip Tensile Test

Wide strip tensile test measures the tensile strain of both machined and cross machine

direction of geotextiles using standardised method AS 3706.2. Test results from a

research study overestimated the failure load of woven material by 10% and

underestimated the failure load of light weight non-woven approximately by

20%(Myles and Carswell, 1986).

2. Trapezoidal Tear Strength


The trapezoidal tear strength of material can be conducted according to AS 3706.3.

This test measures the force required to propagate a tear in geotextiles once a tear has

been initiated (in both machined and cross machined direction) by trapezoid method.

As this test shows little correlation with field performance, this test is no longer applied

and has been removed from the European standards (Watn and Chew, 2002).

3. CBR Burst Strength

The burst strength is measured by applying a normal pressure against a geotextile

sample secured in a ring. The stress against the geotextile at failure gives the value of

the bursting strength. The burst strength represents the strength of individual fibres in

geotextiles.

4. Grab tensile strength

The grab strength test is performed to ensure both the Quality Control (QC) and

Quality Assurance (QA) is achieved for geotextiles of similar structures(Sarsby and

Textile, 2007). Investigation reveals that grab tensile strength has no correlations with

geotextiles’ resistance to damage (Christopher and Elias, 1998).

5. Drop Cone test

Drop cone test is carried according to AS 3706.5 and quantifies the resistance of

geotextiles against damage by measuring the amount of damage by an impact force of

a cone shape object. Lawson’s studies implied the results obtained from the drop cone

test best represents the mechanical stress on geotextile (Lawson, 1992). However, the

accuracy of this research study is limited to geotextiles ranging in weight from 120-

300g/m2, which is too narrow when geotextiles used for coastal protection structures

generally ranges from 300 to 1200 g/m2. Despite this limitation, Lawson developed a

modified version of the drop cone test to accommodate the full range of geotextiles.


However, despite the modification made, this test does not take into account the

installation conditions, for example, strength of subgrade (density) or the moisture

content of subgrade (placing in the dry or under water).

6. Mass per unit area, thickness

Mass per unit area is used as a characteristic relative value that is often correlated with

properties such as tensile strength, tear strength, puncture strength etc. The

heavyweight geotextile with a higher mass per unit area will usually be stronger than

a lightweight geotextile. Mass of geotextiles represents the amount of polymer used in

manufacturing of the product. Mass has been commonly used as an indicator for

mechanical performance but studies suggest that it is does not necessarily reflect

geotextiles’ robustness. Modernised manufacturing techniques with rapid

technological developments and better-quality raw materials could result in better

performing geotextile given the same amount of mass (Palmeira et al., 2008; Wong et

al., 2000b).

The thickness of geotextiles is required in the calculation of filtration properties

such as the permittivity and transmissivity. Correlation to index tests study found that

both mass per unit area and thickness are merely descriptive properties (Christopher

and Elias, 1998) and do not necessarily reflect the performance on-site when

comparing geotextiles of different structures, for example staple fibre and continuous

filament non-woven geotextiles.

7. Static Puncture Test

Test was developed to measure the puncture resistance of geotextiles (ISO 12236:

2006) using a flat end CBR plunger. The use of flat end plunger does not replicate the

same nature and extent of damage during installation. Besides, the applied energy was


static with plunger advancing at rate of 50 mm/minute; this does not reflect the

dynamic impact of armour units imparted on the geotextiles.

These index tests do not consider installation conditions which limits the

accuracy in assessing geotextiles’ field performance. For example, Static Puncture

Test is carried under static load conditions, whilst mechanical actions of primary

armours placement (bulk loading) are dynamic in nature. Despite that fact, industry

designers and engineers depend on specifying minimum mechanical properties and

mass per unit area to minimise the risk of puncture taking place on the laid geotextile

layer. This is a commonly adopted approach by the industry which could lead to

incorrect selection of geotextiles(Hornsey, 2012) .

At present, the only laboratory test that replicates the dynamic impact of falling

armours on geosynthetics is developed by BAW in 1978 (current issue: RPG (1994) ).

This test replicates the dynamic impact by releasing a drop of hammer with a tip edge

onto the geotextile that is laid above a soil sample at determined drop energy

(Heibaum, 2014). Figure 2-7 illustrates the installation guideline referring to drop

height and mass per unit area of geotextiles using the BAW impact test. Though this

is a functional and versatile approach in assessing geotextiles’ robustness, the question

of the suitability of cylindrical drop hammer to represent armour units still invites

contention. A simplified chart with the use of drop energy as a function of rock mass

and drop height should be developed to represent the installation conditions.


Figure 2-7: Installation guideline (NAUE, 2005b)

Hornsey (2012) acknowledged the limitations of index derived values as he

replaced the original specified geotextile with a better grade geotextile as the initial

selected material failed to perform during an on-site performance test. Both Lawson

(1992) and Berendsen (1996) further assert the inadequacy of existing standards in

relation to rock dumping on geotextile filters which led to reporting of many

installation damage cases on site. Watn and Chew (2002) imply that there is still a need

for more research work to be carried out before a good correlation is established

between index derived values and on-site performances. Therefore, engineers and

designers should not solely rely on design guidelines and charts developed by

manufacturers.


• Index derived values are empirical in nature, mass per unit area and thickness

are descriptive properties and

• Poor correlations between index test and on-site performances



• There is a requirement of a new method to replicate construction stress (rock

dumping) to determine geotextiles’ robustness against mechanical stress

2.5.2 Field Trials

The ideal approach would be to perform large scale drop rock tests in real field

conditions as there is no test methods have been developed by which the same nature

and degree of damage can be replicated consistently in the laboratory. This is an

approach which could easily and accurately assess geotextiles’ robustness during the

installation process. At present, there is neither a standard procedure for field trials nor

a standard method to assess the influence of installation (rock dumping) on geotextile’s

robustness, mechanical and hydraulic properties. The procedures in conducting a field

trials varies in many ways and is influenced by many factors, these include (Watn and

Chew, 2002):

• Geotextile characteristics; which includes the polymer type, weave structure,

specific mass and other mechanical properties of the geotextile

• Primary armour stone size, weight, angularity

• Height of the release stone

• Characteristics of the soil base, density, consistency, presence of stones

• Angle of inclination of the base soil

• Whether the base soil is above or below water

• Number of tests that were conducted

Field trials can be conducted with many combinations of the above parameters.

Therefore, results are expected to be misleading and conflicting at times. To


complicate matters , Watn and Chew (2002) imply that occurrence of puncture is a

statistical event. Hence, to study geotextiles’ robustness against construction stress

(rock dumping), adequate number of drop tests are required before the right conclusion

regarding the selection of geotextile can be achieved with high level of confidence.

There are limited studies (Bräu, 1996; Diederich, 2000; Paula et al., 2008) that

conduct extensive research work as full scale investigations require heavy man-power,

large setup, are costly and time consuming. Most engineers and designers would

simply carry out field trials to determine the appropriate installation height to minimise

the risk of punctures inflicted on geotextile filters (Ameraunga et al., 2006; Chew et

al., 1999; Holtz et al., 1997; Wong et al., 2000b). This often leads engineers and

designers to specify thicker geotextile (higher cost) than required or adopt construction

procedure that is not economical and not efficient.

This led Chew et al. (1999) to proposed a standard drop test (SDT) to measure

the robustness of geotextile filters in a quantifiable and empirical manner during

installation. Despite the reproducible results obtained from SDT, results suggest the

risk of puncture is a random event. Though, SDT closely replicate installation

conditions, the variation in overlaying armour rocks in size and shapes and subgrade

types makes it harder to determine the safe puncture threshold. Hence, there is need

for a universal adoptable method that closely replicates the predominant installation

conditions of geotextile in the field.


• Field drop tests method are complex and unable to produce results that are

consistent and repeatable


• Comparison of results from different drop tests is difficult as each field site

has its own distinct character

• Occurrence of punctures is statistical event which requires numerous amount

of testing before selection of geotextiles can be made

• There isn’t a standard field trial method adopted by the industry that

determines geotextiles’ robustness because the variations and complex

nature of installation site

• Field trials are expensive and requires heavy logistics (man power and time)


• There is a requirement of a new drop test method by which the same nature

and extent of damage during construction process can replicated consistently

in the laboratory

• There is a need to develop a testing method that is simplified and yield results

that are consistent and reproducible

2.6 Summary

The subject of installation damage caused by the placement of armour rocks on

geotextiles has led engineers, designers, manufacturers, and users to perform research

work to develop better methods for evaluation damage susceptibility. The mechanical

impact on the geotextiles may reduce or possibly destroy its ability to function as

required. However, it should be noted it is possible that the material may continue to

serve its intended purpose despite the damage. It is therefore important to evaluate the

expected extent of damage and the consequences of the damage in terms of ability of

geotextiles to fulfil its intended function in the structure.


The review of the previous research has noted the influence of installation

damage on geotextiles’ mechanical and filtration properties. Studies evaluated the

properties of geotextile after installation and recorded the extent of mechanical and

hydraulic damage. Long term observational studies and laboratory induced damage

tests were utilised to accurately evaluate the properties of installed geotextiles to

ensure the material’s ability to continue to perform the intended function. Although

both approaches can provide relevant information on geotextiles after installation,

several issues have been discussed in the reviewed literature. The common issues when

observational studies take place are it requires time (mostly in years), high cost and

large setup.

Results gathered from long term observational studies cannot determine the

direct influence of construction stresses (rock dumping) as damage factors act in a

collective manner over the years of installation. The review of the current literature

asserts that rock dumping (installation) imparts the highest mechanical stress onto

geotextiles over its entire life-cycle, surpassing service stresses. Therefore, it is

essential to assess the direct influence of rock dumping on geotextiles’ properties. In

determining geotextiles’ robustness during installation, understanding the site

characteristics, i.e. subgrade type, density and moisture content is crucial. Due to its

complex nature, the assessment of subgrade moisture content as a parameter has not

been given much attention. The influence of subgrade moisture condition (saturated,

unsaturated or dry) on the robustness of geotextiles has yet to be investigated.

The review of research literature enabled understanding of the significant role

played by installation (rock dumping) in geotextiles’ properties. The lack of

appropriate method in replicating construction stress makes it difficult to evaluate

geotextiles’ robustness during installation. Geotextiles’ robustness significantly


affects the susceptibility to damage on geotextiles. The material’s ability to withstand

construction stresses governs the type and extent of damage on geotextile. The damage

(tensile strain/ abrasion/puncture/ tear) and strained/ elongation subjected during

installation affect the mechanical (retained strength) and filtration (pore size and

permittivity) properties. The review of previous literature and existing methods (index

and field approaches) highlights the need for the development of new standard method

that can be carried out in a semi-laboratory environment.

The reviewed research literature has categorized the common methods adopted

by engineers and designers when selecting the appropriate geotextile for coastal

application work. The two main approaches are specifying minimum mechanical

properties of geotextiles and field trials. The first method utilised index tests to specify

geotextiles’ properties which correspond to the ability of material to withstand

mechanical stress during installation and construction process. However, various

studies agreed that index tests measurements are empirical in nature and more work is

required to establish a good correlation between index test and performance on-site.

While the latter refers to real performance test on site which provides accurate results,

but currently, there isn’t a standard performance test that is adopted by the industry.

Poor selection of geotextiles can lead to serious structure instability. Consequently,

more appropriate evaluation methods need to be developed to safeguard against the

consequences associated with structure instability if geotextiles were severely

damaged during installation. Results gathered from the newly developed method can

be considered as the next improvement to the current standards and installation

guidelines on-site.

This research work utilizes a new approach, Drop Rock Test (DRT) to replicate

construction stresses (single rock dumping) on geotextiles. The thesis also presents


the investigation on the influence of construction stresses (rock dumping) on

geotextiles’ mechanical (robustness and retained strength) and filtration (permittivity

and pore size) properties. Specimens are subjected to the DRT for damage simulation

and immediately recovered and assessed with relevant index tests (Static Puncture

Test, Constant Head Test and Bubble Point Test). The selection of the material can be

achieved with greater confidence if engineers and designers can identify the material’s

robustness, strength reduction and filtration properties upon installation.

Research Design 35

Chapter 3: Research Design

3.1 Methodology

To study the behaviour of geotextiles under field conditions (construction stress), a

new test methodology, Drop Rock Test (DRT) has been developed that allows

controlled and repeatable installation. This study separates the research project in two

phases. The first phase is a large-scale semi-laboratory test (DRT) aimed to study the

influence of construction stress (single rock dumping) on the robustness (survivability)

of geotextile. The second phase aimed to study the extent of damage of geotextiles

during installation/construction process which includes visual assessment and index

tests. The index tests included are static puncture test, constant head test and bubble

point test. The tests were performed on control specimens and specimens extracted

after the drop rock tests. The geotextiles used in the laboratory tests are shown in

Figure 3-1 and corresponding to its material properties are listed in Table 3.1 and 3.2.

36 Chapter 3: Research Design

Figure 3-1: Staple Fibre Geotextiles

SF1 SF2

SF3 SF4

Research Design 37

Table 3.1: Staple fibre (SF) Geotextile Properties (typical values)(Kendall et al., 2014b)

Table 3.2: Continuous Filament (CF) Geotextile Properties (typical values )(Kendall et al., 2014b)

3.2 Drop Rock Test

The DRT apparatus consists of a gantry crane with a lifting capacity of 1550kg, an

impermeable subgrade containment unit, gantry crane and concrete blocks. Figure 3-

1 illustrates two A-frames of rectangular hollow sections are bolted to concrete blocks

Properties Geotextiles

Mec

hani

cal

Test * Standard Units SF1 SF2 SF3 SF4 Wide Strip Tensile Elongation

MD

AS 3706.2 %

107 111 110 112

XMD 84 83 81 83

Wide Strip Tensile Strength

MD

AS3706.2 kN/m

11 18 26 37

XMD 21 39 55 83

Trapezoidal Tear Strength

MD

AS3706.3 N

320 477 656 842

XMD 542 977 1264 1774

Grab Tensile Strength

MD

AS2001.2.3 N

686 1161 1753 2469

XMD 1097 1948 2958 4539

CBR Burst Test AS3706.4 N 2719 4522 6526 8824 Physical Mass per unit area g/m2 380 611 846 1224 * MD: Machine direction XMD: Cross-Machine Direction


Mec

hani

cal

Test Standard Units CF1 CF2 CF3

Wide Strip Tensile Elongation

MD

AS3706.2 %

54 58 68

XD 59 62 67


MD

AS 3706.2 kN/m

22 30 58

XMD 21 28 56


MD

AS3706.3 N

540 753 1485

XMD 510 700 1425


MD

AS2001.2.3 N

1430 2100 4290

XMD 1350 1910 4300

CBR Burst Test AS3706.4 N 3600 4800 9696

Physical Mass per unit area g/m2 280 379 740


to provide support to the girder sections. The concrete blocks (weight ranging 93, 438

and 922kg) constructed with a 90o tip facing downwards to deliver the highest

mechanical stress on geotextile placed above the subgrade-filled containment (refer to

Figure 3-2). The concrete block can be released from a selected of drop height between

0.5m to 2.0m. The DRT similarly replicates the construction process of a rip-rap

revetment shown in Figure 3-3.

The selection of the sizes and weight of the three concrete blocks represents the lower

and upper range (size and weight) of armour rocks used for a coastal revetment. The

range of drop height represents the drop heights that are commonly adopted by mid-

range excavators for the construction of a coastal revetment. Note that the sides of the

concrete block are smooth but the 90o tip facing downward creates the worst possible

damage compared to rough-edged concrete block. The testing conditions were

intentionally harsh to ensure damage on geotextiles is in the severest possible state,

thus permitting a comparison and evaluation after extraction.

The DRT procedure in this study is summarised as follows:

i. Geotextiles with dimensions of 1.8m by 2.0m stencilled with a grid of

50mm by 50mm (area of 0.81m2) where the concrete block is targeted to

fall is prepared. The geotextile sample is then laid above the prepared

subgrade (dry, field or saturated) and secured firmly with G-clamps.

ii. The subgrade confined box is filled with natural, field or saturated

subgrade. For natural subgrade, no additional moisture was added to the

subgrade. For field moisture condition (11%), the subgrade is permeated

with a high-pressure hose at flow rate, 50ml/s for duration of 60 minutes.

