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A look into time dependent behaviour of clays at macro and micro scale
Une étude du comportement dépendant du temps des argiles aux échelles micro et macro
Yigit I. & Cinicioglu S.F.
Department of Civil Engineering, Istanbul University, Istanbul, Turkey
ABSTRACT: This paper aims to analyze and combine the mechanisms that occur at micro and macro scales during 1-D consolidation
of clays. In this study, the approach by Yin (1999) was used to interpret time dependent action during creep phase of a specifically
designed 1-D consolidation test program. This phase of the study gave a creep rate parameter, (
ψ
/v) which is itself a characteristic
feature of time dependency and also well-accepted to carry the implications of micromechanical changes. In addition to obtaining the
creep rate parameter, a quantification process was applied on micrographs that were taken at different consolidation stages and load
increments of the 1-D consolidation test. This process was facilitated to find a micro-scale parameter defined as PF (Percent Finer).
The possibility of existence of an interrelationship between
ψ
/v, and PF was then searched for as both of these parameters vary with
respect to creep time. A strong correlation with a direction, either positive or negative, depending on the applied load increment was
found. However, it should be noted that the number of data points are quite a few and a greater number of data points and data sets
may be required to ascertain the validity and reliability of a correlation. Nevertheless the developed relation seems to be promising.
RÉSUMÉ : Le but de ce papier est d'analyser et de combiner les mécanismes de déformation qui se produisent pendant la
consolidation 1-D des argiles aux échelles micro et macro. Dans cette étude, L'approche de Yin (1999) a été utilisé pour interpréter la
réponse dépendant du temps des argiles lors de tests spécialement conçus de fluage-consolidation 1-D. Cette étude nous a fourni le
paramètre (
ψ
/v) qui est une caractéristique de la réponse dépendant du temps des argilites et représentant, comme il est communément
admis, les effets des changements de la microstructure. En plus, une analyse quantitative des micrographies, prises à différents
niveaux de consolidation et de charges, a été menée qui a permis d’identifier un paramètre microstructural PF (Pourcentage de Fines).
Une possible corrélation entre les deux paramètres
ψ
/v et PF qui évoluent au cours du fluage a été recherchée. On a trouvé une forte
corrélation qui peut-être positive ou négative selon l’accroissement de la charge appliquée. Il faut noter que le nombre de points de
donné n’est pas suffisant pour assurer la validité du résultat de manière fiable. Cependant, la relation trouvée entre ces deux
paramètres paraît très prometteuse.
KEYWORDS: creep, time dependency, micromechanics, ESEM, micrographs, clay structure, consolidation.
1 INTRODUCTION. FIRST LEVEL HEADING
Research on the constitutive behavior of clays revealed the
important influence of micromechanical processes on the stress
strain behavior. Current studies mostly focus on either
observing micromechanical processes such as anisotropic fabric
orientation, structuring – destructuring - restructuring
phenomena or using macro-mechanical measurements to search
for the implications of micromechanical behavior.
Research on the microstructural changes during
consolidation attracted special attention and most of these
studies investigated the variation of fabric and/or structure with
respect to the state of stress (Mitchell 1993, Collins and
McGrown 1974, McConnochie 1974, Smart and Tovey 1982,
Delage and Lefebvre 1984, Adamcewicz et al. 1997, Hicher et
al. 2000). In these studies, internal fabric and/or structure were
usually defined in terms of a parameter found through a
quantification method based on image processing applied on
micrographs. These parameters are then facilitated as the tools
to interrelate micro and macro behavior. On the other hand, time
effects on the stress-strain behavior of soils cannot be
overlooked in a study concerning micromechanical changes
occurring in connection with any type of stress path application.
Following Šukjle’s study (1957), recent research provided
significant scientific evidence on the time dependent behavior
of soft clays and showed that the differences in the strain rates
cause changed consolidation processes as an indication of time
dependent behavior. Strain rates are high just after increasing
the load but decreases with time and strain develops. This
implies that as the rate changes fabric also shows variation.
Although, time-dependent behavior is not specific for the
secondary compression stage (Leroueil 2006) creep models are
usually preferred, as, they reflect time dependency directly. In
this context, Yin (1999) proposed a nonlinear creep function,
which involves time dependent creep parameter and has a limit
state at infinite creep time. This modified version of classical
empirical approach is simple and can be used in advanced
general time dependent constitutive models.
Although extensive research has been conducted and
important achievements have been realized regarding time
dependency, there is a lack of consistency in the studies that try
to combine the micromechanical observations with macro-
mechanical measurements. This paper aims to analyze and
combine micro and macro structural mechanisms that occur
during 1-D consolidation of clays. In this study, a new approach
for interpretation of time dependent behavior of clays is
introduced. This method incorporates fabric change due to creep
process with creep rate parameter. In the context of the
presented research 1-D consolidation tests were conducted
during which micrographs were taken at different stages and
load increments. The results of the 1-D consolidation tests were
then analyzed in accordance with the purpose of this study. The
analyses were carried out with a dual approach both by
displaying the applicable time rates encountered during
consolidation and also by quantifying the micrographs taken at
some specific times and stages of consolidation.
, Cinicioglu S.F.
