Basic HTML Version
www.geotechnicalnews.com
Geotechnical News
June 2013
39
THESIS ABSTRACTS
Assessment of Post-compaction
Characteristics of an Unsaturated Silty
Sand
Ana Heitor
Ana Heitor (aheitor@uow.edu.au)
Conventional field compaction control methods are effec-
tive at the time of placement. However, their measurements
are discrete and have a limited depth of investigation, which
may not be suitable for post-construction compaction qual-
ity assessments of deeper fills or larger surface areas. The
use of available non-destructive cost and time effective
methodologies, such as shear wave velocity (Vs) surveys
(i.e. SASW, spectral analysis of surface waves), offers a
valuable alternative to efficiently control compaction during
post-construction stages. In fact, while Vs has been used
for evaluating the quality of compaction the effect of partial
saturation has been neglected. This poses problems for
reclaimed fill areas because the ground water level (GWL)
is located deeper. In addition, high in situ Vs may not truly
represent a higher degree of densification because com-
pacted soil is under unsaturated condition, which means, in
situ suction has an important role in controlling the shear
strength and thus Vs.
This doctoral thesis addresses the effects of partial satura-
tion in the implementation of a field methodology based
on the propagation of Vs and suction for evaluating the
compaction quality. It encompasses the use of both small
and large strain range in relation to laboratory and field
approaches to characterise the behaviour of materials under
different compaction conditions, as well macrostructure
characterization using X-ray CT-scan techniques. The small
strain behaviour was characterized using Bender elements
and suction was controlled and measured using an array
of different techniques. A new empirical formulation for
evaluating the current void ratio or degree of compaction
based on shear wave propagation and suction is proposed.
The performance of the methodology developed is first
calibrated for site-specific silty sand soil in laboratory and
then assessed for field site located in Penrith, in which the
evaluation of the current compaction degree is of paramount
importance for the future redevelopment of the site.
Sponsoring Professor and University: Prof. Buddhima
Indraratna, University of Wollongong, Wollongong, Australia,
T: 0061 (02) 4221 3046, F: 0061 (02) 4221 3238,
E: indra@uow.edu.au
An Experimental Study on the Deformation
Behaviour of Geosynthetically Reinforced
Ballast
Syed Khaja Karimullah Hussaini
Syed Khaja Karimullah Hussaini, Centre for Geomechanics and
Railway Engineering, University of Wollongong, Australia, email:
The ballast layer is responsible for distributing the applied
wheel load to the subgrade soil and maintaining the track
alignment. However, upon repeated loading, the ballast
deforms and degrades thereby significantly affecting the
track performance. Therefore, it is necessary to stabilise
the ballasted tracks so that they can carry high-speed trains
without creating any major track problem. In this research,
an investigation using the large-scale direct shear apparatus
was carried out to study the ballast-geogrid interface behav-
ior and establish the effect of geogrid aperture size on the
interface shear strength. To realistically simulate the ballast
behaviour under cyclic loading, a process simulation test
(PST) apparatus was designed in this study. The influence of
geogrid on the deformation and degradation of ballast was
assessed by conducting the model track tests. Moreover, the
study investigated the possible use of optical-fiber Bragg
grating (FBG) sensors in monitoring the ballast deforma-
tions.
The large-scale direct shear tests reveal that the normal-
ized geogrid aperture size (A/D
50
) has a profound influ-
ence on the shear strength of the ballast-geogrid interfaces.
The ratio A/D
50
is categorized into three key zones: (a)
Feeble Interlock Zone, (b) Optimum Interlock Zone, and
(c) Diminishing Interlock Zone. The best geogrid aperture
size to optimize the interface shear strength is determined
to be 1.20D
50
. The model track tests reveal that the geogrid
successfully arrests the lateral strains in ballast, and that
the ideal geogrid placement location to effectively stabilize
the track is a function of A/D
50
ratio. Two new parameters,
namely, the Lateral Spread Reduction Index (LSRI) and
Geogrid Influence Zone (GIZ) are proposed to assess the
performance of geogrid-reinforced ballast, and lateral strain
profiles determined. The study further highlights the abil-
ity of FBG sensors to capture the deformations in ballast
thereby encouraging their use in the monitoring of track
stability under operating conditions.
Sponsoring Professor and University: Prof. Buddhima
Indraratna,ResearchDirector-CentreforGeomechanicsandRailway
Engineering,
University
of
Wollongong,
Australia.
T: +61 242213046; F: +61242213238; E: