1007
Behaviour of a compacted silty sand under constant water content shearing
Comportement d'un sable limoneux compacté sous cisaillement à teneur en eau constante
Heitor A., Rujikiatkamjorn C., Indraratna B.
Centre for Geomechanics and Railway Engineering, University of Wollongong, Wollongong, Australia ; ARC Centre of
Excellence in Geotechnical Science and Engineering, Australia
ABSTRACT: The structure derived from compacting the soil at different water contents and energy levels can have a substantial
effect on its shear strength. While the shear strength with varying suction can be estimated based on the saturated shear strength
parameters and the unsaturated angle of shearing resistance (
b
), limited studies have explored the variation of shear strength
properties with different compaction states. In this paper, the shear strength of a silty sand soil was investigated using a conventional
direct shear box under constant water content condition. The tests were conducted on specimens prepared by Proctor compaction with
three different normal pressures. The shear strength parameters were obtained and modelled in terms of ultimate states.
RÉSUMÉ : La structure interne d’un sol résultante du compactage à différentes teneurs en eau et niveaux d'énergie peut avoir un effet
important sur sa résistance au cisaillement. Alors que la résistance au cisaillement avec succion variable peut être estimée sur la base
des paramètres de résistance au cisaillement en conditions saturées et de l'angle de résistance au cisaillement non saturé (
b
), peu
d'études ont exploré la variation des propriétés de résistance au cisaillement avec des niveaux de compactage différents. Dans cet
article, la résistance au cisaillement d'un sol de sable limoneux a été étudiée en utilisant une boîte de cisaillement direct classique. Les
tests ont été effectués sur des échantillons préparés par la méthode de compactage Proctor avec trois différentes pressions normales.
Les paramètres de résistance au cisaillement ont été obtenus et modélisés en termes d’états ultimes.
KEYWORDS: Compacted soil, shear strength, constant water content, direct shear test.
1 INTRODUCTION.
Field compaction may not always be uniform (i.e. differences in
hydration time, lift thickness and soil variability). Early studies
have shown that these variations can have substantial effects on
the mechanical behaviour of the compacted fine-grained soils
(Proctor, 1933; Seed and Chan, 1956). The main aspects that
can be affected are related to swelling, wetting induced collapse
compression and soil response due to external loading
(compression behaviour and shear strength).
Research studies on the shear strength behaviour of
unsaturated compacted soil prepared at different compaction
states (i.e. water contents and energy levels) showed that there
is an intimate relationship between shear strength and water
retention properties (i.e. Vanapalli et al. 1996). Furthermore,
Wheeler and Sivakumar (2000) reported that a change in water
content during compaction produces variations in the positions
of the normal compression and critical state lines. Toll and Ong
(2003) modelled the ultimate (critical) shearing behaviour of
soil prepared at different initial compaction states and
introduced the critical stress ratios as a function of the degree of
saturation (
S
r
). Tarantino and Tombolato (2005) investigated the
shear strength and hydraulic behaviour of statically compacted
kaolin and showed that some of stress-strain behaviour features
observed can only be modelled using hydro-mechanical
coupling models.
While there has been an intensive research effort dedicated
to the study of shear strength properties with varying post-
compaction suction, limited research studies focussed on
investigating the shear strength properties for different
compaction states. This is undoubtedly important considering
the lack of field compaction uniformity in terms of water
content and energy. This paper presents the results from direct
shear tests performed on compacted silty sand. The specimens
were prepared at different compaction states in order to explore
a broad range of initial conditions. Tests were carried out using
a conventional direct shear box with the adoption of special
measures to maintain constant water content conditions.
1.1
Constant water content direct shear tests (CWDST)
There are a number of different types of apparatus that can be
used to study the shear strength behaviour of unsaturated soils.
For testing soil under unsaturated conditions, the conventional
apparatus often needs to be modified to enable the suction to be
controlled or measured during the shearing stages, using i.e.
axis translation technique, vapour equilibrium or osmotic
suction control. While these types of control are effective, the
laboratory conditions may not always be truly representative of
those in the field, where shearing typically occurs under
constant water content conditions. The use of the conventional
direct shear box for determining the unsaturated shear strength
parameters is very attractive because it is readily available to
practitioners. Although it requires a careful moisture control, it
can benefit from higher rates of shearing (compare 1
m/min for
suction controlled apparatus with 0.005
1mm/min, i.e. Zhan
and Ng 2006, Oloo and Fredlund 1996). The only drawback is
the absence of an independent system to measure suction,
although, Oloo and Fredlund (1996) and Cokca et al. (2004)
assumed that any changes in suction during shearing would be
minimal provided that a relatively fast rate of strain is adopted.
2 EXPERIMENTAL WORK
2.1
Soil type
The soil used in this study was silty sand classified as SP-SC
(Unified Soil Classification System, USCS). The soil is a by-
product of cobble quarrying activities that has been widely used
to fill low areas at the Penrith Lakes site in Penrith (NSW,
Australia). While the soils present on site are quite variable, for
