1113
Small-strain shear modulus and shear strength of an unsaturated clayey sand
Module de cisaillement en petites déformations et la résistance au cisaillement d'un sable argileux
non saturé
Georgetti G.B., Vilar O.M.
University of Sao Paulo, Brazil
Rodrigues R.A.
Sao Paulo State University, Brazil
ABSTRACT: This paper presents and discuss results of laboratory tests with a compacted clayey sand. Two sets of tests were
performed to analyze the shear strength and the small-strain shear modulus (
G
o
) of this soil. The first set consisted of triaxial
compression tests performed on saturated specimens and multistage triaxial compression tests with controlled suction on unsaturated
specimens, which were sheared under constant water content. The second set consisted of tests to measure the
G
o
of specimens under
constant suction using the bender elements technique. The results allowed observing the development of suction during shearing and
influence of suction and confining stress on the shear strength and on the small-strain shear modulus of the tested soil. A planar failure
envelope could nicely fit experimental data of shear strength. The influence of net confining stress on the small-strain shear modulus
reduced as the suction increases. The shear modulus tended to increase non-linearly with suction, seeming to reach an asymptotic
value.
RÉSUMÉ : Cet article présente et discute les résultats des essais de laboratoire sur un sable argileux compacté. Deux séries
d’essais
ont été effectuées pour analyser la résistance au cisaillement et le module de cisaillement en petites deformations (
G
o
) de ce sol. La
première série comprenait des essais de compression triaxiaux réalisés sur des échantillons saturés et des essais de compression
triaxiale avec succion contrôlée sur des échantillons non saturés, qui ont été cisaillés en teneur en eau constante. Les essais de la
deuxième série ont servi à mesurer le
G
o
d'échantillons sous succion constante en utilisant la technique des
bender
éléments. Les
résultats obtenus ont permis l
’observation de la succion
pendant le cisaillement du sol. Ils ont aussi permis
d’
observer l'influence de la
succion et de la contrainte de confinement sur la résistance au cisaillement et le module de cisaillement du sol testé. Une enveloppe de
rupture plane pourrait bien représenter les données expérimentales de résistance au cisaillement.
L’influence
de la contrainte de
confinement sur le module de cisaillement diminue ave
c l’augmentation de la succion.
Le module de cisaillement a tendance à
augmenter de façon non linéaire avec succion, semblant atteindre une valeur asymptotique.
KEYWORDS: Unsaturated soil, shear modulus, bender elements, shear strength.
1 INTRODUCTION
Unsaturated soils are present in large areas worldwide,
especially in tropical and subtropical zones. Soil water in
unsaturated soil is under suction, which is known to influence
the mechanical and hydraulic properties of the soils. For
instance, it is known that shear strength increases with suction
up to some limit value and that hydraulic conductivity tends to
decrease with suction by many orders of magnitude, depending
on the level of suction considered. Several laboratory
techniques have been developed or adapted to evaluate the
suction influence on key properties of the soils, such as the axis
translation technique (Hilf 1956) used to control soil suction in
triaxial and consolidation tests. In particular, triaxial
compression tests on unsaturated samples are used not only to
evaluate shear strength but also to estimate soil stiffness through
the analyses of stress-strain curves. However, these stiffness
measurements face many drawbacks as pointed out by Atkinson
and Sallfors (1991). An attempt to overcome such difficulties is
using local displacement transducers (Jardine et al. 1984) and
bender elements (Dyvik and Madshus 1985) in conventional
laboratory apparatuses. Specifically, bender elements technique
has increasingly called the attention of researches over the past
decades. This technique intends to be a simple and accurate
method to measure the small-strain shear modulus of the soil
(Dyvik and Madshus 1985). When compared to other methods
of obtaining the small-strain shear modulus, bender elements
technique provided good agreement or slightly overestimated
values in tests performed by Youn et al. (2008) and Hoyos et al.
(2008).
This paper presents test results performed to investigate the
shear strength and the shear modulus at small strains of a
compacted sandy soil formed under tropical environment. The
shear strength was obtained through triaxial compression tests,
which were conventional consolidated-drained for saturated soil
and constant water content tests for unsaturated soil. The small-
strain shear modulus was measured using bender elements in
both saturated and unsaturated soil at different suctions and
under different isotropic confining pressures.
1 SOIL PROPERTIES AND SPECIMENS PREPARATION
The soil used in this investigation is a clayey medium to fine
sand of colluvial origin. It was under the action of pedogenetic
processes typical of tropical environments, where periods of
intense rain and elevated temperature induce severe weathering
process. As a result, bases and silica are removed, remaining the
more resistant minerals such as quartz. Resulting clay fraction is
composed by iron and aluminum oxides and kaolinite. The soil
presents liquid and plastic limits of 32 and 16%, respectively.
The fine to medium sand fraction is 67% and the clay content is
28%. According to the Unified Soil Classification System it is a
SC soil. Standard Proctor parameters are maximum dry density
of 1.80 g/cm³ and optimum water content of 13.8%.
The triaxial compression and bender element tests were
performed with dynamically compacted specimens molded with
