2498
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
the holocene clays were from 5 to 60 kpa. the compression
index changed from 0.1 to 1.0. the coefficient of consolidation
in vertical direction is similar to that in horizontal direction (
c
h
)
for the remoulded dredged mud layer, while
c
v
/c
h
is about 2 for
the holocene clay layer.
WD5A WD5B
WD1
VC2
WD4
WD2
WD3
VC1
155m
35m
70m 41m
84.5m
84.5m
70m
70m
50m
169m
210m
MS15-1
MS17-1
MS18-1
MS16-1
MS22-1 WD5B
VC1-2
VC2-1
Surface settlement plates
Piezometers
VWP2-WD1
VWP1-WD2
VWP4-WD4
MS20-VWP5
MS19-
VWP5
MS28-VC1
MS27-
WD3
Inclinometers
figure 1. site layout for s3a with instrumentation plan
the surcharge preloading system was adopted for the inner
areas (Wd1-Wd5) while, in the outer area (Vc1 and Vc2)
close to the marine park, the technique of vacuum combined
preloading was selected to control lateral displacement in order
to minimise disturbance of the nearby marine habitats. stringent
design criteria were adopted for the design and construction of
emabankment over the soft holocene deposits: (a) service load
of 15-25 kpa, (b) maximum residual settlement less than 250
mm over 20 years after treatment. the surcharge embankment
heights varied from 3.0m to 9.0m. Based on the design criteria,
table 1 presents the pVd characteristics and treatment types
applied to each section. in the non-vacuum areas, both circular
and band shape drains were established in a square pattern at a
spacing in the range of 1.1-1.3m. the length of pVds changed
from 6m to 27.5m across the site as shown in the table 1.
table 1. pVd characteristics and improvement scheme
drain type
drain
spacing fill height
(m)
treatment
scheme
Wd1
circular
drains
1.1
5.2
surcharge
Wd2
circular
drains
1.3
7-7.2
surcharge
Wd3
Band drain
type -a
1.1
4.3-4.6
surcharge
Wd4
Band drains
type -a
1.3
6.1
surcharge
Wd5a
Band drains
type -B
1.2
3.3
surcharge
Wd5B
Band drains
type -B
1.1
5.5
surcharge
Vc1
circular
drains
1.2
3.2
surcharge+
vacuum
Vc2
circular
drains
1.2
2.8
surcharge+
vacuum
the inevitable variation in drain lengths was attributed to
the non-uniform clay thickness. Wick drains (Band drain type-
a and Band drain type-B) had dimensions of 100mm x 4mm,
and the circular drains had an internal diameter of 34mm. the
authors have deliberately omitted the commercial brand names
of all pVds used. to monitor the ground behaviour,
comprehensive instruments were installed e.g. settlement plates,
vibrating wire piezometers, magnetic extensometers, and
inclinometers. in the vacuum area, only circular drains were
employed at a spacing of 1.2m in conjunction with a high
density polyethylene (hdp) membrane, horizontal perforated
pipes and the pumps that represent the vacuum system. the
horizontally pipes offered the desired uniform distribution of
suction beneath the membrane. the measured suction varied
from 60 kpa to 75 kpa, and no air leaks were observed during
vacuum application that ensured the intact seal provided by the
membrane. a vacuum pressure of 70kpa was applied after 40
days.
3 interpretation of field resUlts
the embankment performances including settlements and
excess pore pressures together with the staged construction of
the embankments are depicted in fig. 2. it would be observed
that the trends are very comparable where the settlement
occured more quickly at the early stage of consolidation. the
amount of final settlement depends on the clay thickness and
embankment height. the highest settlement was measured in
the Wd4 area having the greatest clay thickness (19-26m),
whereas the lowest settlement was in the Wd5a area in which
the clay layer was relatively thin (8-12m).
100
200
300
400
2
1.5
1
0.5
Vertical displacement
(m)
WD3
WD5A
WD5B
VC1
VC2
100
200
300
400
0
2
4
6
8
Embankment Height (m)
100
200
300
400
Time (days)
-100
0
100
200
Excess pore pressure (kPa)
(a)
(b)
(c)
figure 2. embankment responses (a) staged construction, (b)
settlements and (c) excess pore pressures
the measured lateral displacement normalized to total
change in applied stress (vacuum plus surcharge load) for two
inclinometer locations is shown in fig. 3. for Wd3 area, the
total surcharge height was 90 kpa, whereas for Vc1 area the
reduced surcharge pressure of 40 kpa was complemented with a
vacuum pressure of 65 kpa. the lateral displacements clearly
lessen in the holocene sand due to its greater stiffness. fig. 3
indicates that the lateral movements are effectively controlled to
minimise the disturbance in the adjacent moreton Bay marine
park, due to the isotropic consolidation by vacuum pressure.
4 settlement and eXcess pore pressUre
predictions
in order to analyse the radial consolidation caused by vertical
drains, the unit cell theory has been employed to predict the
settlement and excess pore pressure. a unit cell theory was
introduced by Barron (1948) and richart (1957) for surcharge
preloading alone. lekha et al. (1998) further extended the radial
consolidation by including time-dependent surcharge loading.
indraratna et al. (2005) introduced the unit cell analysis for
vacuum preloading under instantaneous loading while Geng et
al. (2012) proposed analytical solutions under time-dependent
