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Observed Field Behaviour of Soft Clay
due to Embankment Loading: A Case
Study in Queensland, Australia
Author
Oh, Erwin, Surarak, Chanaton, Balasubramaniam, Bala, Huang, Min
Published
2007
Conference Title
Proceedings of the Sixteenth Southeast Asian Geotechnical Conference
Copyright Statement
Copyright 2007 Southeast Asian Geotechnical Society (SEAGS). Use hypertext link to access the
conference's webpage.
Downloaded from
http://hdl.handle.net/10072/19555
Link to published version
http://www.seags.ait.ac.th/conference.html
Observed Field Behaviour of Soft Clay Due to Embankment Loading:
A Case Study in Queensland, Australia
E. Y. N. Oh
Queensland Dept. of Main Roads, Herston, Queensland, Australia
[email protected]
C. Surarak1, A. S. Balasubramaniam2 & M. Huang3
Griffith School of Engineering, Griffith University, Gold Coast, Queensland, Australia.
2
3
1
[email protected], [email protected], [email protected]
Abstract: In this paper, the field behaviours of a trial embankment in Southeast Queensland are presented. The trial embankment is approximately 90m in length and 36m in width. It has two sections with vertical drains installed at a spacing of 1 m (Section A) and 2 m
(Section C) triangular pattern, and a third section (Section B) without drains (control section). This trial embankment was constructed
to observe the ground response upon loading, and evaluate the effectiveness of ground improvement techniques using vertical drains
on the soft clays in this region. This paper presents the findings obtained from the field observations during construction. Deformation
behaviour and pore pressure response below the instrumented embankment are investigated and the performance of vertical drains is
discussed. This paper gives a classic case study of a trial embankment built on soft clay in Southeast Queensland, Australia.
INTRODUCTION
This paper presents the laboratory results and field behaviour of
alluvial soft clay found in Sunshine Coast, Southeast Queensland. Soft clays are wide spread along the coastlines of Australia. They pose difficult problems in the design and construction
of roads, expressways and motorways. By definition, soft clays
are of low shear strength and high compressibility. Generally,
they are sensitive and their strength is readily reduced by disturbance during sampling and testing.
The test embankment presented in this paper was fully instrumented to measure the settlements, lateral movements and the
development of excess pore pressures and their dissipation with
time under the embankment load. Also, ground improvement
technique using prefabricated vertical drains (PVD) was also
evaluated for their potential applications
ter content of about 80% from 6.5 m to 12 m depth. As such the
weakest soft clay is encountered at 2 m - 6.5 m depth and this
layer is bound to be of low shear strength as revealed from the
natural water content. The liquidity index of the clay is higher
than 1.0 as the natural water content is higher than the liquid
limit. The plasticity index of the clay is uniform with depth and it
is about 40%.
PL, LL &
water content (%)
Soil Layer
0
40
2
12013
14
15
Vane Shear Strength
(kN/m2)
16 0
10
20
30
SOFT CLAY
4
6
8
LL
10
2
80
Unit Weight
(kN/m3)
0
Depth (m)
1
CLAYEY SAND
SITE DESCRIPTION AND SOIL CONDITIONS
Wn
PL
12
The trial embankment area had the deepest soft clay layers extending to approximately 11m depth. In this area, 33 boreholes
were drilled up to 30m depth. Besides the boreholes, standard
penetration tests (SPT), cone penetration tests (CPT), and field
shear vane tests were conducted.
The soil strata can be classified into several layers. Field testing has indicated a substantial deposit of very soft compressible
organic silty clay between 4m and 10m thick. This material is
underlain by a layer of very loose silty sand of approximately 2
m thick. This in turn is underlain by moderately dense to dense
sand (coffee rock) strata of 4 m to 6 m thick.
