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Recent developments in pavement foundation design
Développements récents dans la conception des fondations des chaussées
Brown S.F., Thom N.H.
University of Nottingham, UK.
ABSTRACT: For many years the design of flexible pavements was heavily dependent on use of the California Bearing Ratio (CBR)
of the soil subgrade within an empirical method that dated back to the 1920’s. Recent changes to design practice in the UK have at
last recognised that the key parameters are the resilient stiffness modulus (E
r
) and the resistance to permanent (plastic) deformation
under repeated loading of the subgrade and foundation materials. The foundation platform on which the asphalt layers are placed is
characterised by a ‘Surface Modulus’, which is measurable on site using a dynamic plate test. Resistance to rutting under construction
traffic is determined from proof loading with a truck. The surface modulus is a function of E
r
for the individual foundation layers. To
aid the selection of materials, a simplified version of the repeated load triaxial test has been developed to quantify both resilient and
plastic strain characteristics. This apparatus is known as the PUMA and is similar to the Springbox device that has been successfully
used in recent years. It can accommodate both soils and lightly stabilised materials for use in sub-base construction.
RÉSUMÉ :
Pendant de nombreuses années la conception des chaussées souples a été fortement tributaire de l'utilisation de l'indice
portant californien (CBR) de la plate-forme du sol par le biais d'une méthode empirique qui remontait à 1920. Les changements
récents dans la pratique du dimensionnement au Royaume-Uni ont enfin reconnu que les paramètres clés sont le module de rigidité
élastique (E
r
) et la résistance à la permanente déformation (plastique) sous charge répétée des matériaux du sol de fondation et de
fondation. La plate-forme de fondation sur laquelle les couches d'asphalte sont placées se caractérise par un «Module Surfacique»,
qui est mesurable sur le site en utilisant un essai à la plaque dynamique. La résistance à l'orniérage sous trafic est déterminée à partir
d’un essai de chargement référence avec un camion. Le module de surface est fonction de E
r
pour les couches de fondation
individuelles. Pour faciliter le choix des matériaux, une version simplifiée du test triaxial à charge répétée a été développée pour
quantifier à la fois les caractéristiques de déformation résilientes et plastiques. Cet appareil est connu comme le PUMA et est une
version améliorée de l'appareil Springbox qui a été utilisé avec succès ces dernières années. Il peut être utilisé à la fois pour des sols et
des matériaux stabilisés utilisés comme couche de fondation.
KEYWORDS: Pavement foundations, laboratory testing, repeated loading, resilient modulus, plastic strain, design.
1 INTRODUCTION.
The California Bearing Ratio (CBR) continues to be used to
characterise subgrades in modern flexible pavement design
methods in most parts of the World (eg. Highways Agency,
2006, AASHTO, 2008) despite the fact that it has long been
recognised as, at best, a simple index of undrained shear
strength. The mechanical properties of subgrade soils and of
capping and sub-base materials that are relevent to pavement
design are, however, the resilient stiffness modulus (E
r
) and the
resistance to plastic strain under repeated loading, concepts that
are gradually being incorporated into pavement design methods.
Developments both in laboratory and field testing have assisted
this process. This has allowed a wider choice of materials to be
used in construction within a framework of performance related
specifications. While field testing using some form of dynamic
plate load represents the end-product performance test for
resilient characteristics on the as-built foundation, engineers
also require element tests that can be used on candidate
materials for the individual layers and the subgrade as part of
the design process. Combining this information within an elastic
analysis of the foundation can deliver a target value for what is
known as the ‘Surface Modulus’, which is the parameter
determined from the dynamic plate loading test.
The repeated load version of the triaxial test was developed
in the 1960s to determine resilient modulus but it has proved
time consuming to use and has essentially remained a research
tool. By contrast, simplified methods of testing asphalt were
developed for use in engineering practice in the 1990s against a
sound research background (Brown, 1995). These have been
adopted within European Standards. A similar philosophy has
since been used to provide an appropriate test method for soils,
granular materials and lightly stabilised materials.
2 PAVEMENT FOUNDATION DESIGN
The concept of designing a pavement in two stages was
proposed by Brown and Dawson (1992). The first stage
involves all layers up to and including the sub-base, known as
the pavement foundation, and requires that a relatively small
number of heavy wheel loads should be accommodated from
construction traffic. The second stage involves design of the
completed pavement for the long term incorporating bound
layers over a foundation of known, measured effective stiffness;
the ‘Surface Modulus’.
The UK Highways Agency (2009) have introduced an
interim design guide for pavement foundations following these
principles (Highways Agency, 2009). Four classes of
foundation are defined in terms of minimum values for the
Surface Modulus; Class 1 ≥ 50MPa, Class 2 ≥ 100MPa, Class 3
≥ 200MPa and Class 4 ≥ 400MPA. The top two classes
generally require the use of stabilized aggregates of some type.
A similar approach is used in France (LCPC and SETRA,
1994).
The definitive test method for measuring the Surface
Modulus is the Falling Weight Deflectometer (FWD) which
applies load pulses that are simulative of moving heavy wheel
