1249
Analysis of the influence of soft soil depth on the subgrade capacity for flexible
pavements.
Analyse de l'influence de la profondeur d’un sol mou sur la capacité portante pour les chaussées
souples.
Carvajal E.
Kellerterra S.L., Madrid, Spain
Romana M.
Universidad Politécnica de Madrid, Spain
ABSTRACT: It is presented the analysis of a flexible pavement structure founded on soft soil subgrade, through the finite element
modelling of a multilayered system, with the objective to evaluate the influence of soft soil depth on pavement response. The analysis
also comprises an iterative procedure to take into account the influence of small strains on soil stiffness. A simple static load of a
heavy truck has been used to evaluate the pavement response; furthermore a cyclic loading has been considered in the form of
haversine function in order to simulate the traffick effects on cumulative permanent deformation. The results of permanent vertical
deformation from the followed procedure is compared to an empirical equation, so that rutting failure intensity is estimated. It is
concluded that deep ground treatments should be applied to achieve an allowable capacity of soft soils up to miminum depth of about
6 m, otherwise maintenance cost of pavements might be excessive.
RÉSUMÉ: L’analyse d'une structure de pavement flexible sur un terrain du sol mou est réalisée grâce à une modélisation éléments
finis appliquée à un système multicouches. L’objectif est d'évalué l'influence de la profondeur du sol mou sur la réponse de la
structure. L'analyse comprend un procédé itératif pour tenir compte de l'influence de petites déformations sur la rigidité du sol. Une
charge statique correspondante à un camion poids lourd a été utilisée pour évaluer la réponse du pavement. Une fonction de charge
cyclique de type Haversine a été considérée pour simuler les effets de trafic dans la déformation permanente accumulée. Les résultats
de la déformation permanente verticale obtenue par cette méthode sont comparés à une équation empirique, pour évaluer l’orniérage.
Le constat est la nécessité de traiter sur une profondeur minimum de 6 m le terrain pour atteindre une capacité de charge admissible
dans les sols mous. Dans le cas contraire les prix de maintenance et d’entretien des pavements pourraient être excessifs.
KEYWORDS: Pavement, subgrade, cyclic load, permanent deformation, small strain, stiffness, damping ratio, finite element model
MOTS-CLES : pavement, fondation, chargement cyclique, déformation, raideur, amortissement, éléments finis
1 INTRODUCTION.
Flexible pavements over shallow soft soils could be built
through the application of a wide range of techniques to
improve the low capacity of subgrade, such as lime and cement
stabilisation,
geogrids
and
geosynthetic
treatments.
Nevertheless when the soil layers which compose the subgrade
reach certain depths, the intensity of shallow treatments become
inefficient and it is necessary to evaluate the depth that is
influenced by the load, and the effects on the capacity and long
term behaviour of such deep soft subgrade. Here below is
presented a theoretical procedure to analyze the response of a
flexible pavement on soft soil under static and cyclic loading. It
is also presented an estimation of the rut depth failure based on
permanent deformation of subgrade layers affected by
determined number of load repetitions.
2 PAVEMENTS ANALYSIS
The pavement analysis through mechanistic approaches is
increasingly adopted, with the development of numerical
modelling tools, considering complex behavior of pavement
structure. Actually, because of the amount of variables that have
to be dealt, the design of flexible pavements could be divided in
two parts, one mechanical and the other empirical, thus
nowadays most frequently used methods for the design are often
called Mechanstic-Empirical Methods.
The mechanistic part consists on determining the elastic
response of the pavement structure in terms of stress, strain and
displacement when a heavy truck equivalent load (P) is applied
on the surface.The parameters that determine the properties and
thickness of surface layer are radial strain and tensile strain (
t
;
t
) at the bottom of the surface layer. Whereas, the capacity of
foundation soil is governed by the vertical compressive stress
and deformation in the top of the subgrade (
v
;
v
). Figure 1.
On the other side, the empirical approach is related to
relationships between each components of the elastic structural
response (
,
, y) and a fatigue law. Thus, the damage
accumulation for a given number of the load application could
be estimated. For this purpose, the fatigue law has to take into
account other essential factors that are difficult to assess from
the mechanical point of view, e.g. rainfalls, temperature
changes, drainage conditions, etc. Particurarly, the evaluation of
pavement foundation (subgrade layers) is usually estimated
through an allowable number of load repetitions (Nd) that
produces unacceptable permanent deformation, which is
commonly known as rutting failure mechanism. Otherwise,
after construction stage settlements produced due to weight of
fills and pavement structure has to be considered, although,
even more important matter will be estimation of the time of
consolidation process and determination of the soils stress state
after consolidation and at the begining of traffic operation.
Figure 1. Parameters for pavement design