For every 7 rock drops, the subgrade was re-permeated for 10 minutes with

Research Design 39

the same flow rate to maintain constant water content between tests. For

saturated subgrade, subgrade was permeated from a high-pressure hose at

flow rate of 50ml/s; except water was not drained out from the system

(GCL is installed). Once the test pit show signs of pooling, the subgrade is

considered fully saturated. Moisture content is found to be approximately

28%.

iii. Compaction of subgrade is dependent on the subgrade moisture conditions

selected. For natural condition, subgrade is compacted using a hand

tamping system. A 4.2kg tamper of 200mm x 300mm x 700m (L x W x H)

is released from a height of 0.5m and repeated 30 times (6 x 5 grid pattern).

The centre region of the subgrade will be repeatedly emptied and refilled

due to the falling of concrete block, additional tamping is conducted with

a 3 by 5 grid pattern to ensure consistent compaction. For field and

saturated conditions, the subgrade is deemed hydraulically compacted

(compacted by the weight of water) in nature. However, repeated tests were

conducted; the centre region is compacted with the hand tampered method

(3 by 5 grid pattern) for uniformity.

iv. The concrete block is electrically winched up to the desired drop height and

is laterally moved across with a trolley along the crane rail. The drop height

is measured from the bottom tip of the concrete block to the surface of the

laid geotextile sample with a T-gauge. The block with sides measuring

750mm is then released on a corner point (90o tip) for specified drop energy

onto the prepared subgrade which was overlain with a geotextile.


v. After the drop, the concrete block and G-clamps are removed and samples

are labelled per its test number, geotextile grade, drop height, weight of

concrete block and date of test.

Research Design 41

Figure 3-2: Drop Rock Test(Cheah et al., 2015)

Figure 3-3: Construction process(Hornsey, 2012)


3.3 Visual assessment

Visual inspection after DRT is made to determine the occurrence of any perforations

(punctures/holes/tears) on geotextile sample. Geotextiles’ robustness refers to

geotextiles’ ability to survive (non-puncture) the drop rock test. Any puncture found

on the geotextile is recorded and measured (Figure 3-4). Non-punctured geotextiles

are examined in terms of elongation. Measurements are determined against 6 squares

(which initially were 300mm) from the point of interest (Figure 3-5) at each side of

concrete block. 300mm is chosen because it records the greatest change in elongation.

The change in length is measured and recorded. Non-punctured samples are extracted

and further assessed with relevant index tests.

Figure 3-4: Visual assessment- Puncture size(Cheah et al., 2016)

Figure 3-5: Visual Assessment-Elongation (Cheah et al., 2016)

Research Design 43

3.4 Static Puncture Test

The extracted specimens from DRT are further assessed with Static Puncture (CBR)

test in accordance with ISO 12236: 2006. The extraction locations of specimens

include the rock drop impact zone and the four corners of the geotextile sample (Figure

3-6). It is assumed that the corner material did not experience any damage caused by

the released concrete block and thus were considered as control specimens.

Figure 3-6: Location of specimens extracted (Cheah et al., 2016)

3.5 Constant Head Test

The specimens from DRT are used to evaluate the remaining permittivity after

extraction in accordance with AS 3706.9 (2012). A single layer of specimen is

subjected to unidirectional flow of water (normal to plane) with head loss ranging

between 5-25mm. The results are expressed in term of permittivity 𝜓𝜓 (s-1), coefficient

of permittivity, k (m/s-1), headloss, Δℎ (m) and flow rate, q (m3/s).

3.6 Bubble Point Test

The extracted specimens from DRT are used to determine the apparent opening size

(O95, µm) in accordance with ASTM D6767 (2014). The test subjects the geotextile to

continuous air flow to remove liquid that is retained through capillary action and


surface tension from the geotextiles. O95 corresponds with the geotextile opening size

that 95% of the pores are smaller than that size.

45

Chapter 4: Impact Resistance and evaluation of retained strength

Charmaine Cheah, Chaminda Gallage, Les Dawes and Preston Kendall School of Civil Engineering and Built Environment Science and Engineering Faculty Queensland University of Technology Published: Geotextiles and Geomembranes 44: 549-556

Statement of Contributions of Joint Authorship

The authors listed below have certified that: 1. They meet the criteria for authorship in that they have participated in the conception,

execution, or interpretation, of the least that part of the part of the publication in their field of expertise;

2. They take public responsibility for their part of the publication, except for the responsible author who accepts overall responsibility for the publication;

3. There are no authors of the publication to these criteria

4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the editor or publisher of journals or other publications, and (c) the head of the responsible academic unit, and

5. They agree to the use of publication in the student’s thesis and its publication on the QUT’s ePrints site consistent with any limitations set by publisher requirements

Cheah, C: (Candidate) Established methodology, data analysis, preparation of tables and figures, writing and compilation of manuscript

Signature: Date: Gallage, C: (Principal supervisor) Supervised and assisted with manuscript compilation, editing and co-author of manuscript

Signature: Date: Dawes, L: (Associate supervisor) Supervised and assisted with manuscript compilation, editing and co-author of manuscript

Signature: Date: Kendall, P: (External supervisor) Editing and co-author of manuscript

Signature: Date: Principal Supervisor Confirmation I have sighted email or other correspondence from all co-authors confirming their certifying authorship. (If Co-authors are not able to sign the form, please forward their email or other correspondence confirming the certifying authorship to the RSC)

Name: Signature: Date:

46 Chapter 4: Impact Resistance and evaluation of retained strength

This Chapter is an exact copy of the published journal paper above.

Linkage of paper to Research Methodology and Development

The most critical stress imparted onto the material is the construction stress and it is

essential to assess and estimate any change in mechanical properties of geotextiles upon

installation where construction stress (dropping rocks) is involved. This paper presents a

new methodology that replicates construction stress on geotextiles and investigates the

robustness (survivability) and retained strength of geotextiles. Paper 1 investigates and

identifies the advantages and disadvantages of the current methods (index tests and field

trials) to assess the survivability (robustness) of the geotextile during

installation/construction process. The paper also describes the use of a new methodology,

Drop Rock Test (DRT) to replicate construction stress on geotextile and assesses the

influence of rock dumping on the mechanical performance of geotextile with static

puncture CBR test.

The following objectives were achieved:

Objective 1• Developed Drop Rock Test (DRT) to replicate construction stress

Objective 2

• Assessed robustness of geotextiles during installation• Assessed retained strength of DRT induced damage geotextiles

47

Figure 4-1: Linkage of Paper 1 to research methodology



Evaluate existing methods that evaluates geotextiles' robustness against construction (rock

dumping)

Examine the influence of construction stress (rock diumping) on mechanical and filtration properties





Develop design chart for robustness of geotextile against construction stress using Drop and CBR Energy

Develop design chart to predict the permittivity performance of geotextile upon installation

Develop charts to predict robustness of geotextiles agsinst subgrade moisture condition

Lite

ratu

re a

nd D

eskt

op S

tudy

E

xper

imen

tal a

nd D

ata

Ana

lysi

s R

esea

rch

Out

com

es

Paper 1

Paper 2

Paper 3


Abstract

Over the last few decades, geotextiles have progressively been incorporated into

geotechnical applications, especially in the field of coastal engineering. Geotextile

materials often act as separator and a filter layer between rocks laid above and subgrade

beneath. This versatile material has gradually substituted traditional granular materials

because of its ease of installation, consistent quality and labour cost-efficiency. However,

geotextiles often suffer damage during installation due to high dynamic bulk loading of

rock placement. This can degrade geotextiles’ mechanical strength. The properties

considered in this paper include the impact resistance and retained strength of geotextiles.

In general, the greater the impact energy applied to geotextiles, the greater the potential

for damage. Results highlight the inadequacy of using index derived values as indicators

that determine geotextiles performance on site because test results showed that geotextiles

(staple fibre (SF) and continuous filament (CF)) with better mechanical properties did not

outperform lower mechanical strength materials. The toughest CF product with a CBR

index value of 9696N shows inferior impact resistance compared to SF product with the

least CBR strength (2719N) given the same impact energy of 9.02kJ. Test results also

indicated that the reduction of strength for CF materials were much greater (between 20-

50%) compared to SF materials (between 0- 5%) when subjected to the same impact

energy of 4.52kJ.

KEYWORDS: Geosynthetics, Drop Rock Test, Impact Resistance, Unsaturated

Subgrade, Retained Strength

4.1 Introduction

For more than 40 years, the use of geotextiles in coastal engineering applications has been

increasing. The versatility (bi-dimensional and flexible), ease of installation, consistent

49

material properties (mechanical and hydraulic) and cost-effectiveness of geotextiles offer

great advantages as construction materials (Giroud, 1984; Palmeira et al., 2008). As such,

geotextiles have gradually replaced granular materials as separator and filter layer

beneath revetments armour, gabions and riprap (Christopher and Fischer, 1992).

Unfortunately, geotextiles are often damaged during installation/construction phase. Past

studies and on-site investigations (Chew et al., 1999; Heerten, 2008; Hornsey, 2012;

Wong et al., 2000b)led authors to agree the mechanical action during placement of armour

rocks can cause noticeable damage to geotextiles in one form or another. This justifies

the need to examine geotextiles’ damage during installation to estimate the short-term

performance of geotextiles. Identifying the retained strength of the material straight after

installation allows engineers to be able to isolate installation damage from hydraulic,

physical, chemical, and biological damage that accounts for the deterioration of installed

geotextiles. The expected serviceability of the material can be achieved with greater

confidence when engineers are provided with the retained strength of the initially installed

material.

The key factor ensuring geotextile to perform its function is the impact resistance

of the material to resist the perforation of the rocks during installation. Rosete et al. (2013)

point out that not only can installation damage on geotextile affect mechanical (tensile)

strength, but also the functionality of geotextiles such as separation and/or filtration can

be undermined. In 1992, Christopher and Fischer inferred that any design efforts for

structures with geotextile filters would be useless if the material is damaged during

installation process and further asserted that “Swiss cheese does not make a good filter”.

Similarly, Heerten (2008) asserts that any geotextile filtration design would be

meaningless if the material suffers puncturing during installation.


Engineers and designers often rely on index values specification, design guidelines

and installation height limitations to minimise damage inflicted on geotextiles during

installation process. Specifying minimum index values as survivability requirement is

risky as index tests fail to simulate field conditions. Furthermore, the extensive drop

testing completed in Singapore revealed that geotextiles with better mechanical properties

such as tensile strength and CBR force do not necessarily have better performance(Chew

et al., 1999; Wong et al., 2000b). Wong et al. (2000b)found a negative correlation

between tensile strength and CBR force of geotextiles with the expected damage. Results

reveal that geotextile with superior mechanical strength did not outperform materials of

lower strength properties. Hence, index test values should not govern the impact

resistance of geotextiles.

Design guidelines and charts available are mostly based on physical parameters

such as mass and thickness of geotextiles. Mass of geotextile represents the amount of

polymer used in manufacturing of the geotextile product (Wong et al., 2000b). Though

the mass of geotextile is often associated with its mechanical performance this parameter

is merely a relative indicator. It is only relevant if the same manufactured form of fabric

is compared. Combined technological developments, reformed manufacturing techniques

and improved quality of raw materials could result in better performing geotextile given

the same amount of mass (Palmeira et al., 2008; Wong et al., 2000b). As such, mass does

not necessarily reflect the impact resistance of geotextile.

Engineers and designers would often carry out field trials to determine the

appropriate drop height in installation to minimise the risk of punctures inflicted on

geotextiles(Ameraunga et al., 2006; Holtz et al., 1997). Chew et al. (1999) and Wong et

al. (2000b) agreed that there is neither a standard methodology for field trials nor a

51

standard damage evaluation approach adopted by the industry. This led Chew et al. (1999)

to propose a standardized drop test (SDT) to measure the puncture resistance of

geotextiles in a quantifiable and empirical manner. However, despite the reproducible

results obtained from SDT, it has been shown that geotextile puncture is a random event

and a repeated number of tests may be necessary to capture the risk of puncturing the

geotextile. Though, SDT may closely simulate field conditions, the variation of

overlaying armour rocks in size and shapes and subgrade conditions makes it harder to

determine the safe puncture threshold. Hence, there is a need to design a test apparatus

to isolate and control the parameters so that governing factors of damage are identified.

In 1978, BAW developed a standard impact test to simulate the dynamic impact of

a falling armour rock on geosynthetics (current issue: RPG (1994)) Heibaum (2014)

describes the dynamic impact is simulated by releasing a drop hammer with a tip edge

onto the geotextile sample laid above a soil sample at determined drop energy. The BAW

guideline highlights the use of drop energy as a function of rock size and drop height, a

functional versatile approach to simulate dynamic impact. However, damage simulated

from this approach does not fully replicate the damage sustained by geotextiles during

installation. The cylindrical drop hammer with a tip edge remains in dispute: the question

of the armour rock represented with a cylindrical drop hammer still invites contention.

When designing geotextiles for coastal applications the effect of possible damage

should be taken account, including mechanical damage during installation. Hence long

term observational studies have taken place to determine the extent of damage after a

certain number years in service, this typically ranges from 5-14 years (Christopher, 1983;

Lawson, 1982; Loke et al., 1995; Mannsbart and Christopher, 1997; Wong et al., 2000b).

Studies suggest that there is a substantial deterioration in geotextiles’ mechanical strength


from samples that were exhumed from project sites. Results gathered from these studies

provide engineers the relevant information to develop safety factors and design

guidelines. However, it is difficult to determine the durability of geotextile during

installation as various damage factors are correlated when it is examined after a number

of years. These damages could include tension loading (during installation and/or

operational phase), heavy wave attacks on armour rocks and/or creep under permanent

loading. It will be useful if engineers are able to identify the initial material strength

reduction upon installation, as design engineers could account for the mechanical

deterioration of the material during operational stage.

This study utilizes a new approach, Drop Rock Test (DRT) (Cheah et al., 2015;

Kendall et al., 2014b) to simulate damage on a geotextile during installation. The impact

resistance of the material is examined by recording the number of samples that survived

(no punctures) for a series of drop rock tests. Surviving samples are directly exhumed and

tested for changes in mechanical strength using CBR puncture tests. Results of

experimental investigations on impact resistance and retained CBR strength are presented

in this paper.

4.2 Materials and Experimental Description

4.2.1 Drop Rock Test

Figure 1 illustrates the DRT apparatus which consists of a gantry crane with a lifting

capacity of 1550kg, concrete block and a subgrade containment unit. As shown in Figure

1, two A-frames of rectangular hollow sections are bolted to concrete blocks to provide

support to the girder section. The concrete block was constructed with a 90o tip facing

downwards to represent the worst possible damage inflicted onto geotextile by an armour

53

rock. Three concrete blocks weighing 922kg, 438kg and 93kg are available with this

apparatus. The DRT apparatus has a maximum drop height capacity of 2.0m.

Figure 1: DRT Apparatus(Cheah et al., 2016)


The DRT procedure adopted in this study is summarised as follows:

i. A concrete block of 922kg with a 90o tip was used in DRT to deliver constant

impact energy onto geotextile for a specified drop height. This orientation ensures

the greatest mechanical stress inflicted on geotextile.

ii. Each geotextile sample measured 1.8m by 2.0m and was stencilled with a grid of

50mm by 50mm, where the concrete block is targeted to fall. Any physical

changes can be observed by measuring the change in the length stencilled on the

geotextile.

iii. The subgrade confined box was filled with sand compacted using a hand tamping

system. A 4.2kg tamper of 200mm x 300mm x 700mm (L x W x H) was released

from a height of 0.5m and repeated 30 times (6x5 grid pattern)., The centre region

of the subgrade was repeatedly emptied and refilled due to the falling of concrete

block; this area was tamped again by a 3 by 5 grid pattern to ensure consistent

compaction.

iv. The concrete block was electrically winched up to the desired drop height and was

laterally moved across with a trolley along the crane rail. The drop height was

measured from the bottom tip of the test block to the surface of the geotextile with

a T-gauge. The testing block was then disengaged from the quick release

mechanism once in position.

v. After the drop, the concrete block and G-clamps were removed, any punctures

found on the geotextile sample were considered a failure and the size of damage

was measured (Figure 2a). For survived (non-punctured) samples, elongation

values were measured against 6 grid squares (which initially 300mm, as each

square is 50mm by 50mm) from the point of intersection (Figure 2b).

55

vi. Survived samples were exhumed and further assessed with Static Puncture (CBR)

Test.

Figure 2: (a) Punctured(Cheah et al., 2016)

Figure 2: (b) Elongation (Cheah et al., 2016)

4.2.2 Materials

Experimental investigations using the Drop Rock Test (DRT) method were completed

following the test program listed in Table 1. Table 1 summarizes the parameters in the

DRT program with two types of non-woven polyester geotextiles: staple fibre (SF) and

continuous filament (CF). Four staple fibre and three continuous filament non-woven

polyester geotextiles were selected to perform the damage evaluation of simulated

installation damage using DRT. Mechanical and physical properties, their units, and test

standards are tabulated in Table 2 and 3.

