ISO/TR 18228-5:2025

Design using geosynthetics - Part 5: Stabilization

This document provides a summary of general guidance for the design of geosynthetics to fulfil the function of stabilization of granular layers in contact with natural soils, fills, asphalt or other materials. The concepts of the summarised guidance are based on installed materials, the installation process and on either the strength or deformation behaviour, or both, of geosynthetics. This document provides general considerations to support the design of unbound layers of paved and unpaved roads, working platforms and foundations utilizing the stabilization function of geosynthetics. This is typically for the serviceability limit state (SLS).

Conception utilisant des géosynthétiques — Partie 5: Stabilisation

General Information

• Status: Published
• Publication Date: 09-Jan-2025
• ICS: 59.080.70 - Geotextiles
• Technical Committee: ISO/TC 221 - Geosynthetics
• Drafting Committee: ISO/TC 221/WG 6 - Design using geosynthetics
• Current Stage: 6060 - International Standard published
• Start Date: 10-Jan-2025
• Completion Date: 10-Jan-2025

Overview

ISO/TR 18228-5:2025 - Design using geosynthetics - Part 5: Stabilization is a Technical Report that summarizes guidance for using geosynthetics to stabilize granular (aggregate) layers in contact with natural soils, fills, asphalt or other construction materials. Published in 2025, it is part of the ISO 18228 series and focuses primarily on the stabilization function-typical applications target the serviceability limit state (SLS). The report explains the underlying mechanisms (confinement and tensioned-membrane action), material and installation considerations, and established design approaches used worldwide.

Key Topics and Technical Requirements

• Stabilization mechanisms: Clear distinction and explanation of the two main mechanisms - internal/external confinement (lateral particle restraint) and tensioned membrane (out-of-plane reinforcement) - and when each is applicable.
• Design methods: Summarized methodologies for common applications, including documented approaches for:
  • Unpaved roads (e.g., Giroud‑Han, Leng‑Gabr; geocell-specific methods such as Pokharel and Emersleben)
  • Paved roads (modified AASHTO, MEPDG approaches)
  • Working and load transfer platforms (static and BCR methods)
  • Railways (Network Rail specifications, Ev2, ballast stabilization, geocell designs)
• Materials and properties: Guidance on aggregate characteristics and key geosynthetic properties (geotextiles, geogrids, geocells) relevant to confinement and reinforcement performance.
• Testing and validation: Overview of testing needs, including laboratory tests and accelerated pavement tests (APT), and tests specific to geotextiles, geogrids and geocells.
• Performance drivers: Considerations for loading (cyclic vs static), installation quality, ground and groundwater conditions, climate effects, durability and sustainability.
• Design layout: Practical advice on geosynthetic placement and system configuration to achieve stabilization objectives.

Practical Applications and Who Uses This Standard

This report is practical for professionals involved in ground improvement and pavement engineering:

• Geotechnical and pavement engineers specifying or designing stabilized unpaved/paved roads
• Rail infrastructure designers addressing ballast and sub-ballast stabilization
• Contractors, site engineers and QA teams implementing geosynthetic solutions on working platforms and temporary/access roads
• Manufacturers and suppliers of geotextiles, geogrids and geocells for performance specification
• Asset owners and consultants evaluating life‑cycle impacts and serviceability improvements

Benefits include reduced rutting and settlement, extended design life of aggregate layers, and optimized material use through validated design approaches.

Related Standards

• ISO 18228 series (general guidance and other function-specific parts)
• ISO 10318-1: Geosynthetics - Terms and definitions (referenced for terminology)

For full normative content, design examples and implementation details, obtain ISO/TR 18228-5:2025 from ISO or your national standards body. Keywords: ISO/TR 18228-5, geosynthetics stabilization, geotextiles, geogrids, geocells, pavement design, working platforms, serviceability limit state.

