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Geotechnical News
June 2013
www.geotechnicalnews.com
THESIS ABSTRACTS
Performance of Geogrids Stabilised Fouled
Ballast in Rail Tracks
Ngoc Trung Ngo
Ngoc Trung Ngo, Mobile: 0413.469.521
The railway track network plays an essential role of trans-
portation infrastructure worldwide. During operations, the
ballast becomes contaminated or fouled due to the infiltra-
tion of fines from the surface, mud pumping up from the
subgrade, and ballast degradation under repeated train load-
ing. Geosynthetics have been increasingly used in railroads
to provide reinforcement and confinement pressure to the
layer of ballast. However, the interaction mechanism and
behaviour of the geosynthetics and ballast at their interface
are not well understood, particularly when the ballast is
severely fouled. This research aims to study how the inter-
face between ballast and geogrid copes with fouling by coal
fines.
The shear stress-displacement behaviour of fresh and fouled
ballast, reinforced with geogrids was investigated through a
series of large-scale direct shear tests at low normal stresses
from 15 kPa to 75 kPa. When the ballast was fouled by coal
fines, the benefits of geogrid reinforcement decreased in
proportion to the increasing level of fouling. A novel Track
Process Simulation Apparatus (
TPSA
) was used to simulate
realistic rail track conditions subjected to cyclic loading. A
threshold value of
VCI=40%
has been proposed to assist
practitioners in conducting track maintenance. If the level
of fouling exceeds this threshold the geogrid reinforcement
significantly decreases its effectiveness, and the fouled
ballast exhibits pronounced dilation. The Discrete Element
Method (
DEM
) was used to study the shear behaviour of
fresh and fouled ballast in direct shear testing. Fouled ballast
with various Void Contaminant Index (
VCI
), ranging from
20%
VCI
to 70%
VCI
, were modelled by injecting a speci-
fied number of miniature spherical particles into the voids
of fresh ballast. Based on the research results, an equation
incorporating
VCI
was proposed to predict the deformation
of fresh and fouled ballast. This equation improves track
design and assists in making appropriate and timely deci-
sions on track maintenance.
Sponsoring Professor and University: Prof. Buddhima Indraratna,
Professor of Civil Engineering, Faculty of Engineering, University
of Wollongong, Wollongong, NSW 2522, Australia. T: +61 2 4221
3046; F: +61 2 4221 3238, E:
Performance Validation of a Permeable
Reactive Barrier (PRB) for Treating Acidic
Groundwater
Gyanendra Regmi
Gyanendra Regmi, Email:
,
Mobile: +61413434882
The effectiveness of a permeable reactive barrier (PRB) to
remediate contaminated groundwater from acid sulphate soil
on the Shoalhaven Floodplain, southeast New South Wales
(NSW), Australia was investigated. High concentrations of
dissolved aluminium (Al
3+
), total iron (Fe), and sulphate
(SO
4
2-
) in the groundwater along with low pH were evidence
of acidic conditions due to pyrite oxidation at the study
site. Groundwater manipulation using engineering solutions
such as weirs and modified floodgates drains are not effec-
tive in low-lying ASS terrain, as they cannot remediate the
acidity already present in the soil nor significantly prevent
pyrite oxidation in areas far from nearby drains. This study
combined laboratory, field and numerical analyses in order
to determine the feasibility and performance of a PRB utilis-
ing zero-cost recycled concrete for the remediation of acidic
groundwater in ASS terrain.
Long-term laboratory column experiments were carried out
using synthetic and real groundwater from the study site.
The column experiments investigated the acid neutralisation
reactions occurring within the PRB and the precipitation
of Al and Fe from the acidic groundwater. Three distinct
pH-buffering reactions were ascertained: (i) the dissolution
of carbonate/bicarbonate alkalinity from concrete at nearly
neutral pH, (ii) the re-dissolution of aluminium hydroxide
precipitates at pH ~4, and (iii) the re-dissolution of ferric
oxyhydroxides minerals at pH <3. However, carbonate/
bicarbonate buffering was the most significant because of
the maintenance of near neutral pH and complete removal of
Al
3+
and total Fe from the influent.
Chemical armouring and physical clogging, which are con-
sidered the major factors in reducing the efficiency of any
reactive material, were also studied by evaluating the dura-
tion of buffering periods for maintaining neutral pH and also
the changes in physical parameters (e.g. hydraulic conduc-
tivity and flow rate) due to mineral precipitation. Chemical
armouring by secondary Al- and Fe- precipitates decreased
the ANC of the recycled concrete by ~50% compared to its
theoretical ANC. Furthermore, high concentrations of Al
3+
and total Fe caused a rapid decrease in ANC efficiency due
to accelerated armouring. Application of larger size concrete
aggregates reduced the threat of physical clogging in the
pilot-scale PRB. Furthermore, mineralogical and morpho-
logical analysis was carried out to characterise the recycled
concrete used in the column experiments and the precipi-
tates formed. Correlation between CaO reduction in the
armoured concrete and the reduction in ANC validated the
decline in ANC by chemical armouring. 3D image analysis