Table 1: Test Program

Description of subgrade and concrete block

Types of geotextiles

Drop height (m) No. of test per drop height


Poorly Graded Sand Horizontal base Drop Mass:922kg

SF1, SF2, SF3, SF4

0.5,1.0,1.5,2.0 5

Poorly Graded Sand Horizontal base Drop Mass:922kg

CF1, CF2, CF3 0.5,1.0,1.5 5

Table 2: Staple Fibre (SF) Geotextile Properties (typical values)(Kendall et al., 2014b)

Table 3: Continuous Filament (CF) Geotextile Properties (typical values) (Kendall et al., 2014b)


Mechanical

Test Standard Units SF1 SF2 SF3 SF4


MD

AS 3706.2 %

107 111 110 112

XD 84 83 81 83


MD

AS3706.2 kN/m

11 18 26 37

XMD 21 39 55 83


MD

AS3706.3 N

320 477 656 842

XMD 542 977 1264 1774


MD

AS2001.2.3 N

686 1161 1753 2469

XMD 1097 1948 2958 4539

CBR Burst Test AS3706.4 N 2719 4522 6526 8824

Physical Mass per unit area g/m2 380 611 846 1224


Mechanical

Test Standard Units CF1 CF2 CF3


MD

AS3706.2 %

54 58 68

XD 59 62 67


MD

AS 3706.2 kN/m

22 30 58

XMD 21 28 56


MD

AS3706.3 N

540 753 1485

XMD 510 700 1425


MD

AS2001.2.3 N

1430 2100 4290

XMD 1350 1910 4300

CBR Burst Test AS3706.4 N 3600 4800 9696

57

4.2.3 Static Puncture (CBR) Test

The exhumed samples were further assessed with Static Puncture (CBR) Test, in

accordance with ISO 12236:2006. The location of exhumed samples (four edges and one

damaged (strained)) is illustrated in Figure 3. It was assumed that the edge materials did

not experience any damage caused by the released concrete block and thus were

considered as control specimens. A comparison was made between control and strained

samples.

Figure 3: Location of Exhumed Samples (Kendall et al., 2014b)

4.3 Results and Discussion

4.3.1 Impact Resistance

In order to assess the impact resistance of geotextiles, repeated-measures of Drop Rock

Test (DRT) were used. Figure 4 compares the impact resistance at a range of impact

energy levels for staple fibre (SF) and continuous filament (CF) geotextiles. Impact

resistance represents the probability of installation survivability and is derived based on

the number of samples (out of 5 tests) that did not puncture when the impact load is

applied. The term, impact energy level, is derived from potential energy in DRT, i.e.:

𝐸𝐸𝑖𝑖 = 𝑚𝑚𝑚𝑚ℎ

Physical Mass per unit area g/m2 280 379 740


Where m is the mass of rock, g is the gravitational acceleration and h represents the

installation height. (Kendall et al. (2014a)) point out that impact energy can conveniently

summarize the relevant installation specifications such as rock size and drop height. Thus,

impact energy is chosen to represent the results. In general, the potential for damage

increases with higher impact energy.

It is accepted that with greater drop heights, the greater the impact energy applied

on the laid geotextile, hence, the mechanical stress induced on geotextile is greater as

well. Therefore, it is assumed that the material with greater mechanical properties would

have greater impact resistance. However, Figure 4 depicts that the above concept should

simply be applied for materials for the same manufactured form of fabric. Tables 2 and 3

show that mechanical properties of CF samples are generally greater (indicating better

impact performance) than SF samples however DRT tests showed the opposite trend.

Results showed that CF3 with 9696 N of CBR Burst Strength did not outperform the SF1

geotextile with 2719N in DRT tests. As shown in Figure 4, SF geotextiles have greater

impact resistance compared to CF geotextiles. This highlights the risk on relying on index

test values which is inadequate in reflecting geotextile performance on site. This result

could likely to be related to the fibre characteristics of CF, which are longer in its fibre

length compared to SF geotextiles.

59

Figure 4: Impact Resistance at a range of impact energy for Non-woven Geotextiles(Cheah et al.,

2016)

The long fibrous length of CF geotextile is more likely to tear compared to short

twisted spun SF geotextiles when subjected to a dynamic force. These findings agree with

the views of Chew et al. (1999) and Wong et al. (2000b), who suggested geotextile

characteristics are one of the key parameters influencing geotextiles performance during

a SDT test. By the interpretation of these results, abrasive shearing appears to be a

negative influence in impact resistance of geotextiles. This type of damage was observed

throughout the testing phase. The abrasion to long monofilaments seemed to be more

pronounced than abrasive effects on short staple fibre. The downward movement of the

concrete block induces friction between the geotextile and subgrade as well as between

the rock and the geotextile. The fibre layers of the material are sheared away by the

abrasive effects, which resulted thinning of the material.

In theory, the impact resistance of both SF and CF geotextiles increases with the

increase in mass per unit per area. On the contrary, the heaviest CF3 (740g/m2) did not

outperform the lighter 380g/m2 SF1. From this data, this confirms the mass of the

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

4.52 9.04 13.60 18.07

Impa

ct R

esis

tanc

e(%

)

Impact Energy Level (kJ)

SF1 (380 g/m²)

SF2 (611 g/m²)

SF3 (846 g/m²)

SF4 (1224 g/m²)

CF1 (280 g/m²)

CF2 (379 g/m²)

CF3 (740 g/m²)


material is simply a physical parameter and does not necessarily represent the impact

resistance of the material. Wong et al. (2000b) pointed out the mass of the material

basically implies the amount of polymer used for geotextile manufacturing. Within the

family of non-woven geotextiles, significant differences in performance can be observed

as was seen with CF and SF. The impact resistance of the material is influenced by the

mass but not entirely governed by the mass. Note that one SF2 test result was omitted at

9.02kJ due to experimental error; hence it resulted as an impact resistance of 75%.

Figure 5: Punctured Area for Non-woven Geotextiles(Cheah et al., 2016)

Figure 5 shows the punctured area of CF geotextiles to be significantly larger than SF

geotextiles given the same impact energy level. Even, the heaviest continuous filament

sample (CF3) did not outperform any staple fibre geotextiles despite index values in Table

3 suggesting reliable mechanical performance. Following on from these results, the DRT

has proven to be an effective approach to assess impact performance compared to index

tests that are less likely replicate field conditions.

0.0

250.0

500.0

750.0

1,000.0

1,250.0

1,500.0

1,750.0

2,000.0

2,250.0

4.52 9.04 13.60 18.07

Punc

ture

d Ar

ea (c

m2 )

Impact Energy Level (kJ)

SF1 (380 g/m²)

SF2 (611 g/m²)

SF3 (846 g/m²)

SF4 (1224 g/m²)

CF1 (280 g/m²)

CF2 (379 g/m²)

CF3 (740 g/m²)

61

Lawson (1992) correlates the geotextiles filtration performance with the number of

punctures and the size of puncture and as expected with greater damage, the filtration

efficiency of the material reduces. Lawson suggests there should not be too many

punctures or large punctures inflicted onto the material for the geotextile to function

reasonably. This study is unable to determine the number of punctures on geotextile as

each laid geotextile only experienced one rock drop per installation height instead of the

bulk loading of armour rocks. If the geotextiles punctured, it was a single puncture. Figure

5 shows the size of punctured area of geotextiles. The punctured areas of geotextiles are

shaped in-line with crater of the concrete block. Generally, the size of damaged area is

larger with higher impact energy. The size of punctured area represents the damaged size

inflicted on geotextiles, but since the concrete block is constructed with a 90o sharp edged

to simulate the most critical form of installation damage, it signifies the maximum

possible damaged area.

Over the past few decades, the manufacturing quality, installation practice and

performance expectations of geotextiles have steadily increased. For many years,

installed geotextiles with punctures were deemed stable as long the structure remains

static, but with continuous cyclic wave actions, armour rocks are unable to constantly

remain static under hydraulic action. Hence, the punctured area will not be sealed.

Increased expectations for life time serviceability and filter integrity recognize that a shift

in the armour rock can expose the damaged area and put stability of the structure at risk.

As acknowledged by authors (Christopher and Fischer, 1992; Heerten, 2007, 2008),

filtration designs will be compromised if the geotextile material punctures. Hence,

throughout the testing program, any visible puncture noted was considered a failure.


4.3.2 Retained Strength

Survived (strained) geotextile samples were further examined for residual strength with

the Static Puncture (CBR) Test. This value is compared with the average of the four

unaffected edges of the geotextile sample. Figure 6 compares the retained strength

between control and strained geotextile samples. It is apparent from Figure 6 there is a

decrease in retained strength in strained samples in both staple fibre (SF) and continuous

filament (CF) sample. The correlation between SF and CF geotextiles is interesting

because of their mechanical properties (tabulated in Table 2 and 3) as CF samples at

impact energy of 4.52kJ showed substantial reduction in retained strength compared to

SF samples (1 and 2) at 4.52kJ.

Figure 6: Retained strength and Elongation for Geotextiles(Cheah et al., 2016)

Generally, the gap between control and strained SF samples is fairly similar, except for

SF3. The change in percentage of SF3 retained strength is approximately 30% at 13.60kJ

which is twice the difference at 9.04kJ. The likely reason for the underperformance of

SF3 is it was close to rupture at 13.6kJ impact load, which resulted in relatively low CBR

value. The observed difference between control and strained CF samples is much larger

0.02.04.06.08.010.012.014.016.018.020.0

0.010.020.030.040.050.060.070.080.090.0

100.0

SF1

(4.5

2kJ)

SF1

(9.0

4kJ)

SF2

(4.5

2kJ)

SF2

(9.0

4kJ)

SF3

(9.0

4kJ)

SF3

(13.

6kJ)

SF4

(9.0

4kJ)

SF4

(13.

6kJ)

SF4

(18.

07kJ

)

CF1

(4.5

2kJ)

CF2

(4.5

2kJ)

CF3

(4.5

2kJ)

Aver

age

Elon

gatio

n (%

)

Reta

ined

Str

engt

h (k

N)

Non-woven geotextile and Impact Energy (kJ)

Strained Elongation

63

compared to SF geotextiles. The reduction in retained strength could extend up to 48%

for CF2. The reason for CF2 underperformance is likely due to the previously stated

concept; CF2 was close to rupture at 4.52kJ impact load, thus resulted in low CBR value.

The rigid clamping system in this study represents a conservative approach where

samples are clamped rigidly around the perimeter of the subgrade contained creating a

fixed edge boundary. Geotextile may have more freedom to conform to the soil and

dumped rocks in practice. This implies the residual strength of geotextile sample may not

exceed the presented results when subjected to greater impact energy particularly under

field conditions; nonetheless geotextiles are often subjected to bulk loading of armour

rocks. The results presented in Figure 6 are likely to be conservative compared to rocks

dumped in bulk as the study utilizes a single rock per drop on a confined geotextile.

Contrary to expectations, this study did not find significance correlations between

elongation and impact energy for staple fibre geotextiles. Elongation herein refers to the

strain caused by the deformation of the soil subgrade after the rock has dropped (refer to

Figure 2b). In general, with greater impact energy applied, the greater the deformation in

the subgrade and sequentially this causes the material to experience greater strain Results

in Figure 6 differ with this notion. The difference in elongation (%) between SF samples

remains within a narrow range of 15-18% regardless of the impact energy imposed. A

correlation between various impact energy levels with elongation is not determined for

CF samples, as all the samples punctured at impact energy greater than 4.52kJ. However,

when different grades of geotextiles are subjected to the same impact energy, interesting

results could be observed. Figure 7 presents the average elongation (%) of staple fibre

samples subjected to impact energy of 9.02kJ and continuous filament samples subjected

to impact energy of 4.52kJ.


Figure 7:Av. Elongation (%) of Non-woven Geotextiles subjected to different Impact Energy (kJ)* Note*: Figure 7 did not include elongations of SF products at 4.52kJ because samples with the same properties are no longer available. (Cheah et al., 2016)

Figure 7 shows that SF samples had similar elongation among all product grades

subjected to same amount of impact energy. The higher grade materials (SF3 and SF4)

have significantly superior strength properties but they did not show a significant

difference in ability to elongate compared to the lower grade geotextile, SF1. These

results are likely related to the consistent amount of subgrade deformation occurring

during each impact test. This highlights the inadequacy of index properties being

measured as performance indicators. The mechanical properties obtained from the

various index tests (Table 2 and 3) measure the elongation in both cross-machine and

machine dimension but do not model the deformation experienced by geotextiles caused

by the armour rocks. These index tests do not account for the soil-geotextile interaction.

The sublayer/subsoil beneath the geotextile contributes to the ability of geotextiles to

withstand strain during installation and dumping force caused by the armour rock. It is

important to take account of the soil-geotextile interaction as the stability of geotextile

layer strongly depends on the type and composition of sublayer/subsoil. In contrast, the

elongation behaviour CF geotextile samples upon impact showed a trend with increasing

geotextile grade; the higher grade the material, the lower the expected strain. The

0.02.04.06.08.0

10.012.014.016.018.020.0

SF1

SF2

SF3

SF4

CF1

CF2

CF3

Aver

age

Elon

gati

on (%

)

Non-woven Geotextile

4.52kJ

9.02kJ

65

observed trend could be attributed to the influence of CF’s tensile strength on the amount

of subgrade deformation. Interestingly, though both SF and CF samples are categorised

as non-woven geotextiles with similar mechanical strength, they responded distinctly.

4.4 Conclusion

The long term durability and serviceability of geotextiles in structures depends

significantly on the impact resistance of geotextile during the harsh installation

conditions. Any functions such as separator and filter layer could be compromised when

the material suffers severe damage. The study utilises a new approach, Drop Rock Test

(DRT) to assess the impact resistance and retained strength of geotextiles. The DRT is a

reliable scientific method to simulate controlled installation conditions which is able to

generate consistent and reproducible results.

The experimental investigations in this study illustrate the use of Drop Rock Test

(DRT) to assess the impact resistance and retained strength of geotextiles. The impact

resistance reflects survivability during installation, and the damaged size area can be used

to indicate the worst possible damage that may be inflicted on the geotextiles. Findings

suggest the potential of damage to the material increases with higher impact energy, as

the greater the impact energy, the greater the mechanical stress is induced onto the laid

geotextile. Results are in agreement with Wong et al. (2000b) findings which showed

index derived values are very limited in their ability to quantify the impact resistance of

the material. Results reveal that the lighter staple fibre non-woven geotextile (380g/m2)

outperforms the heavier continuous filament non-woven geotextile (740g/m2) which

implies the use of mass per unit area as a relative indicator for installation robustness

should be discouraged. Hence, neither index derived values nor the mass should solely

govern the impact resistance of geotextiles.


Retained strength of the survived sample represents the retained strength of the

installed geotextile on site. The results are in accord with the long term observational

studies (Christopher, 1983; Lawson, 1982; Loke et al., 1995; Mannsbart and Christopher,

1997) indicating noticeable loss in mechanical strength. However, results from the long

term observation studies are only obtained after a number of installation years which

makes it harder for engineers and designers to accurately isolate factors that accounts

from the mechanical deterioration of installed geotextiles (installation damage/cyclic

waves/weathering). Findings showed that the reduction in retained strength for SF

geotextiles and CF geotextiles could extend up to approximately 30% and 50%

respectively.

Acknowledgements

This study was carried out as part of the first author’s PhD research at Queensland

University of Technology (QUT) in Brisbane and therefore authors would like to

acknowledge QUT Australian Postgraduate Awards (APA) for providing scholarship to

the first author for her PhD study. Authors express their appreciation to Geofabrics Centre

of Excellence in Gold Coast Australia for providing financial support, test materials and

testing facilities for this study. Finally, the support given by QUT technical staff, QUT

undergraduate students and technical staff of Geofabrics Centre of Excellence Centre is

gratefully acknowledged.

References

AS 2001.2.3.2- 2001 Methods of test for textiles-Physical tests- Determination of maximum force using the grab method (ISO 13934-2:1999, MOD).

AS 3706.2-2012 Geotextiles-Methods of Test-Determination of tensile properties- Wide strip and grab method.

AS 3706.3-2012 Geotextiles- Methods of Test- Determination of tearing strength- Trapezoidal method.

67

AS 3706.4-2012 Geotextiles- Methods of test- Determination of burst strength- California Bearing ratio (CBR) Plunger method.