Standards Content (Sample)

Technical
Report
ISO/TR 18228-5
First edition
Design using geosynthetics —
2025-01
Part 5:
Stabilization
Conception utilisant des géosynthétiques —
Partie 5: Stabilisation
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
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Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Concepts and fundamental principles . 1
4.1 General .1
4.2 Benefits .3
4.3 Confinement and particle restraint .4
4.3.1 General .4
4.3.2 Internal confinement — Description of mechanism .4
4.3.3 External confinement — Description of mechanism .6
4.4 Vehicular action .9
4.5 Loading conditions (cyclic and static) .9
4.6 Multi component systems .10
4.7 Static loads — Settlement control and limitation .10
4.8 Reduction in deformation from trafficking .10
4.9 Reduction in unbound aggregate degradation .10
4.10 Extension of design life .11
4.11 Ground and groundwater conditions .11
4.12 Climatic conditions .11
4.13 Sustainability . 12
4.14 Impact of filtration, separation and drainage requirements . 12
5 Typical applications .12
5.1 Stabilized granular (aggregate) layers in trafficked areas . 12
5.2 Stabilized granular (aggregate) layers in railways . 13
5.3 Stabilized granular (aggregate) layers in working platforms .14
6 Design methods .15
6.1 General . 15
6.2 Unpaved roads . 15
6.2.1 Giroud-Han (2004) method . 15
6.2.2 Leng-Gabr (2006) method .17
6.2.3 Pokharel (2010) method for geocells .18
6.2.4 Emersleben (2009) method for geocells .19
6.3 Paved roads . 22
6.3.1 Modified American Association of State Highway and Transportation Officials
(AASHTO) (1993) method . 22
6.3.2 MEPDG methods .24
6.4 Working platforms and load transfer platforms . 25
6.4.1 General . 25
6.4.2 Static method for clay subgrade . 25
6.4.3 BCR method for soft sand subgrade .27
6.5 Railways . 28
6.5.1 Network Rail (UK) design specifications . 28
6.5.2 Ev2 method . 29
6.5.3 Railway ballast stabilization .31
6.5.4 Design method for railway stabilization with geocells .32
6.6 Design of the geosynthetics layout . 33
7 Materials .36
7.1 Properties of aggregate . 36
7.2 Key properties for geosynthetics – Internal confinement . 36
7.3 Key properties for geosynthetics – External confinement . 36

iii
8 Testing.37
8.1 General .37
8.2 Accelerated pavement tests (APT) .37
8.3 Tests for geotextiles and geogrids . 38
8.4 Tests for geocells . . . 38
Annex A (informative) Tensioned membrane design .40
Bibliography .43

iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 221, Geosynthetics.
A list of all parts in the ISO/TR 18228 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
The ISO 18228 series provides guidance for designs using geosynthetics for soils and below ground
structures in contact with natural soils, fills and asphalt. The series contains 10 parts which cover designs
using geosynthetics, including guidance for characterization of the materials to be used and other factors
affecting the design and performance of the systems which are particular to each part, with ISO/TR 18228-1
providing general guidance relevant to the subsequent parts of the series.
The series is generally written in a limit state format and guidelines are provided in terms of partial material
factors and load factors for various applications and design lives, where appropriate.
Ultimate limit state (ULS) design is necessary for some applications, e.g. slab foundation design, working
platform design etc., but usually must be proven separately. This document is a state of practice report and
information is provided in terms of the application of the mechanisms and design methods. A discussion on
separation, filtration and other relevant engineering issues addressed with geosynthetics are addressed in
the separate parts of ISO 18228.
This document includes information relating to the stabilization function. Details regarding design
methodologies adopted in a number of regions are provided.

vi
Technical Report ISO/TR 18228-5:2025(en)
Design using geosynthetics —
Part 5:
Stabilization
1 Scope
This document provides a summary of general guidance for the design of geosynthetics to fulfil the function
of stabilization of granular layers in contact with natural soils, fills, asphalt or other materials.
The concepts of the summarised guidance are based on installed materials, the installation process and on
either the strength or deformation behaviour, or both, of geosynthetics.
This document provides general considerations to support the design of unbound layers of paved and
unpaved roads, working platforms and foundations utilizing the stabilization function of geosynthetics.
This is typically for the serviceability limit state (SLS).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 10318-1, Geosynthetics — Part 1: Terms and definitions
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 10318-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Concepts and fundamental principles
4.1 General
There are two primary mechanisms by which geosynthetics can improve the performance of a granular
layer, the confinement mechanism and the tensioned membrane mechanism. The distinction between these
two mechanisms and their relevant applications must be understood.
The first mechanism provides stabilization by way of particle confinement, or lateral restraint. By
minimizing the movement of aggregate particles, confinement increases the shear resistance and widens the
load distribution angle, improving the mechanical properties of the granular (i.e. aggregate) layer, thereby
controlling deformation under load (i.e. SLS).
The second mechanism provides reinforcement by way of friction, or interlock, or deforming, or a
combination of these three, out of the plane under load. In this case, the geosynthetic material is anchored on

