Geotechnical News June 2011
61
THESIS ABSTRACTS
Analytical and Numerical Study of Soil
Disturbance Associated with the
Installation of Mandrel-Driven
Prefabricated Vertical Drains
Ali Ghandeharioon
Ali Ghandeharioon, Faculty of Engineering, University of Wollon-
gong, Wollongong, NSW 2522, Australia,
email:
Prefabricated vertical drains (PVDs) combined with preloading
have gained in popularity among the most effective ground improve-
ment techniques available to mitigate the unacceptable differential
settlements caused by the heterogeneity and high compressibility of
soft soil deposits. In this thesis the installation of mandrel-driven
PVDs and associated disturbance in cohesive soils were studied by
conducting analytical investigations, laboratory experiments, and
numerical modeling.
An analytical study of mandrel penetration in soft saturated clays
was carried out with a new elliptical cavity expansion theory (CET).
The elliptical CET was developed using modified Cam clay param-
eters to address the undrained analysis that accounts for the rate of
mandrel penetration and the time for predicting internal pressure
in the cavity, corresponding stresses and excess pore pressure in
the soil while driving the mandrel. A more realistic elliptical smear
zone based on the elliptical CET was introduced while the disturbed
soil surrounding the mandrel was characterized by the plastic shear
strain normalized by the rigidity index.
A number of large-scale laboratory tests that incorporated the
field conditions and effects of confining pressures were performed.
A consolidometer specifically designed for the purpose, and a ma-
chine capable of driving mandrels at realistic rates were used in
these experiments. The variations of pore water pressure during
installation of a mandrel-driven PVD were monitored. The smear
zone was then analyzed to establish a relationship between its size
and the in-situ effective stresses.
The installation of a mandrel was simulated numerically using
a commercial finite element software package, ABAQUS. The fi-
nite element models included coupled analyses with a large-strain
formulation. There was agreement between the pore pressures mea-
sured in the laboratory and the finite element predictions. The extent
of smear zone was studied numerically as well.
In addition, a number of case histories taken from Malaysia and
Thailand were analyzed to evaluate the associated soil disturbance
during installation of PVDs. The results of these analyses indicated
that the developments can be applied to field conditions.
Supervisors: Prof. Buddhima Indraratna, and Dr. Cholachat Rujiki-
atkamjorn, University of Wollongong
Cyclic Densification of Ballast and
Associated Deformation and Degradation
Pramod Kumar Thakur
Pramod Kumar Thakur, 6/111 Station Rd., Sunnybank, QLD, 4109,
Australia, Tel: +61 403 702 460,
email:
Railway ballast forms a major component of a conventional rail
track and is used to distribute the dynamic load to the subgrade, pro-
viding a smooth running surface for the train. Heavier cyclic load-
ings on an existing track are inevitable due to increased demand for
freight transport for mining and agriculture industries and for pub-
lic transport via trains due to increased fuel costs. This loading has
caused progressive deterioration and densification of ballast leading
to loss of track geometry and differential track settlement. Conse-
quently, the tracks require frequent maintenance. Understanding the
behaviour of ballast under increasing load conditions is imperative
for the optimum use of ballast and longevity of the maintenance
period. Therefore, this research aims to investigate the densification
and degradation behaviour of ballast under high frequency cyclic
loading.
A series of laboratory experiments were conducted using large-
scale cyclic triaxial equipment under high frequency loading. The
experimental results revealed that both the densification and break-
age of ballast increase with an increase in the frequency and number
of cycles. While the resilient modulus of the ballast was found to
decrease with increasing frequency, it increased with the increas-
ing number of cycles and confining pressures. The Discrete Element
Method (DEM) was employed to study the mechanism of particle
breakage at the particle scale level. The DEM-based software, Par-
ticle Flow Code in 2-Dimensions (PFC2D), was used to simulate
biaxial tests under cyclic loading. The DEM simulation results
revealed that particle breakage is a governing aspect of the actual
deformation behaviour of granular material. Moreover, the DEM
results confirmed that the particles break under tension and that this
breakage is mainly oriented and concentrated in the direction of par-
ticle movement. Finally, a cyclic densification model, based on a
critical state framework and calibrated and validated using labora-
tory experimental results, was proposed using the continuum me-
chanics approach incorporating particle breakage.
Supervisors: Dr. Jayan S. Vinod & Prof. Buddhima Indraratna,
Centre for Geomechanics and Railway Engineering, School of Civil,
Mining and Environmental Engineering, University of Wollongong
Factors Affecting the Capacity of Open and
Closed-Ended Piles in Clay
Paul Doherty
Paul Doherty, University College Dublin, Civil Engineering De-
partment, Newstead Building, Belfield, Dubin, Ireland,
Tel: 0035317163229, fax: 0035317163297,
email:
This thesis describes a series of pile tests conducted at a soft clay
test bed site in Belfast, Ireland. These field tests used both highly in-
strumented model piles, capable of measuring the effective stresses
in the surrounding soil, and full-scale uninstrumented concrete piles.
Two open-ended pile installations and one closed-ended installation
were performed, after which the piles were subjected to cyclic load-
ing and rapid static loading. In addition, two concrete piles installed
as part of a previous body of research were subjected to maintained
load tests following a 10 year ageing period. The testing schedule
was designed to assess the areas of uncertainties inherent to com-
mon offshore pile design practice in cohesive soils. These factors
included: the impact of end condition, cyclic loading, rate effects
and the impact of time on capacity.
An extensive review of the literature was undertaken which
confirmed that offshore pile design has undergone significant ad-
vances in recent years due to a transition from total stress to effec-
tive stress design methods. The improved reliability of recent de-
sign approaches has been confirmed through comparative database
studies. However the conditions offshore differ significantly from
the typical database piles. The databases largely comprise small di-
ameter closed-ended piles in contrast to the open-ended piles used
offshore, resulting in a potential bias due to the end condition. In ad-
dition, the databases are largely populated by maintained load tests
(MLT), whereas the in-situ loading conditions are primarily cyclic
and potentially rapid due to storm events with rise times in the or-