Ameraunga, J., Boyle, P.J., Loke, K.H., Hornsey, W., Stevens, M., 2006. Use of geotextiles to overcome challenging conditions at the seawall project in Port of Brisbane, 8th International Conference on Geosynthetics, Yokohama, Japan.

Cheah, C., Gallage, C., Dawes, L., Kendall, P., 2015. Effect of Simulated Rock Dumping on Geotextile, 12th Australia New Zealand Conference on Geomechanics, Wellington, New Zealand.

Chew, S.H., Karunaratne, G.P., Tan, S.A., Wong, W.K., 1999. Standardized Drop Test (SDT) to evaluate puncture resistance of geotextiles in coastal revetments, Rencontres Geosynthetics 99, Bordeaux, France, pp. 303-310.

Christopher, B.R., 1983. Evaluation of two geotextile installation in excess of a decade old. Transportation Research Record, 79-88.

Christopher, B.R., Fischer, G.R., 1992. Geotextile filtration principles, practices and problems. Geotextiles and Geomembranes 11, 337-353.

Giroud, J.P., 1984. Geotextiles and geomembranes. Geotextiles and Geomembranes 1, 5-40.

Heerten, G., 2007. Revetment Design- Lessons learnt from failures, 10th Australia New Zealand Conference on Geomechanics. Australian Geomechanics Society, Brisbane, pp. 324-329.

Heerten, G., 2008. Needle-punched nonwoven GTX in coastal engineering applications, Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, Darmstadt, Germany.

Heibaum, M., 2014. Geosynthetics for waterways and flood protection structures – Controlling the interaction of water and soil. Geotextiles and Geomembranes 42, 374-393.

Holtz, R.D., Christopher, B.R., Berg, R.R., 1997. Geosythetic Engineering. BiTech Publishers Ltd., Richmond, British Columbia, Canada.

Hornsey, W.P., 2012. Geotextiles in specialise marine applications: an australia prespective over 32 years. Geofabrics Australasia.

Kendall, P., Austin, R.A., Cheah, C., 2014a. Installation Durability Testing of Revetment Geotextiles, 7th International Congress on Enviromental Geotechnics, Melbourne, Australia.

Kendall , P., Austin, R.A., Cheah , C., Lacey , M., 2014b. Large Scale Controlled Testing of Geotextile Puncture Resistance for Rock Impact, 10th International Conference on Geosynthetics. Deutsche Gesellschaft für Geotechnik e.V., Berlin Germany.

Lawson, C.R., 1982. Geotextile requirements for erosion control structures, Recent Developments in Ground Improvement Techniques, Proceedings of the International Sumposium. A. A. Balkema, Bangkok, Thail, pp. 177-192.

Lawson, C.R., 1992. Geotextile revetment filters. Geotextiles and Geomembranes 11, 431-448.

Loke, K.H., Lee, C.H., Lim, C.H., Chin, P.W., 1995. Performance of nonwoven geotextile in coastal protection works with marine clays: a case study. Nonwovens Industry 26, 60-64.

Mannsbart, G., Christopher, B.R., 1997. Long-term performance of nonwoven geotextile filters in five coastal and bank protection projects. Geotextiles and Geomembranes 15, 207-221.


Palmeira, E.M., Tatsuoka, F., Bathurst, R.J., Stevenson, P.E., Zornberg, J.G., 2008. Advances in geosynthetics materials and applications for soil reinforcement and environmental protection works. Electronic Journal of Geotechnical Engineering. Bouquet 8, 1-38.

Rosete, A., Lopes, P.M., Pinho-Lopes, M., Lopes, M.L., 2013. Tensile and hydraulic properties of geosynthetics after mechanical damage and abrasion laboratory tests. Geosynthetics International 20, 358-374.

RPG, 1994. Guidelines for Testing Geotextiles for Navigable Waterways. Bundesanstalt für Wasserbau, Karlsruhe.

Wong, W., Chew, S., Karunaratne, G., Tan, S., Yee, K., 2000. Evaluating the Puncture Survivability of Geotextiles in Construction of Coastal Revetments, Advances in Transportation and Geoenvironmental Systems Using Geosynthetics, pp. 186-200.

69

Chapter 5: Measuring hydraulic properties of geotextiles after

installation

Charmaine Cheah, Chaminda Gallage, Les Dawes and Preston Kendall School of Civil Engineering and Built Environment Science and Engineering Faculty Queensland University of Technology Article submitted to: Geotextiles and Geomembranes (under review)


The authors listed below have certified that: 1. They meet the criteria for authorship in that they have participated in the conception,

execution, or interpretation, of the least that part of the part of the publication in their field of expertise;





Cheah, C: (Candidate) Established methodology, data analysis, preparation of tables and figures, writing and compilation of manuscript

Signature: Date: Gallage, C: (Principal supervisor) Supervised and assisted with manuscript compilation, editing and co-author of manuscript

Signature: Date: Dawes, L: (Associate supervisor) Supervised and assisted with manuscript compilation, editing and co-author of manuscript

Signature: Date: Kendall, P: (External supervisor) Editing and co-author of manuscript

Signature: Date: Principal Supervisor Confirmation I have sighted email or other correspondence from all co-authors confirming their certifying authorship. (If Co-authors are not able to sign the form, please forward their email or other correspondence confirming the certifying authorship to the RSC)

Name: Signature: Date:

70 Chapter 5: Measuring hydraulic properties of geotextiles after installation

This Chapter is an exact copy submitted to the journal paper above.


This paper investigates the influence of construction stresses (rock dumping) on the

filtration properties of geotextiles. Non-woven geotextiles were submitted to damage test

- DRT and further assessed with laboratory tests (Constant Head test and Bubble Point

Test). Findings suggest the filtration properties of severely damaged specimen still meet

the permittivity and retention criterion.


Objective 2

• Assessed permittivity of DRT induced damage geotextiles• Assessed pore size of DRT induced damage geotextiles

Objective5

• Developed chart to predict permittivity of geotextiles after installation

71





dumping)






Develop chart for robustness of geotextile against construction stress using Drop and CBR Energy

Develop chart to predict the permittivity performance of geotextile upon installation


Lite

ratu

re a

nd D

eskt

op S

tudy

E

xper

imen

tal a

nd D

ata

Ana

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s R

esea

rch

Out

com

es

Paper 1

Paper 2

Paper 3


Abstract

Since geotextiles have been progressively incorporated into coastal protection structures,

the influence of installation damage on them has been the primary concern. During

installation/construction, geotextiles are repeatedly subjected to high mechanical stresses

which often exceed service stress. It is therefore vital to evaluate the mechanical and

hydraulic damage and determine the consequences of these damages to better develop

criteria for selection of suitable products. As these damages could reduce the material’s

mechanical strength and hydraulic efficiency, or in the severest form of damage,

puncturing, would end the separation function. The properties investigated in this paper

include the permittivity and apparent opening size (AOS) of geotextiles. Generally, the

greater the drop energy of armour units applied to geotextiles, the greater the potential for

damage. Findings show that the residual permittivity could increase significantly, 45%

during installation. The preliminary design of coastal structures will be optimised as

engineers and designers can better estimate the amount of damage on geotextiles upon

installation.

KEYWORDS: Geosynthetics, Drop Rock Test, Hydraulic Efficiency, Apparent Opening

Size, Unsaturated Subgrade

5.1 Introduction

Geotextiles have been extensively incorporated into coastal and waterway engineering

and are used predominantly as filters or separators for rock revetments, armoured

slope/banks along coastlines and embankments (Abromeit and Heibaum, 1996; Heerten,

1984; Palmeira and Tatto, 2015; Pilarczyk, 2000). The question around the effect of

installation damage on geotextiles has been the focus point for researchers. During

installation, geotextiles are repeatedly subjected to high dynamic bulk loading of armour

73

units. This can degrade geotextile’s hydraulic efficiency. Hydraulic efficiency of

geotextile herein refers the ability to allow free passage of water through rock armour

whilst retaining and protecting soil beneath from washing away from tidal currents and

wave actions. The growing popularity of geotextile as an alternative to conventional

granular filters for hydraulic structures requires research on the hydraulic properties of

the material over the expected design life.

Factors that greatly influence geotextile’s hydraulic efficiency include installation

conditions (subgrade and construction machinery), cover materials (rock armour or

aggregates), climate and geotextile characteristics. In order for geotextiles to remain

durable and continue to perform the intended functions throughout the lifetime of the

structure, geotextiles must first have the ability to withstand construction conditions. The

durability of geotextiles would cease if the materials are severely damaged

(tear/puncture/rupture) during the installation phase. Diederich (2000) points out that the

greatest mechanical stress induced upon geotextiles typically occurs during the loading

and construction phase rather than the service life. For coastal structures, the used of

armour units are often applied to withstand cyclic wave actions(Abromeit and Heibaum,

1996). These heavy units are often dumped onto the geotextile filter layer from a certain

height which induces high dynamic impact onto the material. This would result in the loss

of mechanical strength, hydraulic efficiency or severest, puncturing of the material. It is

therefore important that the material “survives” (does not rupture/puncture) during

installation to ensure both geotextile and structure continue to perform as required.

Extensive research on the hydraulic performance of geotextiles has been conducted

through long-term observational studies in which samples are excavated from sites after

a number of installation years (5-15 years) (Christopher, 1983; Heerten, 1980; Loke et


al., 1995; Mannsbart and Christopher, 1997; Wong et al., 2000a). These studies have

provided valuable information about the hydraulic performance after installation. Though

the material survives, geotextiles still undergo high dynamic stresses (rock dumping) that

would lead to physical changes (strain/elongation). These changes alter geotextiles’

hydraulic properties. Christopher (1983) evaluated the performance of woven geotextile

filter in a rip-rap revetment type seawall at 79th Street Causeway Project in Miami, Florida

that was installed over a decade ago. The permeability of the exhumed samples was found

to be 2x10-2 cm/s whilst new material was noted to be 4x10-2 cm/s. The permeability test

results showed a 50% reduction in its hydraulic efficiency.

Mannsbart and Christopher (1997) evaluated the filtration performance of non-

woven geotextiles at sites across Europe and Malaysia that were installed for 6-14 years.

It is assumed that the percentage of permeability of geotextile would increase as the pore

size increases since the material strains/elongates to conform to the laid armour units. But

their investigation showed a reduction in the permeability. This appears to be correlated

to the re-orientation of geotextile’s micro-structure (internal fibre). The fibre re-

orientation typically occurs during and after installation by loading conditions and

particles, sediments or organic matter deposit that could obstruct the drainage path.

Furthermore, Rollin and Lombard (1988) implied that hydraulic properties can also be

influenced by salt deposition, mineral precipitation and bacterial growth. The

combinations of these factors are likely to clog and decrease the permeability of material.

Loke et al. (1995) presented the results of a field investigation of non-woven

geotextile filter in coastal protection works for marine clays that had been in service for

more than 5 years. The geotextiles were excavated from two sites located in Malaysia.

At Site A, a coastal revetment structure comprised of non-woven geotextile was

75

constructed over marine clay and a sand layer while at Site B, the structure was underlain

with a non-woven geotextile and then laid directly over marine clay. At Site A there was

a substantial permeability reduction of approximately 67% for excavated-dirty and 13%

for excavated-clean specimens whilst at site B, permeability of excavated-dirty specimens

had a reduction of 42%. The opposite trend was observed at site B, with an increase of

60% in permeability for excavated-clean samples. This suggests the reduction in

permeability is likely caused by the deposition of particles, sediments, organic matters as

well as salt deposition, mineral precipitation and bacterial growth in excavated-dirty test

specimens. In contrast, the increase in permeability for Site B – excavated-clean

specimens was likely caused by the increase in pore size. It was found that the pore sizes

for excavated-dirty were generally less than excavated-clean samples. This is in

agreement with the permeability trend where excavated-dirty samples are most likely

affected by environmental deposits.

Wong et al. (2000a) evaluated the performance of woven, polypropylene-based

geotextiles in a reclamation project located in the south-western coast of Singapore island

between Jurong Town Corporation (JTC) and National University of Singapore (NUS).

The geotextiles had been in service for 12 years at the time of excavation. The study

aimed to evaluate the degradation of mechanical and hydraulic properties in different

tropical coastal conditions. It was noted that for both locations, the permeability of both

sites have increased and this observation is in contrast with the two previous case studies.

A broader perspective was adopted by Wong et al. (2000a) who implied that pore size is

not the only determinant of its permeability. It was asserted that the continuity of pores

plays a vital role in allowing water to flow through geotextiles. With the increase in pore

continuity, the number of drainage paths would increase as well, thus allowing greater


flow of water passing through the geotextile. This view is supported by Rawal et al.

(2010b) who asserted the hydraulic efficiency of geotextiles is greatly influenced by the

width and depth of pores.

Undoubtedly, long term studies on hydraulic performance have aided the task of

selecting the appropriate opening size to meet the filtration criteria. But, research work

focuses on the long term observations; hence it would be difficult to isolate the influence

of installation damage itself as there are a great number of environmental factors

involved. Watn and Chew (2002) point out that there is a lack of knowledge for designers

and engineers to know whether a geotextile that is designed for filtration function can

survive the installation process without being damaged. Berendsen (1996) suggests that

a geotextile filter designed for hydraulic structures should fulfil the filtration criteria but

must have sufficient mechanical robustness to resist damage during installation

particularly the dumping of amour units. The ideal approach would be to perform large

scale dumping tests in real field conditions and evaluate the consequences of the damage

on immediately recovered samples. However, there are limited studies (Bräu, 1996;

Diederich, 2000; Paula et al., 2008) that select this study approach as such investigations

requires large setup, is costly and time-consuming. A universal adoptable method would

be most favourable with controlled damage simulation that closely replicates the

predominant installation conditions of geotextile in the field.

Several authors (Carneiro et al., 2013; Paula et al., 2004; Rosete et al.,

2013) have investigated the effect of installation damage on geotextiles using standard

laboratory tests methodology for damage simulation. Carneiro et al. (2013) evaluated the

short term behaviour (tensile and hydraulic) of five non-woven polypropylene geotextiles

that were placed between two layers of granular material and subjected to dynamic

77

loading using a standard installation damage simulation tool (in accordance to EN ISO

10722-1 (BSI, 1998b). Similarly, Rosete et al. (2013) evaluated the tensile and hydraulic

properties of geosynthetics that were subjected to cyclic loading, following the method

described in EN ISO 10722 (BSI, 2007). Rosete et al. (2013) also simulated abrasion

damage on geotextiles following the procedures in EN ISO 13427(BSI, 1998a).

At present, the only standard impact test to simulate dynamic impact of a falling

armour rock on geosynthetics was developed by BAW in 1978 (current issue: RPG

(1994)). The question of the suitability of the cylindrical drop hammer to represent

armour units has raised concerns by others (Cheah et al., 2016). Numerous tests of rock

dumping were conducted in order to propose a new methodology, the Drop Rock Test

(DRT) for this application (Cheah et al., 2015; Cheah et al., 2016; Kendall et al., 2014a;

Kendall et al., 2014b), as shown to Figure 1. This test measures the mechanical robustness

of geotextiles to resist damage during installation/construction. This paper reports on the

investigation of the influence of installation damage on geotextile’s permittivity and

apparent opening size (AOS). Test specimens were first subjected to the DRT and

immediately recovered and further assessed with AS 3706.9 (2012) Permittivity Test and

Bubble Point Test (ASTM D6767-2014).


Figure 1: Drop Rock Test(Cheah et al., 2017)

5.2 Test Program

5.2.1 Materials

This paper studies four (4) grades of staple fibre non-woven geotextiles (codes GTX1,

GTX2, GTX3 and GTX4) subjected to various impact energy. The hydraulic properties

of the geotextile are listed in Table 1. Experimental investigations were completed

following the test program listed in Table 2.

Table 1: Hydraulic Properties (typical values from Geofabrics Australasia specifications)(Cheah et al., 2017)

Grade of Non-woven geotextile

Pore Size (Dry Sieve) (mm)

Permittivity, ψ (s-1)

Coefficient of permeability, k (m/s x 10-4)

Flow rate @100mm head, q (L/m2/s)

Apparent Opening Size (AOS)*, O95

(µm)

GTX-1 <75 1.43 45 143 182 GTX-2 <75 0.69 33 69 124 GTX-3 <75 0.46 24 46 126 GTX-4 <75 0.32 22 32 108 *Pore size (O95) – Capillary Flow Method (ASTM 6767)

79

Table 2: Test Program Description of subgrade and concrete block Types of

Geotextiles (Staple Fibre)

Drop height (m)

No. of test per drop height

Poorly Graded Sand a

• 𝝆𝝆𝒅𝒅: 1486.6 kg/m3 • Gs: 2.53 • Cu=1.58 • Cc=1.02 • D85= 04.mm

Horizontal base Drop Mass:438kg

GTX-1 GTX-2 GTX-3 GTX-4

0.5,1.0,1.5,2.0

3

a 𝝆𝝆𝒅𝒅= Dry density; Gs= Specific Gravity; Cu= Soil coefficient of uniformity; Cc= coefficient of curvature; D85= diameter which 85% of the remaining particles are smaller than that diameter.