each side of the loaded area to create a tensioned membrane. In doing so, it provides support to the granular
(i.e. aggregate) layer, thus decreasing deformations (i.e. SLS) and increasing bearing capacity (i.e. ULS).
Figure 1 illustrates the difference between the two mechanisms.
a) Lateral restraint
b) Tensioned membrane
Key
1 wheel load creates stresses which, unchecked, cause 6 wheel path rut
strains as shown
2 stabilized composite layer of aggregate and geosynthetic 7 geosynthetic
3 geosynthetic can be within or beneath the layer of 8 membrane tension in geosynthetic
aggregate to provide lateral restraint, inhibit strains, and
thereby support the vertical load
4 subgrade 9 vertical support component of membrane
5 wheel load
NOTE Figure 1 shows the primary mechanisms by which geosynthetics can improve the performance of a
granular layer.
Figure 1 — Mechanisms to improve the performance of a granular layer
In stabilization (i.e. confinement), the geosynthetic operates most effectively at relatively low levels of strain.
Stabilization is less influential in designs where high levels of strain are anticipated. Where high levels of
strain are anticipated, the tension membrane effect (reinforcement) is dominant.
There is considerable discussion in the literature about the relative magnitude of the strain within the
geosynthetic in the stabilization and reinforcement mechanisms. The boundary between the operational
strain envelopes of these two mechanisms and the nature of any transition between them has not been
adequately defined by any research to date. This is partly because it is extremely difficult to measure the
level of strain of a buried geosynthetic. This means that, a universally recognised design methodology based
on this parameter is not yet available.
The design life of the project is also suggested as a key consideration and, as a result, the rate of deformation.
Designing for geosynthetic stabilization results in the successful control of the rate and level of system
deformation to that which is tolerable within the design life of the project.
The tensioned membrane mechanism can require large deformations to mobilize the tensile strength of
the geosynthetic for it to be effective. Due to the importance of the tensile strength, the geosynthetic in
a tensioned membrane mechanism is considered to be performing a reinforcement function. This is not
the only mechanism through which geosynthetics perform the reinforcement function and the reader
is referred to ISO/TR 18228-7 where other situations are described. However, because the tensioned
membrane mechanism can be used in the application of unpaved roads, for ease of reference, a discussion of
the tensioned membrane reinforcement mechanism is provided in Annex A of this document.
4.2 Benefits
Geosynthetics are utilized to facilitate construction and improve the performance of unbound aggregate
layers over subgrades of varying strength. The benefits of geosynthetics have been well documented in
numerous case histories. These cover the range of full-scale laboratory experiments to instrumented field
studies. Many of these are highlighted in the Bibliography. In these cases, the geosynthetic and aggregate
together form a stabilized layer.
Further, stabilization of the unbound aggregate leads to an enhancement in both the surface resilient
modulus of unbound layers or subgrade and bearing capacity of the stabilized layer. The composite structure
of aggregate fill, geosynthetic and subgrade must:
a) effectively withstand service-loading pressures;
b) control subgrade and unbound aggregate layer deformation within a range suited to the in-service
requirements;
c) not progressively deteriorate over time through either aggregate deformation, breakdown or
contamination.
The corresponding functions of separation and filtration can also contribute to an improvement in
performance where site conditions require them to be provided.
4.3 Confinement and particle restraint
4.3.1 General
Stabilization by geosynthetics necessitates the minimization of particle movement through confinement.
Minimization of particle movement is achieved by particle restraint. For geosynthetics to provide particle
restraint, they must have adequate tensile stiffness and sufficient interaction with the soil.
Confinement is the dominant stabilization mechanism at low levels of strain (typically less than 1 %,
but possibly up to 2 %) of the geosynthetic within or beneath the aggregate layer, depending also on the
importance of the stabilized system (e.g. highway versus haul road) and the position of the geosynthetic
within the stabilized system (usually at lower levels higher strains are acceptable). If strain values are
expected to be above this, the designer can consider whether other mechanisms need to be included. For
the purposes of this document, two types of confinement have been considered and named internal and
external.
4.3.2 Internal confinement — Description of mechanism
Internal confinement is the intimate interaction of a two-dimensional geosynthetic with aggregate in or
underneath a compacted granular layer thereby creating a pseudo-composite material of improved shear
strength and stiffness. The interaction can occur via interlock, surface friction or both. For interlock to be
effective, the geosynthetic must have apertures (e.g. a geogrid) into which granular particles can penetrate.
While vertically loaded, additional shear stress is transmitted from the aggregate to the geosynthetic which
in turn results in deformation (strain) in the geosynthetic. The shear resistance caused by friction and
mechanical interlock generates a lateral restraint of the aggregate particles. The stiffness provided by the
geosynthetic reduces development of lateral deformation in the base aggregate over a defined height above
the geosynthetic (the stabilized or "confined zone") by preventing the development of explicit displacements
of the aggregate.
The confined zone has a limited thickness. Above it, a transition zone is developed which extends until there
is no influence on the granular (i.e. aggregate) layer from the geosynthetic (i.e. unconfined zone). Figure 2
illustrates the various zones.
The efficiency of confinement and thickness of the confined and transition zones varies with different
geosynthetic and soil types. The details therefore are usually defined for each type of geosynthetic and
soil individually. From Figure 2 it is evident that, when a relatively high aggregate thickness is required,
designing with multiple layers of geosynthetics allows a reduction in or elimination of the unconfined zone,
thus affording a more effective stabilization.
The magnitude by which the horizontal and vertical strain in the aggregate layer can be reduced depends on
the stiffness of the composite layer. This is, in turn, a function of the geosynthetic tensile stiffness required
for the stress equilibrium (especially at low strain levels) as well as on the efficiency of the aggregate and
geosynthetic interaction.
During the application of load to the granular (i.e. aggregate) layer (e.g. trafficking or compaction), the
interaction discussed above distributes stress throughout the stabilized granular (aggregate) layer and
geosynthetic, thus reducing any stresses transmitted to the underlying subgrade. The limitation of movement
under load provided in this way via a geosynthetic is referred to as the provision of lateral restraint.
The creation of the confined zone with limited particle movement naturally limits the deformation of the
granular (i.e. aggregate) layer as a whole. The resultant reduced stress transmission to the subgrade limits
its deformation. It is typically the underlying subgrade that is the weakest material in the construction
section and one of the principle aims in developing a confined and stabilized granular (i.e. aggregate) layer is
to limit any stress and strain transmission to this weaker layer.