Fig.1 indicates typical sub-soil layers in the trial embankment
area. In this figure, silty clay (CH) is found up to about 8m depth
followed by clayey silt (MH), silty clay (CH) and clayey sand up
to 12m depth. The natural water content of these layers were substantially higher than the liquid limit and the highest water content of 120% is found at 2m - 6.5m depth, followed by lower wa-
Fig. 1 Index properties, unit weight and vane shear strength
The voids ratio-effective vertical stress relationships for these
tests are presented in Figs. 2(a), 2(b) and 2(c). From these voids
ratio-effective stress relations, values of the compression index
Cc, the coefficient of compressibility av, the coefficient of volume decrease mv, the constraint modulus D and the coefficient of
consolidation, Cv were established as a function of the effective
vertical stress. The compression indexes for all samples (see Fig.
3) were found to increase with effective vertical stress initially
and thereafter, it remains constant. Depending on the initial water
content of the samples, the Cc vales generally ranged from 0.5 to
1.0 at stresses in the normally consolidated range. The exception
was the sample taken at depth of 8.2 m to 9.0m, which indicated
a Cc value of 1.5 in the normally consolidated range of stress levels.
3.50
3.0
3.00
2.5
2.00
B H 19 Depth 4.0 - 4.8m
B H20 Depth 3.4 - 4.2m
B H 20 Depth 8.2 - 9.0m
B H22 Depth 6.8 - 7.2m
B H 22 Depth 2.7 - 3.2m
B H22 Depth 8.2 - 9.0m
2.0
1.50
Cc
Voids Ratio
2.50
B H 19 Depth 0.8 - 1.6m
B H 20 Depth 1.8 - 2.6m
1.5
1.00
0.50
1.0
BH19: Depth 0.8 - 1.6 m
BH19: Depth 4.0 - 4.8 m
0.00
0.10
0.5
1.00
10.00
100.00
1000.00
Effective Stress (kN/m2)
0.0
0
50
7
6
2.00
m v (m 2/MN)
Voids Ratio
2.50
1.50
BH20: Depth 1.8 - 2.6 m
250
B H 19 Depth 0.8 - 1.6m
B H 19 Depth 4.0 - 4.8m
B H 20 Depth 1.8 - 2.6m
B H 20 Depth 3.4 - 4.2m
B H 20 Depth 8.2 - 9.0m
B H 22 Depth 2.7 - 3.2m
B H 22 Depth 6.8 - 7.2m
B H 22 Depth 8.2 - 9.0m
5
4
3
2
BH20: Depth 3.4 - 4.2 m
1
BH20: Depth 8.2 - 9.0 m
0.00
0.10
200
8
3.00
0.50
150
Fig. 3 Variation of compression index, CC, with effective vertical
stress, σ v′
3.50
1.00
100
Effective Vertical Stress, σ' v (kN/m 2)
Fig. 2(a) Voids ratio – effective vertical stress for BH19
0
1.00
10.00
100.00
0
1000.00
50
100
150
200
250
Effective Vertical Stress, σ'v (kN/m2)
2
Effective Stress (kN/m )
Fig. 4 Variation of coefficient of volume decrease, mv, with effective vertical stress, σ v′
Fig. 2(b) Voids ratio – effective vertical stress for BH20
3.50
3
3.00
The test embankment is approximately 90m in length and 36m in
width. Vertical drains were installed with a crawler-mounted machine from the working platform. Section A and C of the test
embankment are the prime sections representing the design vertical drain spacing (1m of triangular pattern) and no drains respectively. Section C representing an intermediate case with 2m triangular pattern drain spacing and less instruments.