5.2.2 Equipment and Methodology

To investigate the effects of installation damage on the hydraulic properties of geotextiles,

Non-woven Geotextiles (NW-GXts) listed in Table 1 were first subjected to the drop rock

test as summarised in Table 2. Geotextiles retrieved from the drop rock tests were

subjected to hydraulic testing. The permeability of the geotextile for this study was

conducted with the constant head test method. This test quantifies water passes through a

geotextile (normal to plane). The performance of geotextile is expressed by permittivity

(ψ). There are two fundamental properties that govern hydraulic properties; (1)

permittivity, ψ (s-1) and (2) apparent opening size (O95, µm). The O95 of geotextile is

often measured with the dry sieving method (ASTM D7451) but this test is known to

lack precision and results in predominantly one pore size for non-woven geotextile (<75

µm). Hence, it is difficult to gain a better understanding of the opening sizes of the pore

channels in the geotextile using this test. Therefore, this study opted to determine the pore

size characteristics by means of the bubble point test. The bubble point test measures the

pore size by capillary flow; based on the principle that geotextile have discrete continuous


pore from one side to the other side of the geotextile. Detailed procedures for the relevant

test methods are described in the following sections.

5.2.2.1 Drop Rock Test

To simulate installation conditions on site where ripraps, stones or rock armour are

dropped above geotextile during construction of coastal protection structure, the Drop

Rock Test was utilised (Cheah et al., 2016). Tests were carried out by releasing a 90o

acute tip concrete block of 438 kg at a specified drop height onto a geotextile that was

laid above the subgrade with fixed edge boundary (Fig. 1). The elongation of the DRT

samples that undergo various impact energies were measured and immediately recovered

for Permittivity Test and Bubble Point Test. Elongations were measured against 6 grid

squares which were initially 300mm (50mm per square) from the point of impact (Fig.

2).

Figure 2: Elongation measurement(Cheah et al., 2017)

5.2.2.2 Hydraulic Tests

5.2.2.2.1 Constant Head Test The retrieved samples were further assessed with Permittivity Test, in accordance with

AS3706.9 (2012). The location of retrieved samples (edge, partially damage and critical

damage) is illustrated in Figure 3. It was assumed that the edge materials did not

81

experience any damage caused by the released concrete block and thus were considered

as control specimens. Comparisons were made between control and strained specimens

(partially and critical damage), refer to Figure 4. In the constant head method, a single

layer of specimen were subjected to unidirectional flow of water (normal to plane) with

headloss ranging between 5-25mm. Figure 5 illustrates the constant head apparatus. The

results were expressed in terms of permittivity 𝜓𝜓 (s-1), coefficient of permittivity, k (m/s-1

x10-4), headloss, ∆ℎ (m) and flow rate, q (m3/s) was determined. The term permittivity

determined from the following equation:

𝜓𝜓 = 𝑞𝑞 𝑛𝑛𝐴𝐴∆ℎ� (1)

where 𝜓𝜓 is the permittivity of specimen, in reciprocal seconds, q is the flow through the

specimen, n is the number of layers of specimen, A is the exposed cross-sectional area in

square meters and ∆ℎ is the headloss, in meters. The coefficient of permittivity, k was

determined from the following equation:

𝑘𝑘 = 𝜓𝜓 × 𝑡𝑡 (2)

where 𝑘𝑘 is the coefficient of permittivity of specimen in meters per second. Results

presented in Figure 8 also include the change in permittivity in percentage term (%)

between control and critically damaged specimens,

𝑃𝑃𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑛𝑛𝑡𝑡𝑒𝑒𝑚𝑚𝑒𝑒 𝑒𝑒ℎ𝑒𝑒𝑛𝑛𝑚𝑚𝑒𝑒 𝑖𝑖𝑛𝑛 𝑃𝑃𝑒𝑒𝑒𝑒𝑚𝑚𝑖𝑖𝑡𝑡𝑡𝑡𝑖𝑖𝑃𝑃𝑖𝑖𝑡𝑡𝑃𝑃 (%) = (𝑃𝑃𝐶𝐶𝐶𝐶 − 𝑃𝑃𝐶𝐶)𝑃𝑃𝐶𝐶� ∗ 100%

(3)

where 𝑃𝑃𝐶𝐶𝐶𝐶 is the mean permittivity of the critically damaged samples and 𝑃𝑃𝐶𝐶is the mean

permittivity of control specimens.


Figure 3: Location of recovered samples(Cheah et al., 2017)

Edge (Control)

Partially Damaged

Critical Damaged

Figure 4: Test Specimens (Cheah et al., 2017)

5.2.2.2.2 Bubble Point Test Specimens were further examined with the Bubble Point Test in accordance to ASTM

D6767-2014. The bubble point test determines the apparent opening size (O95, µm) as a

EDGE

PARTIAL

CRITICAL

83

function of measuring the pressure required to force the fluid that is held in the continuous

pores of geotextiles by capillary action and surface tension. The test subjects the

geotextile to continuous air flow to remove liquid that is retained through capillary action

and surface tension from the geotextiles. The comparison of air flow rates of both wet

and dry geotextile at the same pressure determine the percentage of fluid passing through

the pores larger than or equal to a specific size measured. O95 corresponds with the

geotextile opening size that 95% of the pores are smaller than that size (µm).

Figure 5: Permittivity Test Apparatus (Texas Research Institute (TRI) Australasia, 2016)


5.3.1 Permittivity

5.3.1.1 Residual permittivity of damaged specimen

The partial and critically damaged specimens were examined for the residual permittivity

with the Constant Head Test. These values are compared with the average of unaffected

edges of the geotextile sample. Table 3 summarised the mean values of permittivity, ψ, s-

1 and the coefficient of variation, % of non-woven geotextiles. All results refer to 3 (valid)

specimens. Figure 6 presents the inter-correlations among the various measures for

installation damage on geotextiles (permittivity, drop energy, non-woven geotextiles, and


the extent of damage). Figure 6 indicates that critically damaged geotextiles have greater

permittivity compared to partial damaged and control specimens. This is not surprising

as the critical zone takes the greatest impact meaning more energy is transferred to the

geotextile upon impact, resulting in greater amount of damage on the geotextile.

It is apparent in Figure 6 that the gap between the control and critically damaged

trend lines is larger for lower grade geotextile, GTX-1. The likely reason for the large gap

in GTX-1s’ permittivity, is it experiences the greatest strain at 6.44kJ drop energy. The

tight gap between control and damaged trend lines for other geotextiles (GTX-2 ,3 and 4)

suggest that the materials could withstand greater mechanical stress (rock drop damage)

with less potential for damage. The unvarying permittivity values of GTX-4 suggest that

the increase in drop energy had little influence on the hydraulic properties. The following

interpretations can be drawn from these findings: (1) residual permittivity is linearly

proportional to the drop energy and (2) the greatest mechanical impact is in the critical

zone.

Table 3: Mean Permittivity (s-1) of Non-woven geotextiles at different Drop Energy (kJ) (Cheah et al., 2017)

Geotextiles Drop Energy (kJ)

Permittivity (s-1)

Control Partial Critical

GTX-1

2.44 1.83 2.09 2.19

4.29 1.89 2.19 2.60

6.44 X

8.58 X

X- Punctures occurred during Drop Rock Test

85

GTX-2

2.44 1.02 1.00 1.09

4.29 0.95 1.02 1.13

6.44 1.00 1.06 1.11

8.58 0.99 1.15 1.29

GTX-3

2.44 0.66 0.75 0.79

4.29 0.73 0.79 0.89

6.44 0.67 0.81 0.903

8.58 0.76 0.80 0.93

GTX-4

2.44 0.37 0.41 0.43

4.29 0.37 0.38 0.43

6.44 0.41 0.43 0.47

8.58 0.40 0.40 0.46


Figure 6: Permittivity(s-1) of geotextiles at different Drop Energy (kJ) (Cheah et al., 2017)

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

2.44 4.29 6.44 8.58

Perm

ittiv

ity (s

-1)

Drop Energy (kJ)

GTX-1-Control GTX-1- Partial GTX-1-Critical GTX-2-ControlGTX-2-Partial GTX-2-Critical GTX-3-Control GTX-3-PartialGTX-3-Critical GTX-4-Control GTX-4-Partial GTX-4-Critical

87

5.3.1.2 Relationship between permittivity and drop energy

Figure 7: Linear relationship between permittivity and drop energy of critically damaged geotextiles (Cheah et al., 2017)

Figure 7 presents the relationship between permittivity and drop energy before and after

rock drop damage tests. The data projected a linear relationship between permittivity of

geotextile and applied drop energy. From Figure 7, after induced damage the rate of

permittivity per drop energy of GTX-4 remains practically unchanged (0.009 s-1/kJ). For

both GTX-2 and GTX-3, the gradient of both trend lines is very similar to each other

(0.291 s-1/kJ and 0.267 s-1/kJ, respectively). For GTX-1, the permittivity increases at a

rate of 0.17 s-1 per drop energy (kJ), the highest rate of change compared to other

geotextile samples (GTX-2, 3, and 4). It is apparent in Figure 7 that the better the grade

of the geotextile, the better the ability of the material to withstand rock drop damage.

5.3.1.3 Effects on the permittivity and apparent opening size (O95)

Figure 8 represents the relationships obtained from the hydraulic test after semi-

laboratory damage tests (Drop Rock Test). The data correspond to the percentage change

y = 0.1723x + 1.8335

y = 0.0297x + 0.9927

y = 0.0261x + 0.73

y = 0.0087x + 0.3985

0.00

0.50

1.00

1.50

2.00

2.50

3.00

-1.00 1.00 3.00 5.00 7.00 9.00

Perm

ittiv

ity (s

-1)

Drop Energy (kJ)



in permittivity and O95 of critically damaged specimens to the corresponding drop

energies. Figure 8 shows that, as expected, the permittivity and O95 increases with drop

energy. For GTX-1 (Figure 8a) both curves projected the expected behaviour and for

GTX-4 (Figure 8d) the permittivity and O95 remained unchanged after rock drop damage.

For both GTX-2 (Figure 8b) and GTX-3 (Figure 8c), the change in permittivity

corresponds exactly with the change in O95 but atypical observation is noted at 6.44kJ.

After rock drop damage at 6.44kJ, the permittivity of GTX-2 decreased about 18% when

compared with the undamaged sample. This is 20% less than the change in permittivity

at the lower drop energy, 4.29kJ. This is a result of two factors: the number of tested

specimen is small (3); and the size of the tested specimen is small and may not fully

represent the greatest extent of damage. For GTX-3 the permittivity and O95 increased

35% and 18% respectively after rock drop damage at 6.44kJ, whereas at the highest drop

energy, 8.58kJ, the permittivity and O95 values decreased to 22% and 13% respectively.

It would be expected that the permittivity and the O95 of the material would increase

because the induced rock drop damage, 8.58kJ is more severe than at 6.44kJ. This is likely

the result of the complex porous structure of the fibrous network in a geotextile and the

number of tested specimen is small (3). The fibre re-orientation in GTX-3 after rock drop

damage at 6.44kJ affected more free paths of the permeant flow through the geotextile

resulting in higher permittivity and O95 values than at 8.58kJ. This is consistent with the

finding from Rawal (2010) in that the alignment of fibres in the geotextiles significantly

affects the pore size in non-woven geotextiles.

89

(a)

(b)

(c)

(d)

Figure 8: Relationship between permittivity (P) and pore size (PS) between control and critically damaged specimens (Cheah et al., 2017)

05

1015202530354045

2.440 4.290

Perc

enta

ge c

hang

e %

Drop Energy (kJ)

GTX1_P

GTX1_PS

05

1015202530354045

2.44 4.29 6.44 8.58

Perc

enta

ge c

hang

e %

Drop Energy (kJ)

GTX2_P

GTX2_PS

0

5

10

15

20

25

30

35

40

2.44 4.29 6.44 8.58

Perc

enta

ge c

hang

e %

Drop Energy (kJ)

GTX3_P

GTX3_PS

-10-505

10152025303540

2.44 4.29 6.44 8.58Pe

rcen

tage

cha

nge

%Drop Energy (kJ)

GTX4_P

GTX4_PS


5.3.1.4 Coefficient of permittivity, k

From the test results, the coefficient of permittivity was determined with Equation 2. The

majority of the design standards require the minimum permittivity of the geotextile in

relation to the permittivity of the adjacent soil to be

𝑘𝑘𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑖𝑖𝑔𝑔𝑔𝑔 ≥ 10 𝑘𝑘 𝑠𝑠𝑔𝑔𝑖𝑖𝑔𝑔

where 𝑘𝑘𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑖𝑖𝑔𝑔𝑔𝑔 and 𝑘𝑘 𝑠𝑠𝑔𝑔𝑖𝑖𝑔𝑔 are the coefficient of permeability of the geotextile and soil,

respectively (Heibaum, 2016). Table 4 presents the mean coefficient of permeability of

critically damaged specimen at various drop energies. In this study, the coefficient of

permeability of subgrade is 2.7 m/s × 10−4 which limits 𝑘𝑘𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑖𝑖𝑔𝑔𝑔𝑔 to 27 m/s × 10−4.

Given the case of very permeable soil, Heibaum (2016) further suggests that 𝑘𝑘𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑖𝑖𝑔𝑔𝑔𝑔 ≥

𝑘𝑘 𝑠𝑠𝑔𝑔𝑖𝑖𝑔𝑔 may be sufficient. Closer inspection of Table 5, shows 𝑘𝑘𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑖𝑖𝑔𝑔𝑔𝑔 for all four grade

of geotextiles were greater than 27 m/s × 10−4, the smallest k value (GTX4- 4.29kJ),

37.67 m/s × 10−4still achieves the minimum requirement. The hydraulic tests showed an

increase of coefficient of permittivity of damaged specimen as compared to the control

geotextiles ranging from a factor of 1.4 to 3.3, and their coefficient of permittivity

exceeded the coefficient of permittivity of the subgrade tested in Table 2.

Table 4: Coefficient of permittivity, k (m/s x10-4) (Cheah et al., 2017)

Coefficient of permittivity, k (m/s x 10-4)

Geotextiles

Drop Energy (kJ)


2.44 87.68 72.62 54.65 40.50

4.29 82.74 79.26 66.29 37.67

91

6.44 x 81.93 60.90 46.40

8.58 x 79.21 59.00 38.66

x- puncture at the drop energy underline- the smallest k value

Summary

From the results above, it is concluded that permittivity of damaged geotextile increases

linearly with greater drop energy applied during installation. This study found that the

greatest mechanical stress is located in the critical zone as the permittivity of specimen in

the critical zone is the greatest. Findings suggest higher grade geotextile (i.e. GTX-4)

have better resistance to damage as the rate of permittivity per drop energy is the lowest

(0.009s-1/kJ) among the four grades of geotextiles. The AOS of damaged specimens are

generally consistent with the permittivity behaviour. Despite the material was severely

damaged, the geotextile still meets the permittivity criterion.

5.3.2 Retention

A safe geotextile filter must meet the retention criterion. The increase in opening size

suggests permittivity increases and possibly retention decrease. Though the result above

indicates the increase of pore size and permittivity, the retention criterion is still fulfilled.

Koerner (2016a) suggests that the apparent opening size (O95) for safe geotextile filter

retention should be equal or lesser than D85 (particle size diameter of the protected soil

where 85% of the particle sizes are finer). D85 of the subgrade used in this study is 0.4mm

and O95 of the critically damaged non-woven geotextiles is less than 0.4mm. This suggests

that despite the material being severely damaged; it still fulfils the retention criterion.

93

5.3.3 Strained/Elongation

Prior studies have noted the importance of pore size variations but little attention has

been given to the strain/elongation condition subjected to the specimen (Christopher,

1983; Loke et al., 1995; Mannsbart and Christopher, 1997). As illustrated in Figure 9,

the specimens (GTX-2 and 3) elongated progressively to a certain extent, but once it

reaches its fibre “breaking point”, the material ceased to elongate further. It is probable

that the micro-fibre component of geotextile has exceeded it plastic limit, resulting in

rips/breaks in the micro-fibre layers. The internal breakage of these fibres possibly will

create new and/or larger internal voids causing permittivity of geotextiles (GTX-1, 2

and 3) to increase. For a lower grade geotextile, e.g. GTX-1, it is likely that the

specimens had exceeded its breaking point at 4.29 kJ while GTX-4 specimens have

yet to approach their elongation limits.