a) Interaction by direct shear only
b) Interaction by direct shear, interlocking or both
c) Interaction with geocell
Key
σ vertical stress applied at the top surface
v0
σh horizontal stress
σv vertical stress at the subgrade interface
τds direct shear stress
σi interlocking confining stress
GTX geotextile
GGR geogrid
GCE geocells
1 unconfined zone
2 transition zone
3 confined zone
Figure 2 — Interaction of granular (aggregate) material with geosynthetics
Reducing the horizontal strain leads to a decrease in the Poisson ratio of the soil and geosynthetic composite
material compared to the Poisson ratio of the unstabilized soil. Reducing the Poisson ratio increases the
horizontal stiffness which means that the geosynthetic stabilized soil layer is able to distribute the vertical
stresses σ applied at the top surface on a wider area, as shown in Figure 3.
v0
In terms of the well-known concept of load distribution angle, for the unstabilized soil layer the vertical
stress on the subgrade σ will have a load distribution angle α [Figure 3a)]; while for the two dimensional
vu u
and three-dimensional geosynthetic stabilized soil layer, the vertical stress on the subgrade σ will have a
vs
much wider and more uniform distribution according to the increased load distribution angle α [Figures 3b)
s
and 3c)].
On the other hand, it is evident that at equal maximum value of σ and σ , the load that the stabilized soil
vu vs
layer can support will be much higher than the load supported by the unstabilized soil layer. In other words,
the improved vertical load distribution on the subgrade affords a higher bearing capacity of the stabilized
system compared to that of the unstabilized system.
These concepts have been demonstrated by research and monitored stabilized versus unstabilized soil
layers under different types of loads and are widely used in the presently available design methods.
4.3.3 External confinement — Description of mechanism
External confinement occurs when a volume of material is confined by a three-dimensional geosynthetic
system, which can be made of a geocell or a factory or in-situ three-dimensional assembling of geosynthetic
components. For ease of use, reference will be made to geocells only in this document, although the details
apply equally to the factory and in-situ 3D assembling of geosynthetic components.
The geocell stabilization mechanism limits horizontal infill soil deformation via the geocell walls, thereby
confining the infill soil. The limitation of horizontal deformation is based on four factors.
a) Hoop tension forces in the cell walls.
b) Resistance from the surrounding cells.
c) Friction between cell walls and infill material.
d) Connection strength of joined walls.
Under vertical load, horizontal earth pressure is restrained by the cell walls. The resulting strains in the cell
walls mobilize hoop stresses within the loaded cell (Figure 4). The magnitude of the activated hoop stress
depends on the geocell material, stress-strain behaviour, magnitude of load, number of load cycles, location
of the applied load, type and properties of infill material, and the foundation characteristics.
The hoop stresses and resistance provided by surrounding cells restrict lateral deformation of the fill by
producing confining stresses σ [Figure 3c)]. The intensity of the confining stresses depends on the height
3D
to diameter ratio of the geocell, the height of surcharge and the tensile properties of the geocell.
The confined zone shown in Figure 3c) includes two parts.
1) Cell height.
2) Limited thickness above and, possibly, beneath the geocell.

