2576
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
0
50
100
150
200
250
300
0
20
40
60
80
Settlement(mm)
Time (Days)
Kh/Ks=2.1 & rs/rm=2 (S1)
Kh/Ks=2 & rs/rm=3 (S2)
Kh/Ks=1.85 & rs/rm=4 (S3)
Kh/Ks=1.75 & rs/rm=5 (S4)
(a)
0
10
20
30
40
50
60
0
20
40
60
80
Excesspore waterpressure(kPa)
Time(Days)
Kh/Ks=2.10 & rs/rm=2 (S1)
Kh/Ks=2 & rs/rm=3 (S2)
Kh/Ks=1.85 & rs/rm=4 (S3)
Kh/Ks=1.75 & rs/rm=5 (S4)
(b)
1.0
2.0
3.0
4.0
5.0
32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70
Permeability ratio (k
h
/k
s
)
Time (Days)
r
s
/r
m
=2 r
s
/r
m
=3
r
s
/r
m
=5
r
s
/r
m
=4
S1
S3 S2
S4
The required time to obtain 90% degree of consolidation for
different smear zone properties is illustrated in Figure 9 using
parametric study results, which presents a better interpretation
of the effects of the smear zone properties on consolidation
time. According to Figure 9, the consolidation time significantly
depends on the smear zone permeability and extent. For
example, assuming r
s
/r
m
=2, for the case with k
h
/k
s
=2 and
k
h
/k
s
=5, the required times to obtain 90% degree of
consolidation are approximately 33 days and 53 days,
respectively, indicating 60% difference. It can be noted that the
difference is more significant for larger values of r
s
/r
m
.
Figure 9. Predicted time to obtain 90% degree of consolidation
Figure 9 clearly indicates that the smear zone extent ratio (r
s
/r
m
)
is an important parameter influencing the consolidation time
and cannot be neglected. Varying r
s
/r
m
in the range of 2 to 5,
assuming k
h
/k
s
as a constant parameter can influence the
required consolidation time by more than 25%. Combined
effects of uncertainties in the smear zone extent and
permeability will result in momentous changes of consolidation
time. Results presented in Figure 9 indicate that the influence of
uncertainties in r
s
/r
m
becomes more important when
permeability of smear zone decreases.
According to the back calculation results presented in
Figure 6, the predicted settlement curve is in the best agreement
with the field measurements considering smear zone properties
of k
h
/k
s
=2 and r
s
/r
m
=3. The required time to obtain 90% degree
of consolidation for this condition is equal to 34 days, which is
highlighted as point S2 in Figure 9. A vertical line is plotted
from t
90%
= 34 days, which intersects the set of lines at points
S1, S2, S3 and S4. Smear zone properties at these points are
summarised in Table 2.
Table 2. Back calculated smear zone properties to achieve t
90%
= 34 days
Point
S1 S2 S3
S4
k
h
/k
s
2.10 2.0 1.85 1.75
r
s
/r
m
2
3
4
5
Numerical analyses applying developed FLAC code have been
conducted to compare the settlement and excess pore water
pressure variations against the consolidation time. Different
combinations of smear zone extent and permeability may result
in the same t
90%
= 34 days and predictions are presented in
Figure 10.
Figure 10. FLAC analysis results for points in Table 2 (a) Settlement
variation; (b) Excess pore water pressure dissipation
Figure 10 shows that the curves for the settlement variations
and the excess pore water pressure dissipations with time follow
the same trend for points S1, S2, S3 and S4. Therefore, smear
zone properties of any of these points can be adopted for the
practical design purposes.
4 CONCLUSIONS
Preloading time during consolidation process can significantly
be affected by formation of the smear zone in the vicinity of the
prefabricated vertical drains (PVDs). Smear zone is a reduced
permeability area induced by mandrel insertion that halts the
consolidation process. Available literature proposes a wide
range for the smear zone extent and permeability and yet there
is no definite prediction method that can be used to estimate the
extent of smear zone and its permeability to be used in the
design procedure. In this study, numerical analyses have been
employed to investigate the effects of uncertainties of smear
zone characteristics on the preloading design. FLAC 2D
software has been employed to develop a numerical code
assisting with the parametric study and back calculating smear
zone properties. The verification exercise on Chittagong port
case history confirms the validity of the developed numerical
code. According to the parametric study results the properties of
the smear zone have key roles on the required consolidation
time to achieve a certain soil strength and stiffness satisfying
both bearing capacity and settlement design criteria. Therefore,
accurate estimation of the properties of smear zone based on the
soil type and the installation method is vital for the ground
improvement projects adopting PVD assisted preloading.
Results of this study indicate that assumptive properties for
smear zone characteristics may result in inaccurate predictions
of ground deformations and pore water pressures. This can lead
to early removal of surcharge in construction process resulting
excessive post construction settlement. Thus, it is recommended
to practising geotechnical engineers to back calculate the smear
zone properties using a trial construction similar to the future
construction procedure.
5 REFERENCES
Dhar, A. S., Siddique, A. and Ameen, S. F. 2011. Ground improvement
using pre-loading with prefabricated vertical drains.
International
Journal of Geoengineering Case Histories
, 2(2), 86-104.
Holtz, R.D., Jamiolkowski, M.B., Lancellotta, R. and Pedroni, R. 1991.
Prefabricated vertical drains: design and performance.
CIRIA,
Butterworth-Heinemann
, London.
Indraratna, B. and Redana, I. W. 2000. Numerical modeling of vertical
drains with smear and well resistance installed in soft clay.
Canadian Geotechnical Journal
, 37(1), 132-145.
Indraratna, B., Rujikiatkamjorn, C. and Sathananthan, I. 2005.
Analytical and numerical solutions for a single vertical drain
including the effects of vacuum preloading.
Canadian Geotechnical
Journal
, 42(4), 994-1014.
Shang, J.Q., Tang, M. and Miao, Z. 1998. Vacuum preloading
consolidation of reclaimed land: a case study.
Canadian Geotechnical
Journal
, 35(5), 740-749.