One would expect an increase in drop energy applied onto the laid geotextiles to

result in greater elongation/straining which would result in the increase in permittivity

due to larger pore size/voids. It is apparent that the finding is not fully born out in the

results. It can be seen from Figure 9, the increasing drop energy applied (2.44kJ to

6.44 kJ) resulted in greater elongation for GTX-2 but a decrease in permittivity is found

at 6.44kJ (Figure 6). In other words, the material experienced greater deformation but

showed less permittivity. These findings may be somewhat limited by the sample size

retrieved for the hydraulic tests as comparatively the elongation measurement

considers a larger damaged area. The size of damage (shown in Figure 2) was

measured against 6 grid squares (which initially 300mm, as each square is 50mm by

50mm) from the point of intersection.


Figure 9: Relationship between Elongation (%) and Drop Energy (kJ) (Cheah et al., 2017)

5.3.4 Abrasion Damage

Note that the change of permittivity may also be attributed to the creasing of material

and abrasive wear on geotextiles. The surface fibre layers of the geotextiles are sheared

away as illustrated in Figure 10. The creasing of the material may affect the thickness

of the material resulting in the change in permittivity.

Figure 10: Abrasive wear and creasing specimen (Cheah et al., 2017)

10

11

12

13

14

15

16

2.44 4.29 6.44 8.58

Elon

gatio

n (%

)

Drop Energy (kJ)

Relationship between Drop Energyand Elongation

GTX-1

GTX-2

GTX-3

GTX-4

Creasing

Abrasive wear

95

5.4 Limitations

The ability to generalise these results is subject to certain limitations. For instance, this

study only takes into account clean samples, whereas in field case studies, exhumed

samples are often dirty and clogged. The current study is based on a small sample size;

further work needs to be carried out to provide more definitive evidence. The

conventional approach to evaluate the permeability of material is the use of water

which is free from turbidity, ions, foreign matter and microorganism; whilst permeant

in a geotextile filter design would be different.

5.5 Conclusion

The present study was designed to determine the hydraulic properties of geotextiles

after installation. The investigation has shown the greater the drop energy applied, the

increase in permittivity of geotextile is greater. Findings suggest for damaged samples

(critical and partial) have greater permittivity than control specimens and the increase

could extent up to 45% during installation. Critically damaged specimens in the critical

zone were subjected to the greatest impact load; they experience the severest form of

damage caused by the acute 90o tip of the concrete block. Because of the impact, the

material undergoes strain/elongation and the opening size of the material increases.

Findings also indicate that the increase in permittivity and opening size of the material

still fulfils the permittivity and retention criterion for an effective geotextile filter. The

opening size of all four grades of non-woven geotextiles is less than the D85 of the

subgrade that was underlain beneath the geotextile. The apparent opening size (O95)

characteristics of critically damaged samples are consistent with the permittivity trend

of geotextiles at varying drop energies. The results of this investigation show that the

relationship between elongation of geotextile and drop energy may not be directly

proportional. The results suggest that the material is likely to experience strain to a


certain extent and once its micro-structure approaches breaking point, the material

ceased to elongate further and is on the verge of puncturing. Barring a puncture in the

geotextile, the rock drop damage observed did not significantly affect the hydraulic

performance (permittivity and retention criterion) of the geotextile. From a design

perspective, the most important outcome is the development of the linear relationship

chart which gauges the amount of damage (in terms of permittivity) prior construction.

It is economical and effective approach as the preliminary design of the coastal

structures is optimised when engineers and designers can better estimate the amount

of damage on geotextiles upon installation.

Acknowledgements

This study was carried out as part of the first author’s PhD research at Queensland

University of Technology (QUT) in Brisbane and therefore authors would like to

acknowledge QUT Research Training Program (RTP) for providing scholarship to the

first author for her PhD study. Authors express their appreciation to Geofabrics Centre

of Excellence in Gold Coast Australia for providing financial support, test materials

and testing facilities for this study. Finally, authors would like to show their gratitude

to TRI Australasia for providing the testing facility for this study and express thanks

to the technical staffs for sharing insight and expertise that greatly assisted the

research.

Notations

AOS Geotextile Apparent opening size Cc Soil coefficient of curvature (dimensionless) Cu Soil coefficient of uniformity (dimensionless) D85 Soil particle diameter for which 85% in mass of the remaining particles have

smaller diameters than that value (mm) Gs Specific Gravity k Soil coefficient of permeability (m/s) q Flow under a constant 0.1m head of water 𝜌𝜌𝑑𝑑 Dry density of subgrade

97

Ψ Geotextile permittivity (s-1)

References

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Berendsen, E., 1996. Dumping of rock on geotextiles. Geosynthetics: applications, design and construction. Proc. 1st European geosynthetics conference, Maastricht, 1996, 919-924.

Bräu, G., 1996. Damage of Geosynthetics During Installation- Experience from Real Sites and Research Works, in: De Groot, M.B., Den Hoedt, G., Termaat, R.J. (Eds.), Proceedings of the First European Geosythetics Conference- Eurogeo 1. Balkema, Maastricht, Netherlands, pp. 145-150.

BSI, 1998a. EN ISO 13427 (1998) Geotextiles and Geotextile-related Products. Abrasion Damage Simulation (sliding block test), London, UK.

BSI, 1998b. ENV ISO 10722-1 (1998) Geotextiles and Geotextile-related products - Procedure for Simulating Damage During Installation - Part 1: Installation in Granular Materials, London, UK.

BSI, 2007. EN ISO 10722 (2007) Geosynthetics, Index Test Procedure for the Evaluation of Mechanical Damage under Repeated Loading - Damage caused by Granular Material, London, UK.

Carneiro, J.R., Morais, L.M., Moreira, S.P., Lopes, M.L., 2013. Evaluation of the Damages Occurred During the Installation of Non-Woven Geotextiles. Materials Science Forum 730-732, 439-444.




Diederich, R., 2000. Evaluation of Installation Damage of Geotextiles. A Correlation to Index Tests. Fifth conferece on Geosynthetics, YiChang, China, 24-31.

Heerten, G., 1980. Long-term experience with the use of synthetic filter fabrics in coastal engineering. Coastal Engineering Proceedings 1.

Heerten, G., 1984. Geotextiles in coastal engineering—25 years experience. Geotextiles and Geomembranes 1, 119-141.

Heibaum, M., 2016. Geotextiles used in filtration, Geotextiles- From Design to Applications. Elsevier, p. 257.



Koerner, R., 2016. Geotextiles: From Design to Applications. Elsevier S&T Frontlist Promotion 2016, GB.




Palmeira, E.M., Tatto, J., 2015. Behaviour of geotextile filters in armoured slopes subjected to the action of waves. Geotextiles and Geomembranes 43, 46-55.

Paula, A.M., Pinho-Lopes, M., Lopes, M.d.L., 2004. Damage during installation laboratory test. Influence of the type of granular material, Proceedings of 3rd of European Geosynthetics Conference, Munich, pp. 603-606.

Paula, A.M., Pinho-Lopes, M., Lopes, M.d.L., 2008. Combined effect of damage during installation and long-term mechanical behavior of geosynthetics, 4th European Geosynthetics Conference.

Pilarczyk, K., 2000. Critical review of geosystems in hydraulics and coastal engineering application, Atti II European Geosynthetics Conference, Bologna, pp. 65-76.

Rawal, A., Shah, T., Anand, S., 2010. Geotextiles: production, properties and performance. Textile Progress 42, 181-226.

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Wong, S., Chew, S., Tan, S., Karunaratne, G., Yeo, C., 2000. Geotextile Performance After 12 Years Of Service In Coastal Environment, ISRM International Symposium. International Society for Rock Mechanics.

99

Chapter 6: Investigation of installation robustness of geotextiles in

relation to subgrade moisture condition

Charmaine Cheah, Chaminda Gallage, Les Dawes and Preston Kendall School of Civil Engineering and Built Environment Science and Engineering Faculty Queensland University of Technology Article submitted to: Geotextiles and Geomembranes (under review)


The authors listed below have certified that:

1. They meet the criteria for authorship in that they have participated in the conception, execution, or interpretation, of the least that part of the part of the publication in their field of expertise;





Cheah, C: (Candidate) Established methodology, data analysis, preparation of tables and figures, writing and compilation of manuscript Signature: Date:

Gallage, C: (Principal supervisor) Supervised and assisted with manuscript compilation, editing and co-author of manuscript Signature: Date:

Dawes, L: (Associate supervisor) Supervised and assisted with manuscript compilation, editing and co-author of manuscript Signature: Date:

Kendall, P: (External supervisor) Editing and co-author of manuscript Signature: Date:

Principal Supervisor Confirmation I have sighted email or other correspondence from all co-authors confirming their certifying authorship. (If Co-authors are not able to sign the form, please forward their email or other correspondence confirming the certifying authorship to the RSC) Name: Signature: Date:

100 Chapter 6: Investigation of installation robustness of geotextiles in relation to subgrade moisture condition

This Chapter is an exact copy submitted to the journal paper above.


This paper examines the influence of site characteristics, specifically subgrade

moisture content on the robustness of geotextiles during installation process. This

research study contributes to a better understanding of how subgrade moisture

condition (natural, field, and saturated) can affect the robustness of geotextile during

installation process. The study suggests that damage found on installed geotextile is

more severe in the case of natural subgrade than in the case of field and saturated

subgrade.


Objective 1

• Revised Drop Rock Test methodology to simulate saturated, field and dry subgrade

Objective 3

• Examined the influence of subgrade moisture content on geotextile's robustness during installation

• Assessed the effect of different subgrade moisture content on the retained strength of geotextiles

Objective 6

• Develop chart for geotextiles' robustness against subgrade moisture condition

101





dumping)






Develop chart for robustness of geotextile against construction stress using Drop and CBR Energy

Develop chart to predict the permittivity performance of geotextile upon installation


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Paper 1

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Abstract

Research in relation to installation robustness of geotextiles is important to study.

Previous studies have attempted to evaluate the puncture mechanism of geotextiles

during installations with a new testing method, Drop Rock Test (DRT) developed at

Geofabrics Australasia in 2012. The effects of the drop rock energy and the types of

geotextile on the installation damage of geotextile have been investigated. However,

little research has been carried out on the influence of site characteristics on the

robustness of geotextile during installation procedures. Moisture content, density and

soil type of the subgrade have significant effects on the installation damage of

geotextiles. This paper presents a modified DRT approach to evaluate the installation

damage on geotextile with different subgrade moisture conditions (natural, field and

saturated). Findings of this study conclude that damage on installed geotextile is more

severe in the natural subgrade case than in the saturated subgrade case.

KEYWORDS: Geosynthetics, Rock Drop Test, Subgrade Moisture content,

Installation Damage, CBR Puncture Resistant, Coastal Protection

6.1 Introduction

Since geotextiles have been used in coastal engineering applications, the question of

installation damage has been a key concern. Though installed geotextiles have served

successfully as filters/drainage layer and separators in numerous projects, there are

still cases where the material failed to serve its intended function due to damage

created during installation. Mechanical damage often occurs to a large extent during

installation in the construction process. Literature studies demonstrate that many

authors have attempted to establish the optimal approach to determine the robustness

of geotextile by investigating its mechanical and physical properties (Bräu, 1996;

Carneiro et al., 2013; Elvidge and Raymond, 1999; Heibaum, 2014; Rosete et al.,

103

2013). Robustness refers to geotextiles’ ability to survive (non-puncture) under impact

loads that are applied by dropping rocks onto geotextiles laid on subgrades. Watn and

Chew (2002) reviewed relevant index test methods (mass per unit area, thickness, wide

width tensile, burst resistance, abrasion, CBR plunger, rod plunger, cone drop,

modified cone drop and pendulum impact test) to evaluate geotextiles’ robustness and

examined which of these tests contributed towards the evaluation of damage

susceptibility. They asserted that to date, no laboratory testing method could provide

a comprehensive assessment of the damage susceptibility (Watn and Chew, 2002).

This is attributed to the inability of laboratory testings to fully comprise factors

influencing the damage mechanisms which includes subgrade characteristics,

construction equipment and procedures, and material properties.

The role of site characteristics in geotextiles damage susceptibility has not

received much attention as it is often difficult to characterize the variability in

complete detail. Both Lopes et al. (2001) and Abu-Farsakh et al. (2007) reported that

subgrade characteristics in terms of particles size and distribution, moisture content,

and density have an influence on the soil-geosynthetic interface behaviour. Chew et

al. (1999) further suggested that subgrade properties along with geosynthetic

parameters often play a critical and complex role during geotextiles’ installation in the

field.

Wong et al. (2000b) conducted large scale experiments using the Standard Drop

Test (SDT) method to investigate the robustness of geotextile filter during the

construction of coastal revetments. These authors concluded that density of subgrade

play a significant role in the occurrence of punctures and holes on the geotextile. They

observed that, for the same geotextile and at the same drop height, severe damage is

found in the case of dense sand subgrade than in the case of loose sand subgrade. Wong


et al. (2000b) highlighted the need to investigate the effect of subgrade on puncture

mechanism of geotextiles. In their study, Wong et al. (2000b) considered the type and

density of subgrade soil but have not taken into account the moisture content of

subgrade soil as a parameter. The question then becomes, under what conditions could

geotextiles experience the least form of damage, saturated, unsaturated or natural

subgrade conditions? To answer this question, it is necessary to study the installation

of geotextile under different moisture conditions of subgrade. Construction of coastal

revetments is commonly conducted on subgrade that could either be natural, semi-

dry/wet (field) condition or under water.

Figure 1 shows a revetment covered with armour units and they are placed above

the geotextile filter in a coastal zone. Heibaum (2016) reported that the impact energy

experienced by the geotextile installed under water is only about 15% compared with

the same geotextile that is installed with same drop height in the natural. However,

installation sites are often difficult, challenging and complex. Installations may not

occur in a fully dried or under water state; they could also be installed in partially

saturated state. Therefore, robustness against impact taking into account the subgrade

moisture condition is needed. To evaluate the effect of subgrade moisture condition on

the puncture mechanism of geotextiles, a controlled Drop Rock Test (DRT) regime

was conducted where subsoil conditions were altered. An evaluation of geotextile

puncture mechanism in relation to subgrade condition provides relevant information

for site conditions which can have varying levels of moisture depending on the tidal

effect and the climate.

105

Figure 1: Geotextile installation (Tessilbrenta, 2013)

This paper aims to assess the robustness of geotextiles against different subgrade

moisture condition. The robustness of four types of non-woven geotextiles were tested

with three different subgrade moisture conditions with the Drop Rock Test. The three

selected of subgrade moistures conditions are natural (1.4%-7%), partially saturated/

field (11%) and saturated (27.8%). After each test, geotextile is first visually observed

and then the specimens are cut out and subjected to CBR puncture test to quantify the

damage.

6.2 Materials and Experimental Description

The experimental set up is similar to the Drop Rock Test (DRT) developed by Cheah

et al. (2016) which simulates installation damage on geotextiles with the placement of

armour rocks with the additional variable of subgrade moisture condition. The test

apparatus consists of an impermeable barrier subgrade containment unit, gantry crane

and concrete blocks. For saturated conditions, the impermeable barrier subgrade

containment unit was constructed by placing a layer of Geosynthetic Clay Liner (GCL)

within the subgrade containment unit. Dimensions and construction of GCL are

illustrated in Figure 2. Upon completion of the GCL installation, sand is shovelled into

the subgrade containment unit. Once the unit is sand filled, the geotextile is laid above

the sand and its’ edges are clamped along the unit to create fixed edge boundary


conditions. This is similar to field installation conditions where a rock is dropped on

an area which is surrounded by already dropped rocks and this situation induces greater

stress in the geotextile. For partially saturated (field) and natural subgrade conditions,

the GCL is removed from the experimental setup.

(a) GCL dimensions

(b) GCL construction

(c) GCL prior to installation

(d) GCL with subgrade containment unit

Figure 2: Impermeable barrier subgrade containment unit

Each subgrade moisture condition (natural, field and saturated) is prepared differently.