Above the confined zones, a transition zone is developed which extends until there is no influence on the
granular (i.e. aggregate) layer from the geocell (i.e. unconfined zone). Figure 3c) illustrates the increased
load spread angle under load.
a) Unstabilized granular (aggregate) layer
b) Two-dimensional confinement of granular (aggregate) layer

c) Three-dimensional confinement of granular (aggregate) layer
Key
σv0 vertical stress applied at the top surface
σ vertical stress on the subgrade with unstabilized granular (aggregate) layer
vu
σ vertical stress on the subgrade with stabilized granular (aggregate) layer
vs
α load spreading angle with unstabilized granular (aggregate) layer
u
α load spreading angle with stabilized granular (aggregate) layer
s
α load spreading angle within three-dimensional structure
3D
1 confined zone
Figure 3 — Increase of the load distribution angle for bearing capacity increase
a) External vertical stress
b) Lateral stress on cell walls
c) Lateral confinement or restraint — Hoop tensile stress in cell walls
Figure 4 — Confinement mechanisms in geocells: Development of cell hoop stress by external
vertical stress
4.4 Vehicular action
Vehicular loads applied to the road surface create stresses within the aggregate. As the wheels approach and
then pass a given location along the pavement, the main principal stress for an individual aggregate particle
is increasing and additionally rotating. This stress rotation is linked to a strong horizontal force component
(i.e. shear stress), which finally leads to lateral spreading (i.e. shear strain) of the base aggregate particles.
Geosynthetics are used to limit these deformations and the adverse effects related to it (e.g. surface rutting
and decrease of bearing capacity).
When the unbound aggregate is installed and compacted on top of geosynthetics, shear resistance is
generated between the two components because of frictional interaction. Other modes of interaction can
be based on aperture interlocking (for geogrids) or cell and confinement fixation (for geocells). In such
cases, granular soil particles can partially penetrate through the geogrid apertures or into the geocells and
interlock with them.
One or both interaction mechanisms, friction or interlocking, are a prerequisite for limiting lateral
movement of the aggregate particles on top of the geosynthetic. At the initial stage, where the fill material
above the installed geosynthetic is compacted, the geosynthetic can be put into tension which causes
development of corresponding strain and stress. The amount of strain depends on several factors, with the
main factors being the stiffness of the subsoil, the degree and efficiency of interaction between aggregate
and geosynthetic and intrinsic properties of the geosynthetic, in particular the stiffness.
4.5 Loading conditions (cyclic and static)
For some of the systems under discussion, the loading conditions immediately beneath the surface are cyclic.
As vehicles continue to traffic a pavement overlying unbound aggregate the stress distribution angle within
the unbound aggregate typically decreases (see Figure 5) due to cyclic loading. The accumulation of plastic
deformations due to loading and unloading cycles normally leads to a reduction of the shear strength of the
base course material. As a result, the maximum stress at the base and subgrade interface tends to increase
over time.
Key
1 wheel load, P 6 distribution at failure
2 tyre contact area 7 base
3 surface 8 geosynthetic
4 initial distribution 9 subgrade
5 distribution after N passes
NOTE See Reference [1] for further information.
Figure 5 — Stress distribution angle
Bearing capacity failure of the subgrade occurs when the stress distribution angle decreases to a point at
[1]
which the stress at the interface exceeds the mobilized shear strength of the subgrade. The utilised shear

