1357
Evaluation of roadbed potential damage induced by swelling/shrinkage
of the subgrade
Effet du retrait-gonflement des sols sur les structures de chaussées
Simic D.
Head of Geotechnical department. Ferrovial-Agromán
ABSTRACT: The expansive soils in arid and semi-arid regions are subject to seasonal moisture variations that trigger changes in
volume. These movements are reflected in swellings along the wet months and shrinkages along the dry months; seasonal movements
that induce significant damages in the pavements. Traditionally, the construction of pavements on expansive clays has resulted in
roads with a poor comfort level for the customers and a great maintenance cost for the administration. Such facts make very
problematic the construction of road pavements in expansive soils. This paper analyzes the behavior of the pavement subject to
deformations due to swelling and shrinkage of the subgrade, in order to evaluate some of the published design methods for the
protection of the pavement against the swelling phenomena of underlying clays. To introduce the design methods, this paper will
describe first the usual pathologies due to swelling and shrinkage, and their explanation by means of the analysis of some
instrumented sections in existing roads. The different design methods will be summarized, showing also some limitations of the
assumptions adopted in each analysis method.
RÉSUMÉ : Les sols gonflants situés dans des régions au climat aride sont soumis à des variations en teneur d’eau accompagnées de
changements volumétriques : des gonflements en période humide et rétraction en période sec. Ces déformations se propageant au
niveau de la chaussée donnent lieu à d’importants coûts de maintenance. Ces coûts rendent problématique la construction si ces
problèmes ne sont pas correctement cernés et gérés. Dans cet article, le comportement de la chaussée soumise aux déformations de
gonflement est décrit et les pathologies et méthodes d’analyse existantes dans la littérature sont évaluées. Des exemples sont montrés
ainsi que les limitations des hypothèses retenues dans les procédés analysés
KEYWORDS: expansive soils, roads, semi-arid regions.
MOTS-CLÉS: sols expansifs, routes, regions semi-arides.
1 INTRODUCTION.
Expansive soil is a term usually applied to any soil that has a
potential for shrinking or swelling due to changes in its
moisture content. It is recognized that there are two main factors
that provides the potential of the soil to swell and/or shrink: the
properties of the soil and the environmental conditions of the
area. The main soil parameters that are included within the first
factor are the clay mineralogy, the soil water chemistry, the soil
suction, the structure of the soil (fabric) and its dry density.
Within the environmental conditions of the area the initial
moisture condition, the moisture variations and the stress
conditions are the factors believed to control the soil movement.
2 MECHANISM OF SWELLING/SHRINKING
The mechanism of the development of longitudinal cracks at
the pavement in arid environments has been described by
Zornberg, J. G.; Gupta, R. and Ferreira, J. A. Z. (2010). Tensile
stresses induced by flexion of the pavement during settlements
caused by the dry season leads to the development of
longitudinal cracks. See Figures 1 and 2 below.
During the dry season there is a drop off in moisture content
of the soil in the shoulders of the pavement structure. The
consequence of this reduction in moisture is a settlement in the
shoulders that does not take place in the centre of the pavement
where the moisture of the soil remains stable thorough the year.
The appearance of cracks in the shoulder of the pavement
accelerates the evaporation of the interstitial water of the soil
reaching also greater depths.
Figure 1. Mechanism of longitudinal crack development on pavement
over expansive clays during dry season. (Modified from Zornberg,
Gupta & Ferreira, 2010).
Figure 2. Longitudinal cracks near the edge of pavement
Topographical surveys along the longitudinal cracks show that
there is a tendency of the soil to settle when the cracks develop,
confirming the model explained above.