For natural conditions, no additional moisture was added to the subgrade. The natural

subgrade had 1.4-7% of moisture content (Fig. 3). For field moisture condition, the

subgrade was permeated with a high pressure hose at flow rate, 50ml/s for duration of

60 minutes to achieve water content of approximately 11% (Fig.4). For every 7 drops,

subgrade was re-permeated with the same flow rate and duration of 10 minutes to

107

maintain constant water content between tests. For saturated moisture conditions,

subgrade was permeated from a high pressure hose at flow rate of 50ml/s, except water

was not drained out from the system (as the GCL is installed). Once test pit shows

signs of pooling (Fig. 5), the subgrade was considered fully saturated. Moisture content

was found to be approximately 28%.

Figure 3: Natural conditions

Figure 4: Field condition (Partially saturated)


Figure 5: Saturated condition

Compaction of subgrade depended on the different subgrade conditions. For natural

conditions, compaction was done manually with a hand tamping tool (Figure 6a). The

tool was released at 0.5m height 6 times, evenly spaced laterally, and the pattern is

repeated 5 times horizontally to form a 6 by 5 grid pattern. However, due to the target

zone created in the centre of the grid, caused by the falling concrete block, is repeatedly

emptied and refilled, this area is tamped again by 3 by 5 grid pattern to ensure

uniformity between tests. For field and saturated conditions, the subgrade was

hydraulically compacted (compacted by the weight of water) similar to what happens

in a coastal environment. However, as tests were conducted continuously, the centre

is repeatedly emptied and refilled; the hand tampered method was applied for

uniformity. Figure 6b shows the hand tampered compacted subgrade under saturated

condition.

109

(a)

(b)

Figure 6: Compacted subgrade under (a) natural and (b) saturated conditions

Figure 7: DRT apparatus(Cheah et al., 2015; Cheah et al., 2016)

Hand tamping tool


6.2.1 Drop Rock Test

6.2.1.1 Damage during installation (DDI)

A single concrete block (922kg) with a 90o tip is used in the test program to deliver

constant impact energy onto geotextiles for a specified drop height (Fig.7). Geotextiles

with dimensions of 1.8m by 2.0m stencilled with a grid of 50mm by 50mm (Fig. 8),

indicating where the concrete block is subjected to fall, is cut out using an electric

fabric cutter. The prepared sample is then laid directly above the prepared subgrade

(natural, field or saturated condition) and clamped securely around the perimeter using

a timber frame and G-clamps (Fig. 7).

Figure 8: Stencilled Geotextile Sample

The concrete block is electrically winched to the desired drop height whilst a trolley

on the crane rail is used to move the concrete block laterally above the target zone as

illustrated in Figure 9. Drop height is measured from the bottom tip of the concrete

block to the surface of the laid geotextile with a T-gauge. The concrete block is then

disengaged from the quick release mechanism once it is in position. Once DDI is

complete, G-clamps are removed and samples are labelled according to its test number,

geotextile grade, drop height and date of test.

111

Figure 9: Target zone

6.2.1.2 Measurement of damage on geotextiles

Visual inspection is made to determine the occurrence of any puncture on geotextile

sample. Impact survivability measures the survival rate of non-punctured samples.3

tests were tested for each geotextile material. Any puncture found on the geotextile is

considered failure and the size of damage (puncture area) is measured (Cheah et al.,

2016). Figure 10a illustrates the measurement of punctured samples. Damaged

geotextiles (non-punctured) are examined in terms of strain/elongation. Measurements

are determined against 6 squares (which initially were 300mm) from the point of

interest (Fig. 10b) at each phase. 300mm was chosen because it records the greatest

change in elongation. The change in length is measured and recorded.

(a)

(b)

Figure 10: (a) Punctured and (b) Damaged sample


6.2.2 Static Puncture (CBR) test

The retained mechanical strength of geotextiles was measured with California Bearing

Ratio (CBR) Puncture test in accordance to EN ISO 12236:2006. To carry out the field

damage test, DRT was used to simulate impact loading caused by falling armour units.

The evaluation of the damage induced on the geotextile was carried out with the

recovered samples from DRT.

6.2.3 Materials and test program

Testing of geotextile materials were completed according the test program in Table 1

and the study consists of four types of staple fibre non-woven geotextile (SF1, SF2,

SF3 and SF4) and a range of drop energy and the number of test carried out for each

geotextile. The subgrade, mechanical and physical properties of material, their units,

and test standards are tabulated in Table 2 and 3.

Table 1: Test program Description of subgrade and concrete block Types of

Geotextiles (Staple Fibre)

Drop height (m)

No. of tests per drop height

Poorly Graded Sand a

• 𝝆𝝆𝒅𝒅: 1486.6 kg/m3 • Gs: 2.53 • Cu=1.58 • Cc=1.02 • D85= 04.mm

Horizontal base Drop Mass:438kg


0.5,1.0,1.5,2.0

3

a 𝝆𝝆𝒅𝒅= Dry density; Gs= Specific Gravity; Cu= Soil coefficient of uniformity; Cc= coefficient of curvature; D85= diameter which 85% of the remaining particles are smaller than that diameter.

Table 2: Hydraulic Properties (typical values from Geofabrics Australasia specifications) Grade of Non-woven geotextile

Pore Size (Dry Sieve) (mm)

Permittivity, ψ (s-1)

Coefficient of permeability, k (m/s x 10-4)

Flow rate @100mm head, q (L/m2/s)

Apparent Opening Size (AOS)*, O95

(µm)

GTX-1 <75 1.43 45 143 182 GTX-2 <75 0.69 33 69 124 GTX-3 <75 0.46 24 46 126

113

GTX-4 <75 0.32 22 32 108 *Pore size (O95) – Capillary Flow Method (ASTM 6767)

Table 3: Index properties for staple fibre non-woven geotextile (typical values) (Kendall et al., 2014b)


6.3.1 Impact survivability of non-woven geotextiles

The results obtained from the analysis of impact survivability of non-woven

geotextiles under different subgrade moisture conditions are presented in Figures 11,

12 and 13. From the data, it can be seen that the impact survivability of geotextile for

the natural condition resulted in the highest rate of puncturing during installation,

whilst the impact survivability of geotextiles for saturated condition resulted in the

lowest occurrence of damage during installation. The most interesting aspect of this

set of data is Figure 12, where the correlation between drop energy and impact

survivability is shown. Theoretically, with the increase in drop energy levels, the

impact survivability rate of the material would be less. This is true in general, except

at higher drop energies of 13.6kJ and 18.07kJ. The result for SF1 and SF2 in Figure


Mechanical

Test Standard Units SF1 SF2 SF3 SF4 Wide Strip Tensile Elongation

MD

AS 3706.2 %

107 111 110 112

XD 84 83 81 83


MD

AS3706.2 kN/m

11 18 26 37 XMD 21 39 55 83


MD

AS3706.3 N

320 477 656 842 XMD 542 977 126

4 1774


MD AS2001.2.3 N

686 1161

1753

2469

XMD

1097

1948

2958

4539

CBR Burst Test AS3706.4 N 2719

4522

6526

8824

Physical Mass per unit area g/m2 380 611 846 1224


12 is somewhat counterintuitive. Both SF1 and SF2 punctured at the third highest drop

energy of 13.6kJ but managed to survive the highest drop energy, 18.0kJ.

This may be attributed to the incompressibility of the pore water in the subgrade

beneath the geotextile. At 18.07kJ, Figure 12 suggests that the pore water in the

subgrade takes up the applied force which reduces the damage impact on the

geotextile. According to compaction curve theory for soil (Das, 2015), as the soil state

approaches beyond the optimum moisture content, water occupies the volume where

soil could be present and density decreases. Since water is incompressible, it could

take up the applied force. Figure 14 illustrates the relationship between compaction

energy, water content and zero-air-voids (ZAV) line. The higher compactive effort

reduces the optimum moisture content and approaches closer to the ZAV line. Given

that the moisture content for field subgrade remains the same (e.g. 11%) for all tests,

it is hypothesised that moisture content could have exceeded the optimum moisture

content, resulting in incompressible excess pore water, thus the drop rock impact (drop

energy) is partially taken up by the pore water in the subgrade. This could result in the

greater survivability rate of geotextile at 18.07 kJ, as shown in Figure 12.

Figure 11: Survivability of geotextile for saturated condition

SF1SF2

SF3SF4

0.0

20.0

40.0

60.0

80.0

100.0

4.52 9.04 13.6 18.07

Geotextile

Impa

ct S

urvi

vabi

lity

rate

(%)

Drop Energy (kJ)

Saturated condition (w=27.8%)

SF1

SF2

SF3

SF4

115

Figure 12: Survivability of geotextile for field (partially saturated) subgrade

Figure 13: Survivability of geotextile for natural condition

Figure 14: Soil compaction curve (Bodó and Jones, 2013) *mo: moisture content origin

SF1SF2

SF3SF4

0

20

40

60

80

100

4.52 9.04 13.6 18.07

Geotextile

Impa

ct S

urvi

vabi

lity

rate

(%)

Drop Energy (kJ)

Field condition (w=11%)

SF1

SF2

SF3

SF4

SF1SF2

SF3SF4

0

20

40

60

80

100

4.52 9.04 13.6 18.07

Geotextile

Impa

ct S

urvi

vabi

lity

rate

(%)

Drop Energy (kJ)

Natural condition (w=1.4-7%)

SF1

SF2

SF3

SF4


6.3.2 CBR Puncture Strength

In reviewing the literature, no data was found on the association between subgrade

moisture condition and material’s strength loss during installation. Figure 15 presents

the strength loss of SF2, SF3 and SF4 non-punctured samples at the highest drop

energy 18.07kJ. These samples were further examined with CBR static puncture test.

Non-punctured (damaged) samples were recovered from the target zone (refer to

Figure 9) and non-damaged samples were retrieved from the edges of the same

material. Comparisons of CBR puncture strength of both initial and retained strength

were compared using the damaged and non-damaged samples of the same parent

material.

The results in Figure 15 are an average of 3 tests, but in some cases 2 tests as,

not all geotextile survived or punctured. For saturated subgrade condition, SF3 and

SF4 tests were not carried out at drop energy of 18.07kJ as SF2 survived 100% at that

drop energy, hence the assumption was made that SF3 and SF4 would fully survive

with the same drop energy. SF3 and SF4 have superior mechanical properties than

SF2; hence the assumption was made. For natural subgrade condition at the highest

drop energy (18.07kJ), SF2 and SF3 geotextiles did not survive. Hence, there is only

one result (SF4) presented.

117

Figure 15: Strength loss (CBR) of material for different subgrade condition subjected to highest drop energy, 18.07kJ

The most visible finding from the analysis is that there is a reduction in retained

strength after installation in any given subgrade moisture condition. Comparison

cannot be made across the three non-woven geotextiles for saturated subgrade

condition as drop rock tests were only carried out for SF2 at 18.07kJ. It can be seen

that SF2 has a slight strength loss, 1.5% when subgrade is fully saturated compared to

field subgrade condition, 29.1% reduction. The significant difference between the two

moisture condition of soil perhaps suggest that subgrade with less moisture content

could potentially bring more damage upon geotextile during installation.

It is apparent from this figure that when subgrade has moisture content of 11%,

all materials experienced strength loss (SF2-29.1%, SF3-12.3% and SF4-10.2%) The

greatest reduction in strength is observed in the weaker grade, SF2. These results

suggest materials with greater mechanical properties are more likely to be more robust

than lower grade materials. Results presented in Figure 13 revealed that only SF4

0

5

10

15

20

25

30

350

2

4

6

8

10

12

Initi

al (k

N)

Reta

ined

(kN

)

Stre

ngth

Los

s (%

)

Initi

al (k

N)

Reta

ined

(kN

)

Stre

ngth

Los

s (%

)

Initi

al (k

N)

Reta

ined

(kN

)

Stre

ngth

Los

s (%

)

Saturated (28%) Field (11%) Natural (1.4-7%)

Stre

ngth

Los

s (%

)

CBR

Punc

ture

Str

engt

h (k

N)

Subgrade moisture condition

SF2

SF3

SF4

SF2

SF3

SF4


survived at 18.07kJ under natural sand condition. SF4 showed a similar trend as other

materials with a decrease in retained strength after installation. Though there is only

one set of data for reference, one notable comparison can be made between field and

natural subgrade installation condition; that there is a greater strength reduction (15%)

at natural than at field subgrade condition (10%). Taken together, these results suggest

there is a strong association between subgrade conditions with puncture mechanism

and CBR puncture strength of non-woven geotextiles. It is postulated that the natural

subgrade condition is the harshest environment to install geotextile and will cause

substantial damage.

6.4 Limitations

Several limitations of the DRT approach need to be acknowledged. The intention of

DRT is to simulate the most severe form of damage; the addition of a secondary armour

layer in the DRT system was disregarded due to difficulty replicating. Secondary

armour layer is a common practice for the construction of coastal protection structures,

as it cushions the impact dumped onto the laid geotextile. The tests were carried out

on a horizontal plane, whilst on site; many geotextiles are not always installed in this

way.

6.5 Conclusions

This study set out to investigate the influence of subgrade moisture content on

the puncture mechanism of geotextile filter. These findings suggest that for the same

geotextile and same drop energy, the damage on the geotextile is more severe in the

case of natural subgrade than in the case of saturated subgrade. These data suggest that

moisture content of soil does play a significant role in the robustness of geotextile

during installation. Findings suggest that there is a correlation between pore water and

geotextiles’ robustness. As the compaction of soil approaches beyond the optimum

119

moisture content, resulting in incompressible excess pore water, the drop energy of the

released concrete block could have been partially taken up by the pore water in the

subgrade. This could result in the greater survivability (robustness) rate of geotextile

when higher drop energy (18.07kJ) was applied under field condition (11%). The

extent of damage (mechanical strength reduction) due to moisture content of soil was

further investigated with CBR static puncture test. This research provides a baseline

for the exploration of the influence of subgrade on the robustness of geotextile in

construction of coastal revetment. Findings highlight the use of DRT as an effective

installation assessment tool of geotextile filters for coastal revetment construction. It

is often challenging and difficult to derive meaningful information from samples

retrieved from site due to the lack of information during installation.

Acknowledgements

This study is part of the first author’s PhD research at Queensland University of

Technology (QUT) in Brisbane and therefore authors would like to acknowledge QUT

Australian Postgraduate Award (APA) providing the scholarship to the first author for

her PhD study. The authors express their appreciation to Geofabrics Australasia for

providing the geosynthetics materials and financial support in this study. Finally, the

authors wish to thank undergraduate project students who helped in these testings;

their work is gratefully acknowledged.

References

Abu-Farsakh, M., Coronel, J., Tao, M., 2007. Effect of Soil Moisture Content and Dry Density on Cohesive Soil–Geosynthetic Interactions Using Large Direct Shear Tests. Journal of Materials in Civil Engineering 19, 540-549.

Bodó, B., Jones, C., 2013. Introduction to Soil Mechanics. Wiley-Blackwell, GB. Bräu, G., 1996. Damage of Geosynthetics During Installation- Experience from Real

Sites and Research Works, in: De Groot, M.B., Den Hoedt, G., Termaat, R.J. (Eds.), Proceedings of the First European Geosythetics Conference- Eurogeo 1. Balkema, Maastricht, Netherlands, pp. 145-150.





Das, B.M., 2015. Principles of foundation engineering: SI edition. Cengage Learning, Boston.

Elvidge, C.B., Raymond, G.P., 1999. Laboratory survivability of nonwoven geotextiles on open-graded crushed aggregate. Geosynthetics International 6, 93-117.

European Standard Online, 2006. EN ISO 12236 Geosynthetics-Static Puncture Test (CBR Puncture Test).




Lopes, P.C., Lopes, M.L., Lopes, M.P., 2001. Shear Behaviour of Geosynthetics in the Inclined Plane Test – Influence of Soil Particle Size and Geosynthetic Structure, Geosynthetics International, pp. 327-342.


Tessilbrenta, 2013. Coastal Erosion Prevention. Tessilbrenta S.r.l. Watn, A., Chew, S., 2002. Geosynthetic damage-from laboratory to field,

Geosynthetics: State of the Art- Recent Developments. Proceedings of the Seventh International Conference of Geosynthetics,7 ICG held 22-27 September 2002, Nice, France. Volume 4.

Wong, W.K., Chew, S.H., Karunaratne, G.P., Tan, S.A., Yee, K.Y., 2000. Evaluating the puncture survivability of geotextiles in construction of coastal revetments, Sessions of Geo-Denver 2000 - Advances in Transportation and Geoenvironmental Systems Using Geosynthetics, GSP 103, August 5, 2000 - August 8, 2000. American Society of Civil Engineers, Denver, CO, United states, pp. 186-200.