Voids Ratio
2.50
2.00
1.50
1.00
0.50
BH22: Depth 2.7 - 3.2 m
BH22: Depth 6.8 - 7.2 m
3.1 Instrumentation in Test Embankment
BH22: Depth 8.2 - 9.0 m
0.00
0.10
TEST EMBANKMENT
1.00
10.00
100.00
1000.00
Effective Stress (kN/m2)
Fig. 2(c) Voids ratio – effective vertical stress for BH22
Fig. 4 illustrates the variation of the coefficient of volume decrease mv, with increase in effective vertical stress. The mv values at the lower stress range seem to have a large variation from
1 to 5 MN/m2, but as the effective stress increases from 50 to 100
kN/m2, the mv values ranged from 1 to 3 MN/m2 for most samples. In this figure, the mv value of 2.2 MN/m2 obtained from
back-calculation based on the centreline settlement of Section A
of the test embankment is also shown. This will be discussed at a
latter section under the interpretations of settlements below the
test embankment.
The details of the cross sections and the instrumentations used
are shown in Figs. 5, 6 and 7. It is noted that sensitive soft silty
clay extends to about 10m depth followed by a 6 m thick layer of
sand and below this is a 2m thick layer of sensitive clay which
again is followed by sand.
3.2 Interpretation of Settlement Data
Horizontal profile gauges were installed in Section A and Section
B to monitor the settlements across the sections with varying
time period ranging from one day to 724 days. These hydraulic
profile gauge data are presented in Fig. 8 for Section A and Section B, and they were interpreted extensively.
In the interpretation of these hydraulic profile gauges, there
are 2 observations. Firstly, Section A with PVD, showed higher
settlement at any time when compared with Section B without
PVD. This is illustrated in Fig. 8 for time period of approximately, 1 day, 62 days, 93 days and 533 days. At all times, Sec-
tion A experienced higher settlement than Section B. Also individual settlement–log time plots are plotted for all sections show
that Section A experienced greater settlement than Section B.
Fig. 5 Cross section showing instrumentation layout plan for Section A (PVD at 1m spacing)
Fig. 6 Cross section showing instrumentation layout plan for Section B (control embankment)
Fig. 7 Cross section showing instrumentation layout plan for Section C (PVD at 2m spacing)
Distance (m)
0
5
10
15
20
25
30
35
40
45
0
Settlement (mm)
-200
-400
-600
Sec A: Day 1
Sec A: Day 62
Sec A: Day 93
Sec A: Day 533
Sec B: Day 1
Sec B: Day 64
Sec B: Day 95
Sec B: Day 536
-800
-1000
-1200
-1400
-1600
-1800
Fig. 8 Surface settlements from horizontal profile gauge in Section A and Section B
Time (Day)
1
10
100
1000
10000
0
-200
Settlement (mm)
-400
-600
-800
cate that the section to the right of the centre line were unable to
achieve 100% primary consolidation even with the use of PVD in
closer spacing of 1.0m. This observation is rather difficult to
comprehend.
The above observation was established systematically, by
plotting the settlement-log time plot of each location separately
to the left and to the right of the centre line of Section A. The settlements for the centre line and the locations to the left are shown
in Fig. 9; those corresponding to the right are shown in Fig. 10.
The location marked EE corresponds to the centreline of the embankment. The locations DD, CC, BB and AA were taken from
the centre line to the left at distances of 5m, 10m, 15m and 20m
respectively. Similarly the locations FF, GG and HH were on the
right hand side and at distances of 5m, 10m and 15m respectively
away from the centreline.
The settlement-log time plot of each separate location to the
left and to the right of the centre line of Section B is given in Fig.
11.
When the settlement-log time plot do not show an S curve,
and where the Casagrande method cannot be applied to estimate
the 100% primary consolidation, the Aasoka (1978) method was
used to estimate the 100% primary consolidation. It was observed later that the Asaoka Method at times give lower values
for the ultimate settlement.
