2942
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
used for the recent dredged mud fill is given. In addition,
laboratory tests results are discussed which were conducted on
reconstituted dredged mud specimens prepared simulating the
sedimentation and consolidation process at the reclamation site.
Figure 1: Aerial view of land reclamation site at Port of Brisbane,
Queensland, Australia
2 SITE CONDTIONS
The Holocene clay layers include upper and lower Holocene
clay layers. The upper Holocene layer consists of sand layers
with interspersed soft clays and silts, thus the pore water
pressure dissipation and the settlement rate is relatively fast.
The lower Holocene clay layer, where the sand and silt layers
are relatively few, controls the rate of settlement at the site
because of its large compressible thickness. As both the in-situ
Holocene clays and dredged mud fill are highly compressible,
settlement due to filling alone could be as high as 2 m even
before any service loads are imposed. It is predicted that it
would take as much as 50 years for the area to be consolidated
considering preloading as the only soil treatment option.
Therefore vertical drains are incorporated to speed up the
consolidation process. Ground improvement by combined
preloading and vertical drains is designed to accelerate the
majority of expected primary settlement and limit the long term
postconstruction settlement. According to the design
requirement of the Port of Brisbane, the long term residual
settlement should not exceed 150 mm over a period of 20 years
for applied pressures up to 50 - 60 kPa in areas where the
Holocene clay thickness is less but the settement limit is greater
for the deeper Holocene clay areas (Ameratunga et al. 2010a).
Maximum vertical stress exerted under the development
loads (i.e buildings, traffic) can vary over the site between 15
kPa and 60 kPa depending on the different purposes the land
would be used. In addition, the total thickness of compressible
clays is variable over the site. The above two factors decide the
amount of preloading to be applied at the ground level.
Preloading is applied by both sand capping and vacuum
preloading. Thickness of sand capping layer varies from 6 m to
9 m across the site. Initially, a number of vacuum trials was
conducted at several test sites within the reclamation area itself
in order to assess the effectiveness of wick drains to be used as
a ground improvement measure (Ameratunga et al. 2010b)
The subsoil layers at the reclamation site is subjected to a
preloading higher than the expected post construction design
load, so that the underlying soil will generally be in an over
consolidated state under design loads. In the over consolidated
stage (recompression range), the settlements in both the primary
consolidation and secondary compression range are
significantly less than in the normally consolidated stage, which
will be discussed later.
2.1 Design Parameters of dredged mud fill
The design strength and consolidation parameters of dredged
mud fill used at the site are estimated from both in-situ and
laboratory tests. In the absence of the above, correlations with
physical properties of the clays are used for the preliminary
assessment of properties. Atterberg limit values of dredged mud
are in the range of 80-85% (Liquid limit- LL), 34-37% (Plastic
limit- PL), 18-19% (Linear shrinkage- LS) and 44-46%
(Plasticity Index- PI). The major constituents in the PoB
dredged mud are 50% clay and 40% silt. From the Atterberg
limits and particle size distribution, the PoB dredged mud can
be classified as high plasticity clayey soil.
The undrained shear strength of recent dredged fill is
evaluated from in situ vane shear and piezocone (CPTu)
dissipation tests. In some instances, the shear strength
parameters are estimated from the following empirical
correlations incorporating PI values.
c
u
/ σ
v
´
= 0.11 + 0.0037 * PI
(1)
Sin ø
´
= 0.8 - 0.094 * ln (PI)
(2)
Brisbane River
Containment paddocks
Containment bunds
Seawall
where
c
u
and
ø
’
are the undrained shear strength and drained
friction angle respectively.
Piezocone dissipation test results are also used to estimate
the consolidation properties such as coefficient of consolidation
c
v
or
c
h
and approximate permeability. The
c
v
values calculated
from the in situ tests are verified from the back calculations
using Asaoka’s method from field monitoring. A
c
v
value of 1
m
2
/yr is used for the dredged mud fill and the coefficient of
consolidation in the vertical and horizontal directions (
c
v
and
c
h
)
are assumed to be equal.
At the PoB reclamation site, there are insufficient records yet
for long time period settlements, thus the secondary
compression parameters are estimated only from the laboratory
tests and correlations. The subsoil is subjected to preloading
higher than the expected post construction design load. As a
result, the underlying soil will generally be in an over
consolidated stage under design loads. The coefficient of
secondary compression
C
αe
depends on the over consolidation
ratio (
OCR
), and it drops quickly with a small increment in the
OCR
ratio (Ameratunga et al. 2010b; Alonso et al. 2000; Wong,
2007). For the reduction of
C
αe
with the
OCR
the following
exponential law has been adopted (Eq.3).
C
αe
(OC) /C
αe
(NC) = [(1-m)/e
(OCR-1)n
]+m
(3)
m
is taken as 0.1, which is equivalent to ratio of
C
r
/C
c
(Mesri,1991) and
n
is equal to 6. At the PoB reclamation site
the underlying soil is generally over consolidated to an
OCR
ratio of 1.1-1.2. An average value of 0.008 was adopted for
design
C
αe
.
The design compression ratio
CR
, given by
C
c
/(1+
e
o
) (
e
o
-
Initial void ratio), is taken as 0.2 to 0.3 based on laboratory
tests. Recompression ratio
RR
(=
C
r
/1+
e
o
) is generally taken as
0.1 times the compression ratio.
3 LABORATORY TESTS
The sedimentation and consolidation of dredged mud were
simulated in the laboratory using the dredged mud samples
obtained from the PoB reclamation site. The objective of the
laboratory tests is to evaluate some of the consolidation
parameters (
c
v
and
c
h
) and compressibility properties (
CR
and
RR
) of reconstituted dredged mud sediment and make
comparison with the design values. In addition, potential
anisotropy that can exist between the horizontal and vertical
coefficients of consolidation and permeability was investigated.
Series of oedometer tests were conducted in the present
laboratory studies.