strength of the subgrade depends on the undrained shear strength of the subgrade, the surface deformation
[1]
or rut depth, the tyre contact area, and the thickness of the base.
Under static loading conditions, the load distribution angle remains the same over time for an uncontaminated
layer of unbound aggregate.
4.6 Multi component systems
Multi-component geosynthetic systems typically increase the thickness of the stabilized layer and provide
improved confinement. As a result, a multi-component geosynthetic system normally results in improved
performance.
A multi-component system can be either multiple layers of the same geosynthetic which provide either
internal or external confinement only, or one which contains layers of different geosynthetics, thereby
providing both internal or external confinement (or both) within a single system.
4.7 Static loads — Settlement control and limitation
The degree and extent to which loads are transferred from the surface of a stabilized layer to the underlying
foundation soil is directly related to the amount of overall settlement that will occur over the life of the
structure. For building loads and paved or unpaved roads constructed on embankments, refer to the
appropriate national standard that highlights the design approach for estimation of settlement and the
limitations of the approach. For each of these cases, the designer normally considers the loading area and
amount of load transferred to the subgrade. However, for paved or unpaved roads, rutting potential or
reduction in surface deformation is generally of greater concern.
4.8 Reduction in deformation from trafficking
As stated, confinement has been identified as the geosynthetics stabilization mechanism. In simplest terms,
the geosynthetic restricts the lateral movement of aggregate fill placed upon or inside it. Many full-scale
trials have shown that geosynthetic stabilized aggregate layers are significantly more resistant to surface
deformation than non-stabilized layers when subjected to repeated trafficking loads. Furthermore, it has
also been shown that deformation of the subgrade beneath the stabilized layer is also significantly reduced.
This deformation reduction relies heavily on effective lateral restraint of the aggregate particles and the
resulting confinement which creates a stiffer or higher modulus composite layer. Since elastic stiffness or
resilient modulus of unbound aggregate is proportional to its confining pressure, the net effect of increasing
the fill’s modulus or stiffness is a spreading of the vertical stress distribution and a corresponding reduction
[2]
in the deformation on top of the subgrade. So, under repeated traffic loading, the geosynthetic not only
restricts the movement of aggregate particles and thereby reduces deformation of the aggregate layer, the
stiffer composite stabilized layer also restricts deformation of the subgrade.
4.9 Reduction in unbound aggregate degradation
The results of modelling and those of full-scale laboratory testing and traffic studies have demonstrated a
positive improvement in the amount of trafficking that can occur over stabilized layers in comparison to
control sections of non-stabilized granular structures. Comparing a non-stabilized and stabilized aggregate
layer in full-scale study has shown both life extension and improved aggregate performance for the
[3]
stabilized layer. More aggregate degradation normally occurs in the control sections than the stabilized
[4]
test sections.
It is accepted that degradation of granular (i.e. aggregate) material increases the content of fines, reducing
the water permeability and increasing the sensitivity of the granular (aggregate) material to reduced
bearing capacity by increased water content and pore water pressure. For this reason, in addition to
lateral confinement, the positive influence of separation, filtration and drainage geosynthetics can become
significant.
4.10 Extension of design life
It follows from the preceding sections that retention of unbound aggregate particle shape, size and
grading as well as minimization of particle movement will occur through lateral confinement, and where
required, separation and filtration. The lateral confinement is provided by both particle to particle and
particle to geosynthetic interaction. The degree of interaction is affected by aggregate quality, geosynthetic
characteristics, placement conditions and level of compaction.