Time (Day)
-1000
1
Location AA
-1200
10
100
1000
0
Location BB
-1400
Location CC
-200
Location DD
-1600
-400
Location EE (CL)
Fig. 9 Variation of settlement with time along the centerline and
the locations to the left in Section A (100% Consolidation Completed)
Time (Day)
1
10
100
1000
10000
0
Settlement (mm)
-1800
-600
-800
-1000
Location AA
Location BB
Location CC
-1200
-200
-1400
-400
-1600
-600
-1800
Location DD
Location EE (CL)
Location FF
Location GG
Settlement (mm
Location HH
Fig. 11 Variation of settlement with time in Section B
-800
-1000
-1200
-1400
-1600
-1800
Location FF
Location GG
Location HH
-2000
Fig. 10 Variation of settlement with time at the locations to the
right of centerline in Section A (100% Consolidation Not Completed)
Secondly, Section A with the PVD, the settlement-log time
plots indicated that 100% primary consolidation is over virtually
for all sections, to the left of the centre line. But these plots indi-
Tables 1 and 2 summarise the 100% settlement estimated from
the Casagrande and Asaoka methods for Section A and Section B
respectively. These values indicate that even Section B is having
proportionately higher settlements, even though no PVD were
used. The major reason for this was perhaps due to the fact that
the PVD Section A and Section C are on either side of the section
B which have no PVD. Earlier work carried out at other sites in
Southeast Asia and elsewhere indicated a better arrangement
would have been to separately locate the PVD Sections and the
no-PVD Section so that there is no interference effect. By not doing so at the test embankment presented here, the lateral drainage
from the Section with no PVD (Section B) and through silt and
sand lenses to the PVD in the drained sections have possibly occurred.
3.3 Lateral Deformation from Inclinometer
Lateral deformation profiles were determined at locations XX
and YY (see Figs. 5, 6 and 7). However, only in Section A and
Section B inclinometer casings were installed at the location XX.
While for Section C, as well as for Section A and B, inclinometer
casings were installed in YY. Fig. 12 illustrates the lateral deformation profiles for Section A and Section B at location XX after one day, 62 days, 93 days and 526 days. Initially, Section A
with PVD was found to develop more lateral deformation. Perhaps this may be due to the disturbance created by the
installation of PVD. However at 526 days time both Section A
and Section B have similar lateral deformation profiles at XX.
PVA 14. These measurements are in accordance with the excess
stress at these points due to the embankment loading.
For Section B, excess pore pressures are plotted in Fig. 15
along the centre line as indicated by piezometers PPB21 and
PPB23 located at 4.5m and 9.5m depth respectively. Both piezometers indicate similar development of excess pore pressures
and also dissipation pattern. Unlike the piezometers in Section A
with PVD, the piezometers PPB21 and PPB 23 in Section B with
no PVD did not indicate faster dissipation of excess pore pressures.
Lateral Displacement (mm)
-40
60
160
260
360
460
0
Table 1 Ultimate settlement (100 percent consolidation settlement) in Section A
Asaoka’s Method
Location
Casagrande’s
100 % Settlement
Method
(mm)
100 % Settlement (mm)
1570
CL
1320
5m Left from CL
885
10m Left from CL
380
15m Left from CL
20m Left from CL
1506
5m Right from CL
1316
10m Right from CL
970
15m Right from CL
Table 2 Ultimate settlement (100 percent consolidation settlement) in Section B
Asaoka’s Method
Location
Casagrande’s
100 pc Settlement
Method
(mm)
100 pc Settlement (mm)
-
Depth (m)
10
15
Sec A: Day 1
Sec B: Day 1
20
Sec A: Day 62
Sec B: Day 62
25
Fig. 12(a) Variation of lateral displacements in Section A and
Section B (before end of construction) at location XX
Lateral Displacement (mm)
-40
60
160
260
1200
1050
480
290
1200
1060
830
460
5
10
15
Sec A: Day 93
Sec B: Day 93
20
Sec A: Day 526
3.4 Excess Pore Pressure Development and Dissipation
Sec B: Day 526
25
Fig. 12(b) Variation of lateral displacements in Section A and
Section B (after end of construction) at location XX
Time (Day)
1
Embankment Loading (kN/m2)
The locations of the piezometers in Section A, Section B and
Section C were shown in Figs. 5, 6, and 7. It should be noted that
the designation PV is used for the vibrating wire piezometers and
the symbol PVA means the vibrating wire piezometer PV in Section A. Similarly, the symbol PP means pneumatic piezometer,
and thus PPA refers to the pneumatic piezometer in Section A.
