2497
Performance and Prediction of Vacuum Consolidation Behavior at Port of Brisbane
Avantages et prédictions de comportement due a la consolidation sous vide au port de Brisbane
Indraratna B., Rujikiatkamjorn C., Geng X.
Centre for Geomechanics and Railway Engineering, University of Wollongong, Wollongong City, NSW Australia, ARC
Centre of Excellence in Geotechnical Science and Engineering, Australia
Ameratunga J.
Coffey Geotechnics, 47 Doggett Street, Newstead, QLD. 4006, Australia
aBstract: due to a projected increase in trade activities at the port of Brisbane, new berths on fisherman islands at the mouth of
the Brisbane river will be constructed in the outer area (235ha) close to the existing port facilities via land reclamation. a vacuum
assisted surcharge load in conjunction with prefabricated vertical drains was choosen to reduce the required consolidation time. the
features of the combined vacuum and surcharge fill system and the construction of the embankment are described in this paper. a
comparison of the performance of the vacuum combined surcharge loading system with a standard surcharge fill emphasizes the
obvious advantages of vacuum consolidation. field data is presented to show how the embankment performed during construction.
an analytical solution for radial consolidation incorporating both time-dependent surcharge loading and vacuum pressure is
employed to calculate the settlements and associated excess pore pressures of the soft holocene clay deposits.
rÉsUmÉ : l’augmentation des activités de commerce au port de Brisbane nécessite la construction, à proximité des terminaux
existants, de nouveaux postes de quais dans les iles fisherman a l’embouchure de la rivière de Brisbane sur une superficie de 235 ha
gagnée sur la mer. Un chargement sous vide contrôle, associe a des drains préfabriqués, a été appliqué pour réduire le temps de
consolidation. l’article décrit les caractéristiques de la technique de consolidation sous vide associe au chargement par remblaiement
et la construction du remblai. Une comparaison entre la consolidation sous vide associée au remblaiement et le pré chargement
classique montre clairement les avantages en faveur de la consolidation sous vide. les données enregistrées sur le site illustrent le
comportement du remblai durant la consolidation. Une solution de consolidation horizontale tenant compte du chargement et de la
pression sous vide est présentée en vue de prédire le tassement et l’excès de la surpression interstitielle du dépôt d’argile molle de
l’holocène.
KeYWords: consolidation, soil improvement, vertical drains, vacuum.
1 introdUction
the port of Brisbane is one of the australia’s largest
commercial ports located at the entrance of the Brisbane river
at fisherman islands. With demand in commercial activities, a
new outer area (235ha) is being reclaimed for major expansion
to maximise the available land, and to provide the maximum
number of berths suitable for container handling for servicing
regional importers and exporters. in this area, the soil profile
mainly consists of compressible clay deposits over 30m in
thickness with very low undrained shear strength (<15 kpa at
shallow depth). the strength of dredged mud had a much lower
strength depending on the placement time and the thickness of
capping material. Without surcharge preloading, it is estimated
that the consolidation time could be more than 50 years with
overall settlements of 2.5-4.0m. therefore, vacuum
consolidation with prefabricated vertical drains (pVds) was
suggested to accelerate the consolidation process and to
minimise lateral deformation adjacent to the moreton Bay
marine park (indraratna et al. 2011).
the effectiveness of the vacuum preloading assisted by
pVds has been illustrated by chu et al. (2000) and chai et al.
(2005). in this technique, vacuum pressure can propagate to a
greater depth of the subsoil via pVd length. also, extended
consolidation time due to stage construction can be minimized
(indraratna et al. 2005). the surcharge fill height can be reduced
by several metres, if a vacuum pressure (at least 70 kpa) is
applied and sustained (rujikiatkamjorn et al. 2008). the
embankment construction rate can be increased and the number
of construction stages can be reduced (Yan and chu 2003).
once the soil has increased its stiffness and shear strength due
to consolidation, the post-construction settlement will be
significantly less, thereby eliminating any risk of differential
settlement of the overlying infrastructure (shang et al. 1998).
to the authors’ knowledge, there is no comprehensively
reported case history where both the conventional surcharge
preloading and vacuum technique have been applied in the same
area with distinct variation of drain types and spacing.
in this paper, the performance between the vacuum and
non-vacuum areas has been compared based on the measured
settlements, excess pore pressures and lateral displacements.
the influences of drain spacing, drain types and type of soil
improvement are discussed based on the observed degree of
consolidation. the analytical solutions for radial consolidation
considering both time dependent surcharge loading and vacuum
pressure are proposed to predict the settlement and associated
excess pore pressure.
2 General description of emBanKment
characteristics and site conditions
at the port of Brisbane, to evaluate the performance of the
vacuum consolidation system with the non-vacuum system
(pVd and surcharge load), a trial area (s3a) shown in fig. 1
was partitioned into Wd1-Wd5 (non-vacuum areas) and Vc1-
Vc2 (Vacuum areas). after placing the dredged fill, the mud
was capped off with a 2-3m layer of dredged sand, which acted
as a working platform for pVd installation machine, whilst
serving as a drainage layer.
the upper holocene sand beneath the reclaimed dredged
mud was about 2m thick, followed by the holocene clay layer
with different in thickness from 6m to 25m. a pleistocene
deposit containing highly over-consolidated clay underlies the
softer holocene clay layer. site investigation techniques
including cone penetration/piezocone tests, dissipation tests,
boreholes, field vane shear tests and oedometer tests were
carried out to assess the relevant consolidation and stability
design parameters. the water contents of the soil layers were
similar to or exceed their liquid limits. the vane tests show that
the undrained shear strength of the reclaimed dredged mud and