For some design situations, there is a direct correlation between one or more geosynthetic characteristics
and performance. However, for other design situations, or for design conditions which are different from
those in the specific design method, an extension in design life can only be determined by performing full
scale or laboratory scale validation trials, or both. The aim of the trials is to calibrate the performance of a
specific geosynthetic and aggregate combination for specific stress conditions. Refer to local and national
guidance that can apply for pavement design. There are numerous local and national design approaches
utilized around the world.
4.11 Ground and groundwater conditions
For stabilization, the geosynthetic must work in combination, as a composite, with aggregate fill to support
applied loads (see Reference [5]). With respect to building over soft soils, either bearing-capacity-based (see
References [2] and [6]) or serviceability-based (see References [7], [8] and [6]), design methodologies yield
appropriate aggregate fill thickness for a given set of input parameters. Likewise, presuming the subgrade
is soft because of moisture content, it is important to keep in mind that the aggregate fill is usually clean
(cohesionless, with preferably single-digit percent fines) and drainable (see Reference [9]). Insufficient
thickness, poor quality, or both can compromise the beneficial functions of the geosynthetic, and jeopardize
stability of the system.
In all cases where a geosynthetic stabilized layer is incorporated within a pavement structure, the
groundwater typically must remain below the influence of surface loads within the structural layer as far as
practicable. Free drainage of the pavement is normally essential. If groundwater rises within the layers, the
ability of the aggregate to transfer shear, and consequently maintain bearing capacity, is compromised. As a
result, proper drainage and inclusion of lateral drains is normally essential in the proper functioning of the
pavement system.
To ensure the aggregate fill is not compromised, it is normal to consider including a correctly designed
separation or filtration geotextile that does not clog and allows free movement of water. Drainage
geocomposites can also be used to provide separation, filtration and drainage functions.
4.12 Climatic conditions
With regard to both environmental and climatic conditions, Reference [10] reviews 140 long term pavement
performance (LTPP) sites for paved roads covering environmental and climatic conditions throughout New
Zealand. The findings suggest that roads in more sensitive climatic zones have an increased rut rate (by
approximately 0,1 mm/year) as compared to the more stable climatic areas. For example, at an equivalent
single axle load (ESAL) of 400 axles per day, the rut rates are:
— 0,16 mm/year for low sensitivity (dryer regions on more stable geological formations) areas;
— 0,28 mm/year (0,12 mm difference) for high sensitivity (wet regions on less stable geological
formations) areas.
However, this research established the fact that the condition and presence of drainage, where needed, was
much more important than just the environmental conditions alone. Observations from the data revealed
that the rut rate of low volume roads was 2,5 times higher on poor drainage sections compared to sections
where adequate drainage was provided throughout the pavement section. The researchers also established
that sections having poor or inadequate drainage deteriorate much faster under heavy traffic volumes.

4.13 Sustainability
Sustainability is often described as considering three primary principles: social, environmental, and
economic. The goal of sustainability is the satisfaction of basic social and economic needs, both present and
future, and the responsible use of natural resources, all while maintaining or improving the well-being of
the environment on which life depends (see Reference [11]).
Paved and unpaved roads constructed with geosynthetics meet all these objectives by reducing construction
time, the amount of resources required to build these structures and the extension in use of or reduction in
maintenance intervals.
Further, since one role of a stabilized layer is to protect the subgrade, the future rehabilitation of these roads
are normally limited to restoration of the surfacing layers. The carbon emissions and embodied energy
costs of incorpo
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