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Hardening process of clayey soils with high water content due to thixotropy effect
Processus de durcissement des sols argileux à forte teneur en eau causé par un effet thixotropique
Tanaka H.
Hokkaido University, Sapporo, Japan
Seng S.
Institute of Technology of Cambodia, Phnom Penh, Cambodia
ABSTRACT: Hardening due to thixotropy for clayey soils with high water content was studied by change in stiffness by time, using
Bender Element Test. Shear strength was also measured to be compared with the increase of the stiffness measured by the bender
element. Following interested findings are obtained: 1) the effect of thixotropy is found significantly at around the liquid limit state
and less remarkable at the lower and higher ranges than the liquid limit; 2) the shear modulus at liquid limit after 24 hours resting is
around 200 kPa; 3) the increment of the shear modulus developed in the thixotropy process appears noticeably higher than that in the
secondary consolidation process. It is believed that these findings are very useful to establish a new theory for the consolidation of the
ground filling by very soft clays or dredged soils with extremely high water content as well as to understand the effects of ageing on
consolidation properties of natural soils.
RÉSUMÉ : Le durcissement de sols argileux, ayant une teneur en eau élevée et causé par la thixotropie, a été étudié
à l’aide de
plaques de piézo-céramiques permettant de suivre les changements de rigidité du sol en fonction du temps. La résistance au
cisaillement a aussi été mesurée et comparée aux changements de rigidité. Ceci a permis les constatations intéressantes suivantes : 1)
l’effet du gain thixotropique devient significatif lorsque l’état du sol est proche de la limite de liquidité lequel serait mo
indre aux
valeurs supérieures et inférieures à la limite de liquidité; 2) le module de cisaillement à la limite de liquidité, après 24 heures au repos,
est d’environ 200 kPa; 3) l’augmentation du module de cisaillement c
ausée par la thixotropie appara
t, d’
une façon notable, plus élevée
que celle pouvant provenir de la consolidation secondaire. Nous croyons que ces constatations sont très utiles pour établir une
nouvelle théorie de la consolidation pour les cas de remblayage avec des argiles très molles ou des sols de dragage tout autant que
pour comprendre les effets du vieillissement sur les propriétés de consolidation des sols naturels.
KEYWORDS: Thixotropy, Stiffness, Bender Element, Consolidation, Liquid limit.
1 INTRODUCTION
The concept of “effective stress” has been widely accepted by
geotechnical engineers and it is believed that geotechnical
parameters such as strength or stiffness can be simply correlated
with the effective stress. In this concept, these parameters are
independent of time and the apparent time dependency during
consolidation can be understood by changing in the effective
stresses in the process of the dissipation of the excess pore
water pressure. On the other hand, some behaviors opposite to
the effective stress approach are also recognized, i.e.,
thixotropic hardening. Soil loses strength or stiffness by
remolding, but they are recovered in time. It may be considered
that this factor should play an important role in the process of
settlement. Thus, this paper will try to understand the
thixotropic hardening phenomenon of clays with high water
content.
Although some experimental data have been reported on this
field, the conventional destructive testing tools such as triaxial,
unconfined compression, and laboratory vane apparatus have
been employed by previous investigators to measure strength
hardening due to thixotropy. Normally, these methods do not
guarantee the same testing condition because of using different
samples and relatively long preparation time for testing, i.e.,
difficult to measure properties at very early time, and also
disturbance effect occurs during insertion of the vane blade, for
example. To overcome these drawbacks, a nondestructive
bender element test, which provides the measurement at a very
low strain level as small as 10
-5
(0.001%), is introduced in this
paper to study the thixotropic behavior of very soft clays,
together with a vane shear test measuring the undrained
strength. In another approach, oedometer test under very low
pressure was also carried out, being equipped with the bender
element to examine the development of soil stiffness due to
consolidation. Correlation between the stiffness and the
effective stress will be established, and also study on the
changing in the stiffness during the secondary consolidation.
Using these test results, the increment of stiffness caused by
thixotropic behavior is tried to be evaluated.
2 SAMPLE AND TESTING METHOD
The tests were run on very soft clays using samples obtained
from three commercial powder clays and three natural clays.
Commercial clays are Fujinomori, Kasaoka, and NSF. The
natural ones are Ariake, Hachirogata and Tokuyama. Their
index properties are summarized in Table 1. Their liquid limits
vary in a large range from low (48.6 %) to very high (246.0 %)
value that allows this study to cover the wide different
characteristic of materials.
After mixing at a target water content, the sample was
poured into a mold with various sizes for different purposes: a
plastic cylindrical mold with the height of 10 cm and the
diameter of 5 cm for the bender element test, a gallon bucket
with 20 cm of the diameter and heights of specimen varied from
7 to 8 cm for the vane test, and a consolidometer cell with
diameter of 10 cm and adjustable height from 4 to 15 cm for
consolidation test. Vibration was gently applied to drive out air
bubbles from a slurry specimen and a sample was compacted
when its water content was small and the sample was stiff. The
mold with the specimen for the thixotropy test was wrapped by
