211
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Characteristics of structure evolution of expansive soil and loess during loading
and wetting
Caractéristiques de l'évolution structurale du sol expansif et du loess lors du chargement
et du mouillage
Chen Z.
Logistical Engineering University, Chongqing 400041, China
ABSTRACT: A CT-triaxial apparatus was used to study systematically the meso-structure evolution of expansive soil and loess. All
soil samples were unsaturated. A group of CT-triaxial drained tests with net cell pressure and suction controlled and a group of wet-
dry circles for expansive soil samples were conducted. A series of CT-triaxial inundation-collapse tests were conducted under certain
suction and net mean stress as well as deviator stress for undisturbed Q
3
loess. A number of CT images and CT date of meso-structure
evolution of the samples were obtained. Some regularity of holes and cracks developing in samples were discovered. The structure
damage evolution equations for expansive soil and loess were proposed, respectively. A method to determine the yield stress in
triaxial test was suggested.
RÉSUMÉ : Un appareil CT-triaxial a été utilisé
pour étudier systématiquement l’
évolution méso-structurale du sol expansif et du
loess. Tous les échantillons de sol étaient insaturés. Un groupe
d’
essais CT-triaxial de drainage ont été faits sous le contrôle de la
pression et de la succion de la salle blanche, ainsi qu’un groupe d’essais d’alternance humide
-sèche pour les échantillons de sol
expansif. Une série d’
essais CT-triaxial de mouillage ont été effectués sur le loess Q3 intact sous une valeur définie de succion, de
contrainte moyenne nette et de contrainte d
éviatorique.Un certain nombre d’images et de données CT de l’évolution méso
-structurale
des échantillons ont été prélevée. Quelques régularités du développement des trous et des fissures dans les échantillons ont été
découvertes. Les équations d'évolution des dégâts structuraux du sol expansif et du loess ont été proposées respectivement. Une
mé
thode de déterminer la limite d’
élasticité en essai triaxial a été suggérée
KEYWORDS: CT-triaxial apparatus; suction; loading; wet-dry circle; collapse; structure evolution.
1 INTRODUCTION
Each type of soils, such as collapsible loess and expansive soil,
has its own special structures. The mechanical properties of
soils are related to their interior factors (i.e., mineralogical
composition and structure) and the outer factors (i.e., loading
and environmental changes). For example, the strength of
collapsible loess and expansive soil are very high during the dry
seasons. However, the strength will decrease drastically when
they are inundation.
Since 2000, a series of researches have been conducted on
the meso-structure analysis of loess and expansive soil using the
CT-triaxial apparatus developed by the author. The meso-
structure and evolution laws of expansive soil and loess under
the loading and moisture changes were mainly investigated.
2 TEST EQUIPMENT
In recent years, great effort has been paid to study meso-
structures and constitutive relations of special soils such as loess
and expansive soil, et al, by the authors. A multifunctional
triaxial apparatus of unsaturated soil combining with CT
machine developed successfully (Chen, et al, 2001; Chen, et al,
2007), and over 30 of stress path tests on saturated soil,
unsaturated soil and special soils can be conducted with the
equipment (Figure 1). The CT machine made by GE is a spiral
scanner type of Prospeed AL. Its spatial resolution is 0.38 mm
and its density resolution is 0.3 % (3 Hu).
3 STRUCTURE EVOLUTION OF EXPANSIVE SOIL
Crack is one of important structural feature, which influences
the mechanical properties of expansive soil. In order to discover
the characteristics of the crack evolution of expansive soil, 9
groups of tests including 55 samples were conducted. These
tests were CT-triaxial tests in loading and wet-dry circles. A
total of 666 CT images and a number of CT data of the test
samples were obtained (Lu, 2000; Wei, 2007; Yao, 2009; Chen,
2011; Yao, 2011; Wang, 2012). However, only a part of test
results are presented in the paper because space is limited.
Figure1. CT-triaxial apparatus
3.1
The characteristics of structure damage evolution of
intact expansive soil during loading
Fifteen CT-triaxial drained tests of intact expansive soils with
net cell pressure and suction controlled were conducted. The
size of samples was 39.1mm in diameter and 80mm in height.
The initial conditions and index of samples are shown in table 1
(only the data of sample 2
#
are listed ).
The scanning cross-sections which were marked 1 and 2
located at 1/3H and 2/3H from the top of specimen. Each
sample was scanned 6 times and the scanning pictures were
marked as a, b, c, d, e and f successively. 1a, 1b, 1c, 1d, 1e and
1f were corresponding to the cross-section of 1/3H (Fig. 2).
The CT images of sample 2
#
clearly illustrate that the
structures of the samples change obviously from initial
deformation to failure and several cracks gradually developed.
The test results of the samples indicate similar crack evolution
rules. 1) The initial damage of structure of intact expansive soil
is great and heterogeneous in space. 2) Shear makes an obvious