121

Chapter 7: Discussion

7.1 Introduction

The overall aim of the research has been achieved through the ongoing development

of an experimental method to replicate construction stress called the Drop Rock Test

(DRT). The influence of construction stresses (rock dumping) on geotextiles’

robustness, mechanical and filtration properties were investigated with the newly

developed DRT methodology. The influence of subgrade moisture condition on

geotextile’s robustness was also achieved using DRT.

The following research objectives have been attained:

1. Developed new experimental method to replicate construction stress (rock

dumping) on geotextiles for coastal application

2. Examined the influence of construction stress on geotextiles properties

(robustness, mechanical strength, physical and filtration)

3. Examined the influence of subgrade characteristics (i.e. moisture condition) on

geotextiles’ robustness during installation

4. Developed design chart to predict robustness of geotextile during installation

5. Developed design chart to predict the hydraulic/ filtration performance

(permittivity) after installation

6. Developed chart for robustness of geotextile against subgrade moisture

condition

122 Chapter 7: Discussion

7.2 Major Outcomes

7.2.1 Drop Rock Test Methodology (Objectives 1 and 2)

Conducting a field drop test to evaluate the robustness of geotextile can be a complex

task. Based on the various parameters described in Chapter 2.5.2, the results gathered

can be significantly different and conflicting. While index tests may produce consistent

and reproducible results, these values do not necessarily correspond with field

performance of geotextile. Findings from Chapter 4 reveal that the lighter and lower

CBR burst strength staple fibre non-woven geotextile (380g/m2, 2719N) outperforms

the heavier and stronger continuous filament non-woven geotextile (740g/m2, 4800N)

in its resistance to construction stress without puncturing. This implies that index

derived values are limited in their ability to quantify the robustness (survivability) of

the material. It is therefore desirable to develop a drop test replicating the way which

construction takes place and at the same time yield results that are repeatable and

reproducible.

The experimental setup of Drop Rock Test (DRT) is designed to replicate the

construction stress (single rock dumping) where geotextiles must survive the impact

of a falling armour using a custom made concrete cube with a 90o tip that imparts the

highest mechanical stress onto geotextile sample placed on a test soil (natural,

unsaturated/field, saturated) at determined drop energy. There were three concrete

cubes constructed to be chosen corresponding to the stone size used for coastal

protection structure, weighing 93, 438 and 922kg. Amongst the three concrete cubes,

the largest cube has sides measuring 750mm and weigh approximately 1000kg. Any

visible perforations (holes) and tensile strain (elongation) measured indicating

reduction of mechanical strength and filtration functions are regarded as damage. The

experimental investigation in Chapter 4 illustrates the use of DRT to represent field

123

conditions and controlling a wide a range of experimental variables (mass and

angularity of concrete, subgrade characteristics and drop height). Induced damage on

geotextiles from DRT is examined with the Static Puncture test and data recorded is

used to develop a geotextile selection design chart for coastal protection applications.

Several limitations of the DRT approach need to be acknowledged. First, the test

is carried out on a horizontal plane, whilst on-site; geotextiles could be installed on

various inclinations. Second, results gathered with a single mass rock, whereas bulk

loads of armour rocks are often dropped on geotextile during installation. Third, this

study was limited by the absence of a secondary armour layer which are applied on

geotextiles to cushion the impact dumped on the geotextiles.

7.2.2 Geotextiles’ robustness against construction stress (rock dumping) (Objective 4)

The important outcome of Chapter 4 is the development of preliminary design chart of

geotextiles’ robustness against construction stress. This chart aids engineers and

designers in selecting the appropriate product and will minimise the risk of puncture

in their application. Figure 7-1 presents the relationship between survivability

(puncture/no puncture) of staple fibre non-woven geotextiles, drop energy and CBR

deformation energy. Survivability (robustness) of geotextile refers to the number of

samples that survived (no puncture) for a series of drop rock tests. The term drop

energy is derived from potential energy,

𝐸𝐸𝑑𝑑𝑑𝑑𝑔𝑔𝑑𝑑 = 𝑚𝑚𝑚𝑚ℎ

Where m is the mass of the concrete block (armour unit), g is the gravitational

acceleration and h represents the installation height. Drop energy conveniently

summarizes the relevant installation specifications such as rock size and drop height.

Thus, drop energy is selected to represent the results. CBR energy refers to the area


under the CBR strength curve up to the point of peak load. The value reflects the plastic

deformation energy engaged by the CBR plunger as it extends into the secured

geotextile specimen.

Figure 7-2 is the preliminary chart for robustness against construction stress for

staple fibre non-woven geotextiles that is derived from Figure 7-1. The three zones

from the chart represents: (i) critical zone where puncture is highly likely, (ii)

intermediate zone where punctures is somewhat unlikely and (iii) subcritical zone

where punctures are very unlikely. The size and occurrence of punctures should be

less in the intermediate zone than in the critical zone. Results found that the mean size

of puncture is 22% lower than in the intermediate zone when compared to the mean

size of puncture in the critical zone. Note that Figure 7-2 may be limited by the small

sample size; further work needs to be done to provide more definitive relationships.

The results may also vary depending on angularity of concrete block (armour unit),

geotextile and subgrade characteristic.

Figure 7-1: Survivability of staple fibre non-woven geotextiles (Kendall et al., 2014a)

0

5

10

15

20

25

30

0 50 100 150 200 250 300

Drop

Ene

rgy

(kJ)

CBR Energy Level (J)

No puncture Puncture Subcritical Critical

125

Figure 7-2: Robustness chart for staple fibre non-woven geotextile

7.2.3 Damage assessment (permittivity) upon installation (Objectives 2 and 5)

Figure 7-3 presents the damage assessment (permittivity) at varying drop energies.

The results were extracted from Chapter 5. The x-axis is represented by Drop Energy,

EDrop, which represents the potential energy of the concrete block imparted on

geotextiles, in kilo-joules. The y-axis shows the permittivity(s-1) after induced damage

from DRT. Figure 7-3 clearly shows that the amount of damage on all four staple fibre

geotextiles (described in Table 1 in Chapter 4.2.2) is proportional to the drop energy

of the concrete block imparted. This is not surprising as higher drop energy imparts

more energy onto geotextile upon impact, resulting in greater amount of damage on

the geotextile. The important outcome of this investigation is the development of

design guidance for engineers which gauges the amount of damage upon installation.

0

5

10

15

20

25

30

0 50 100 150 200 250 300

Drop

Ene

rgy

(kJ)

CBR Energy Level (J)

Subcritical

Critical


Figure 7-3: Design chart to predict permittivity for staple fibre non-woven geotextiles

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

-1.00 1.00 3.00 5.00 7.00 9.00

Perm

ittiv

ity (s

-1)

Drop Energy (kJ)

GTX-1(380g/m2)GTX-2(611g/m2)GTX-3(846g/m2)GTX-4(1224g/m2)Linear (GTX-1(380g/m2))Linear (GTX-2(611g/m2))Linear (GTX-3(846g/m2))Linear (GTX-4(1224g/m2))

127

7.2.4 Geotextiles’ robustness against subgrade moisture condition (Objective 3 and 6)

The test configuration in Chapter 6 is identical except for the moisture content of the

subgrade on which geotextile was tested. Comparing the results of saturated,

unsaturated (field), and natural subgrades will reveal the influence of moisture content

of subgrade on the robustness of geotextiles. Figure 7-4, 7-5 and 7-6 shows the impact

survivability rate (no puncture, %) against Drop Energy, EDrop with a constant dry

density of 1486.6 kg/m3. The damage on geotextile is more severe when the test is

conducted on natural subgrade. This observation is true for all four staple fibre non-

woven geotextiles tested. It was also found that geotextile tested on field (unsaturated)

subgrade, the impact survivability rate was not directly proportional to the drop energy

of the concrete block imparted (Figure 7-5). This may be attributed to the

incompressibility of the pore water in the subgrade. At 18.07kJ, the pore water in the

subgrade absorbs the applied force which reduced the drop energy of the concrete

block imparted. According to the soil compaction theory (Das, 2015), when soil

moves beyond optimum moisture content (OMC), water occupies the volume where

soil could be present. Since water is incompressible, it could absorb the drop energy

applied. Since water content of field subgrade remains constant at 11% for all tests,

the increase in drop energy applied means the soil is compacted beyond optimum

moisture content (OMC) allowing water to absorb some of the drop energy of the

concrete block, resulting in higher survivability rate as shown in Figure 7-5.


Figure 7-4: Survivability chart for staple fibre non-woven geotextiles - Natural subgrade

Figure 7-5: Survivability chart for staple non-woven geotextiles- Field subgrade

Figure 7-6: Survivability chart for staple fibre non-woven geotextile - Saturated subgrade

SF1

SF30

50

100

4.52 9.04 13.6 18.07

Geotextile

Impa

ct S

urvi

vabi

lity

rate

(%)

Drop Energy (kJ)

Natural condition (w=1.4-7%, Sr=5-25%)

SF1

SF2

SF3

SF4

SF1

SF30

50

100

4.52 9.04 13.6 18.07

Geotextile

Impa

ct S

urvi

vabi

lity

rate

(%)

Drop Energy (kJ)

Field condition (w=11%, Sr= 40%)

SF1

SF2

SF3

SF4

SF1

SF30.0

50.0

100.0

4.52 9.04 13.6 18.07

Geotextile

Impa

ct S

urvi

vabi

lity

rate

(%)

Drop Energy (kJ)

Saturated condition (w=27.8%, Sr=100%)

SF1

SF2

SF3

SF4

129

7.2.5 Research Impact for other forms of geosynthetics

The methodology to simulate construction stress for coastal applications can be

extended for other geotextile elements such as geotextile bags, geotextile containers,

geotextile mattress as well geotextile tubes. One disadvantage of designing coastal

protection structure with theses geotextile elements is that they are susceptible to

damage during installation. These damages often occur due to poor installation

practice such as rock dumping upon geotextile. These geotextile elements can be used

in combination with armour rocks. The functional requirements of these geotextile

elements can be examined with the Drop Rock Test (DRT).

Non-woven geotextiles are often used as a drainage and protection layer for

geomembranes or geotextile elements (bag/containers/mattress/tube) against sharp

objects like rip raps or gravel in either the soil/rock layer above or below. Non-woven

geotextiles are generally manufactured with Polyester (PET) and Polypropylene (PP).

Up to this date, little focus is given on the different combination percentage of PET

and PP against puncture resistance. The DRT could also be used as a tool to optimise

manufacturing process of non-woven geotextile. Using different composition of

material (PET and PP) and DRT results, manufacturers can optimise both cost and

quality.

130 Chapter 8: Conclusion

Chapter 8: Conclusion

8.1 Conclusions

Past research points out that the heavy construction stresses imparted onto geotextile

material in coastal protection applications usually exceeds the service stresses. This

suggests that the critical period for geotextiles is during the installation/ construction

process. Therefore, it is essential to assess and estimate any change in mechanical and

filtration properties of geotextiles upon installation where construction stress

(dropping rocks) is involved. The damages inflicted on geotextiles during construction

and failure to consider the extent of damage upon installation in the design process can

hinder the long-term performance of geotextiles. Typically, these issues are related to

the inadequate method of selecting geotextiles and the lack of experimental methods

to replicate installation conditions in repeatable and controlled manner. To select the

appropriate geotextiles, more scientifically robust methods of assessing geotextiles

robustness and their mechanical and filtration properties upon installation is essential.

The focus of this research project was to develop a methodology to replicate the

installation condition and to assess the robustness of geotextiles under coastal

protection application. Both construction stress and subgrade characteristics factors

were integrated to examine the robustness of geotextile upon installation. The

developed method, Drop Rock Test was used to induce similar on-site construction

stresses on geotextiles to assess the influence of rock dumping on the retained strength

and filtration properties of geotextiles. The research objectives have been achieved as

demonstrated below:

1. Developed new experimental method to replicate construction stress (rock

dumping) on geotextiles for coastal application- The Drop Rock Test was

131

designed to replicate the construction stress on geotextiles during the installation

process. The rock armour layer that is often laid onto the geotextiles during the

construction process is replicated with the release of a concrete block constructed

with a 90o tip facing downward from a specified height. The selection of the

sizes and weight of the three concrete blocks (93, 438, 922kg) represents the

lower and upper range of armour rocks used for a coastal application. The

concrete block can be released from a selected drop height between 0.5m to

2.0m. The selected range of drop heights represents the drop heights that are

commonly adopted for mid-range excavators.

2. Examined the influence of construction stress on geotextiles’ properties

(robustness, mechanical strength, physical and filtration)- The

experimental investigation in this research illustrated the use of DRT to

assess the robustness, retained strength, elongation, permittivity and pore

size of geotextiles. Findings in Chapter 4 suggest the potential for damage

to the geotextiles increase with higher drop energy, as the greater the drop

energy, the greater the mechanical stress is induced onto the laid material.

The reduction in retained strength for staple fibre (SF) and continuous

filament (CF) geotextile could extend up to 30% and 50%, respectively. The

hydraulic performance of geotextiles upon installation reflects the drop

energy applied; the greater the drop energy, the increase in permittivity and

pore size of geotextile is greater. The results in Chapter 5 suggest that the

relationship between elongation of geotextiles and drop energy may not be

directly proportional. Findings suggest that geotextiles are likely to

experience strain to a certain level and once the micro-structure approaches


breaking point, the material ceased to elongate further and is on the verge of

puncturing.

3. Examined the influence of subgrade characteristics (i.e. moisture

condition) on geotextiles’ robustness during installation- Chapter 6

investigated the robustness of geotextiles against different subgrade

moisture conditions. The findings suggest that for the same geotextile and

same drop energy, the damage on geotextiles is more severe in the case of

natural subgrade than in the case of saturated subgrade. Chapter 6 provided

a baseline for the exploration of the influence of subgrade moisture

condition on the robustness of geotextile in construction of coastal

revetment. It is also hypothesised that as the compaction of soil approaches

beyond the optimum moisture content, resulting in incompressible excess

pore water, the drop energy of the released concrete block could have been

partially taken up by the pore water in the subgrade. This could result in the

greater survivability (robustness) rate of geotextile when higher drop energy

(18.07kJ) was applied under field conditions.

4. Developed design chart to predict robustness of geotextile during

installation- The robustness chart developed in Chapter 7.2.2_Figure 7-2

allows engineers and designers to select the appropriate staple fibre non-

woven geotextile to minimise the risk of damage during installation process

using drop energy (function of drop height and weight of armour stone) and

CBR energy of geotextiles. This presents a cost effective method as it would

not require large scale field trials, which are often costly and time-

consuming.

133

5. Developed design chart to predict the hydraulic/ filtration performance

(permittivity) after installation- The design chart in Chapter 7.2.3_Figure

7-3 allows engineers and designers to predict the permittivity of geotextiles

upon installation by applying the drop energy of the release armour stone

and the undamaged permittivity value of staple fibre non-woven geotextile.

The amount of damage on the geotextiles in terms of filtration performance

can be easily gauged prior to construction. This provides an economical and

effective approach as the preliminary design of coastal revetments is

optimised.

6. Developed chart for robustness of geotextile against subgrade moisture

condition- The charts in Chapter 7.2.4_Figure2-4 showed the evaluation of

geotextiles’ puncture mechanism in relation to subgrade condition. This

provides relevant information for engineers and designers for different site

conditions which have varying levels of moisture depending on tidal effect

and the climate. Findings suggest that the risk of damage will be minimised

when construction process take places when the subgrade is saturated.

8.2 Recommendation for future studies

The following recommendations are suggested to enhance the development of

knowledge and understanding to quantify installation damage on geotextiles’

properties for coastal protection work:

• Investigate the influence of construction stress on geotextile with different

subgrade material.

• Investigate filtration properties of geotextile of induced elongation via Drop

Rock Test versus elongation formed with Wide Strip Tensile Test.


• Develop predictive numerical model to simulate damage (puncture/elongation

on geotextile) at any given drop energy by considering weight and angularity

of armour unit, subgrade and geotextile characteristics and validate the

numerical results with the results collected from the visual assessments from

the Drop Rock Test.

• Investigate puncture resistance for different geotextile elements such as

geotextile bags, containers, mattress and tube.

• Investigate the different composition of PET and PP against puncture

resistance.

Bibliography 135

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DEVELOPMENT OF METHODOLOGY TO QUANTIFY INSTALLATION DAMAGE ... Yi Ting_Cheah_Thesis.pdf · Development of methodology to quantify installation ... values are derived from mechanical