219
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
The relationship between swelling and shear strength properties of bentonites
La relation entre les propriétés de résistance au cisaillement de l'enflure et des bentonitiques
Domitrović D., Kovačević Zelić B.
University of Zagreb, Faculty of Mining, Geology and Petroleum Engineering, Zagreb, Croatia
ABSTRACT: Clay Geosynthetic Barriers are manufactured hydraulic barriers. The mineral component usually consists of bentonite,
which belongs to a group of expansive soils. The aim of this study was to establish the influence of swelling on mechanical properties
of bentonites. For this purpose, granular bentonite Volclay was chosen for laboratory testing. Its mineralogical composition was
predominantly montmorillonite (80% - 85% by mass). The correlation between swelling and water content on one side and changes in
shear strength i.e. shear strength parameters on the other were tested using a direct shear device. During several series of testing,
different levels of swelling of bentonite were simulated under the conditions of changing normal stress and hydration times. Swelling
behaviour of bentonite was determined through the long-term swelling tests using oedometer, under changing normal stress. Shear
strength testing show decreasing cohesion with longer hydration of bentonite. The friction angle increases with hydration that lasts
from 7 to 14 days. There is no significant change in the friction angle with hydration time longer than 14 days.
RÉSUMÉ : Les géosynthétiques bentonitiques sont des produits manufacturés pour les barrages hydrauliques. La composante
minérale est le plus souvent l’argile bentonitique qui appartient à la catégorie des sols gonflants. L'objectif de cette recherche est de
déterminer l’influence du gonflement sur la propriété mécanique de l’argile bentonitique. Pour la recherche est choisie l’argile
bentonitique en granulés Volclay, de composition minérale où domine la montmorillonite 80-85 %. La corrélation du gonflement et
des changements d’humidité avec les variations de la résistance au cisaillement et les paramètres de résistance au cisaillement a été
étudiée dans le dispositif de cisaillement direct. Voici la série de tests à différentes contraintes normales, à des temps variés
d’hydratation, simulant divers degrés de gonflement de l’argile bentonitique. La courbe de gonflement d’argile bentonitique est
définie par les expériences de gonflement prolongées à différentes contraintes normales dans l’œdomètre. Les résultats montrent une
réduction significative de cohésion avec la prolongation de l’hydratation de l’argile bentonitique. L’angle de frottement augmente
avec l’hydratation (de 7 à 14 jours). Il n’y a pas de changement significatif dans la valeur de l’angle de frottement lors d'une
prolongation de l’hydratation supérieure à 14 jours.
KEYWORDS: Bentonite, Swelling, Direct shear test, Shear strength.
1
INTRODUCTION
Clay Geosynthetic Barriers (GBR-C) - a member of the family
of synthetic materials - are extensively used nowadays as
sealing barriers or their components in a wide range of
engineering applications (Guyonnet et al. 2009, Kang and
Shackelford 2010, Shackelford et al. 2010). In the clay
geosynthetic barrier, bentonite makes the mineral component
that ensures low hydraulic conductivity (Koerner 1996,
Shackelford et al. 2000, Bouazza 2002, Katsumi et al. 2008),
while layers of geotextile make the supporting component.
Layers of geotextile are usually needle-punched or stitch-
bonded in so called reinforced GBR-Cs. There are also
unreinforced GBR-Cs, but their use are limited to relatively flat
slope applications that do not present significant shear stress.
In deciding about the most suitable design for the sealing
system, stability is one of the important factors to consider.
When using clay geosynthetic barriers, it is important to pay
attention to interface and internal shear strength. The interface
strength is developed at the contact between the clay
geosynthetic barrier and adjacent material, be it soil or other
geosynthetic material. The internal shear strength is different
with reinforced and unreinforced clay geosynthetic barriers. The
internal shear strength of reinforced clay geosynthetic barriers
depends on the strength of their components, more precisely the
ultimate tensile strength of reinforcement yarns and the shear
strength of bentonite. Hydrated bentonite has very low shear
strength, which is why its impact on the internal shear strength
will be reduced. The internal shear strength of unreinforced clay
geosynthetic barriers is identical to the shear strength of
bentonite clay (Gilbert et al. 1996, Kovačević Zelić 2000,
Zornberg and McCartney 2009, Fox 2010).
Bentonite clays fall into the group of expansive soils. It was
the bentonite clay swelling property the key mechanism that
ensures very low levels of hydraulic and gas conductivity, and
therefore its role of the clay geosynthetic barrier's sealing
component. The cause for swelling is found in the fact that
smectite clay minerals are the main component of bentonite,
with montmorillonite as the predominant mineral. Electrical
charge and the colloidal particle size are the reasons why this
group of minerals is hydrophilic. Their ability to attract
molecules of water allows them to increase volume several
times, and it plays an important part in the mechanical
properties of bentonite, including strength, deformability and
hydraulic conductivity. What contributes further to the low
hydraulic conductivity of bentonite is an increase in the content
of montmorillonite; specific surface area (decrease in the size of
particles); electrical charge deficit and sodium content (Na
+
) in
the system of exchange (Mitchell and Soga 2005, Guyonnet et
al. 2009). The influence of these factors on the quality of
bentonite can mostly be seen macroscopically through increases
in ion exchange capacity, plasticity and swelling capacity (in the
presence of fluid), against decreases in hydraulic conductivity
and strength. It should be stressed here that sodium bentonite
clays demonstrate a very low level of hydraulic conductivity
and a high swelling capacity.
For the purpose of quantifying the role of bentonite within
the clay geosynthetic barrier, laboratory tests were conducted
with samples of Volclay granular bentonite, using direct shear
and oedometer tests. As a part of this study, a detailed
characterization of bentonite was carried out. Mineralogical
composition of bentonite was: montmorillonite 80-85%,
cristobalite around 5%, quartz around 5%, plagioclase 5%.
Index properties of bentonite were as follows: liquid limit