Thus the last symbol of each piezometer designation indicates the
relevant section in which the piezometer is located.
The excess pore pressures were determined along three alignments in Section A. The construction sequence adopted is shown
in Fig. 13. The excess pore pressures as indicated by piezometers PVA4, PVA10 and PVA 14 are shown in Fig. 14. These piezometers are located around 5.5m depth and PVA4 is along the
centre line of the Section, while PVA10 is along the location XX
and PVA 14 is between locations XX and YY. It is noted that the
piezometer PVA4 along the centre line indicates maximum excess pore pressures and this is followed by piezometer PVA 10
and the least excess pore pressure was indicated by piezometer
360
0
Depth (m)
CL
5m Left from CL
10m Left from CL
15m Left from CL
20m Left from CL
5m Right from CL
10m Right from CL
15m Right from CL
5
10
100
1000
10000
60
50
40
30
20
10
0
Fig. 13 Embankment loading (kN/m2) with time (days) in Section
A
50
40
40
Excess Pore Pressure (kN/m2)
Excess Pore Pressure (kN/m2)
50
30
20
20
10
0
0
10
100
Time (Day)
1000
1
10000
10
100
Time (Day)
1000
10000
Fig. 16 Variation of excess pore pressure with time in Section C
(Piezometers PPVC39 and PPC40)
- PVA4 (5.5m, CL)
’ PVA10 (5.5m, Cross Section XX)
+ PVA 14 (5.5m, Betwee Cross Section XX and YY)
Fig. 14 Variation of excess pore pressure with time in Section A
(Piezometers PVA4, PVA10 and PVA14)
50
4
CONCLUDING REMARKS
This paper presents the laboratory and field behaviour of soft
clay deposit at a test site in Southeast Queensland, as well as the
behaviour of embankment on soft ground constructed with and
without ground improvement. From the different analyses performed in this study, the following conclusions can be made:
- PPB21 (4.5m, CL)
# PPB23 (9.5m, CL)
Excess Pore Pressure (kN/m2)
30
10
1
- PVC39 (7m, CL)
’ PPC40 (10m, CL)
40
1. The laboratory results indicated that soft clays deposit in the
studied areas is very soft and highly compressible. The under
lying soils below the trial embankment can be considered as
normally to slightly overconsolidated soil.
30
2. Maximum lateral displacement of the order of 400mm was
observed and the lateral displacement is contained in the upper 8 – 10m of soft silty clay, which is susceptible to shear
failure
20
10
3. The pore pressure dissipation indicated that the settlement
measured is largely of the consolidation type.
0
1
10
100
Time (Day)
1000
10000
Fig. 15 Variation of excess pore pressure with time in Section B
(Piezometers PPB21 and PPB23)
For Section C, the excess pore pressures and their dissipation
with time are shown in Fig. 16 based on the excess pore pressures measured by piezometers PVC39 and PVC40 located at
depths of 7m and 10m respectively are shown.
The pore pressure dissipation in Section A with closer PVD is
faster than the corresponding dissipation in Section C with wider
PVD spacing and this again is faster than Section B with no
PVD. The minimum acceptable drain spacing as reported elsewhere is also in the range of 1.0 m; this is to prevent excessive
smearing effects at very close spacing lesser than 1.0m.
Due to space limitations, finite element analysis and creep effects have not been discussed and will be reported elsewhere.
ACKNOWLEDGMENTS
The authors wish to express appreciation to Geotechnical Branch
of Queensland Department of Main Roads, for their support in
providing the soil samples, and the permission to use the data
presented in this paper.
REFERENCES
Asaoka, A., (1978). Observation procedure of settlement prediction. Soils and Foundations, Vol. 18, No. 4, pp. 87-101.