440
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
From comparison between the Osaka Bay clay and Pisa clay,
one of the main reasons of those different long-term
consolidation tendencies seems to be the existence of
microfossils, which are found abundantly in Osaka Bay clay,
but barely in Pisa clay. However, it is not true. Because the
unique approximation curve was determined for worldwide
clays with various characteristics with or without microfossils,
it cannot be said that the main reason is the existence of
microfossils. From X-ray diffraction, clay minerals in most of
the clays examined in the previous studies were smectite and
kaolinite. As mentioned above, the Osaka Bay clay also mainly
consists of smectite and kaolinite, but only the Pisa clay among
the clays examined mainly consists of illite. Therefore, it
indicates that illite results in a small strain dependency; i.e., one
of the main reasons of different strain dependency is the clay
minerals.
For the Pisa clay, because strain rate dependency is not
significant, long-term consolidation settlement can be
approximately predicted based on the compression curve
corresponding to a strain rate of 1×10
–7
s
–1
. In fact, settlement of
the Leaning Tower of Pisa has been successfully predicted
without regard to strain rate effect (Burland et al., 2003), even
after about 800 years since its construction. In contrast, because
strain rate dependency of the Osaka Bay clay is significant,
long-term consolidation settlement predicted based on the
compression curve corresponding to a strain rate of 1×10
–7
s
–1
results in underestimation. In situ compression curves observed
by sublayered measurements of settlement and pore-water
pressure at the first phase of the Kansai International Airport are
drawn in Figure 8 together with isotache curves deduced from
24-h incremental loading oedometer test. The in situ measured
data crosses the isotache compression curves diagonally from
higher to lower strain rates, indicating that the behavior is
consistent with the isotache modeling.
Because the strain rate dependency of the Osaka Bay clay is
similar to the other worldwide clays, it can be said that the
strain rate dependency has to be considered in prediction of
long-term consolidation settlement for most of the clays.
Prediction method of field settlement from the overburden
effective stress
'
v0
to a certain consolidation pressure
p
1
is
illustrated in Figure 9. The compression curve observed in the
laboratory long-term consolidation test follows the path
A
C
D
E. At the end of primary consolidation, strain rate
is in an order of 1.0×10
–5
s
–1
, then it passes the point D at
1.0×10
–7
s
–1
, and then it reaches the point E at a much smaller
strain rate. When long-term consolidation test is conducted,
strain rate easily decreases to 1.0×10
–9
s
–1
in 2–4 weeks, but it
hardly decreases to 1.0×10
–10
s
–1
because it requires several
months. In situ strain rate for the Osaka Bay clay is in an order
of 1.0×10
–11
s
–1
, which is much smaller than the laboratory
strain rate of 1.0×10
–9
s
–1
, and the compression curve follows a
path A
B
E
F. According to a conventional method, the
field settlement is predicted as point D based on the result of 24-
h incremental loading oedometer test; however, the real filed
settlement could be point E. In practice, in situ consolidation
settlement can be predicted corresponding to the strain rate,
which is predicted in association with the thickness of the clay
layer. In addition, ultimate consolidation settlement possibly
reaches point F. Using the initial void ratio
e
0
and compression
ndex
C
c
, the additional consolidation strains from D to E
(
D
E
) and from D to F (
D
F
) can be calculated from
geometric relationship (equations are shown in Figure 9). These
in situ additional strains in association with the strain rate
dependency are significant, particularly in a case of thick clay
layer.
i
6 SUMMARY
In the present study, the isotache concept observed in the long-
term consolidation behavior was successfully modeled by
Equation (1), in which preconsolidation pressure decreases with
decrease of strain rate and converges to a lower limit at an
infinitesimal strain rate, then the difference in long-term
consolidation behavior between the Osaka Bay clay and Pisa
clay were compared and discussed using this model. In the
long-term consolidation in an overconsolidation domain,
significant delayed settlement was observed in the Osaka Bay
clay, but not in the Pisa clay. The significant delayed settlement
seems strange; however, it can be said that this strange behavior
is inherently natural because it is caused by the common strain
rate dependency for the worldwide clays. Therefore, the little
delayed settlement observed in the Pisa clay is rather strange.
Using the isotache model, the long-term consolidation
settlement can be quantitatively predicted in association with
the strain rate dependency.
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
100
1000
Visco-plastic strain
ε
vp
Consolidation pressure
σ'
v
(kPa)
Lab. data
1.0
×
10
–7
s
–1
24 h oedometer
1.0
×
10
–8
s
–1
1.0
×
10
–9
s
–1
1.0
×
10
–10
s
–1
1.0
×
10
–11
s
–1
3.3
×
10
–10
s
–1
1.0
×
10
–10
s
–1
3.3
×
10
–11
s
–1
In situ data
Figure 8. Measured field compression curves in Kansai International
Airport Phase 1 (Ma12).
0.3
0.2
0.1
0.0
s
1
3
x
10
11
6 REFERENCES
Burland J.B., Jamiolkowski M. and Viggiani C. 2003. The stabilisation
of the Leaning Tower of Pisa,
Soils and Foundations
, 43(5), 63–80.
Leroueil S., Kabbaj M., Tavenas F. and Bouchard, R. 1985. Stress-
strain-strain rate relation for the compressibility of sensitive natural
clays,
Géotechnique
, 35(2), 159–180.
Šuklje L. 1957. The analysis of the consolidation process by the
isotache method,
Proc. 4th Int. Conf. on Soil Mech. Found. Engng.
,
London, Vol.1, 200–206.
Watabe Y., Udaka K. and Morikawa Y. 2008. Strain rate effect on long-
term consolidation of Osaka bay clay,
Soils and Foundations
,
48(4), 495–509.
Watabe Y., Udaka K., Nakatani Y. and Leroueil S. 2012. Long-term
consolidation behavior interpreted with isotache concept for
worldwide clays,
Soils and Foundations
, 52(3), 449–464.
0 (=1
x
10
100
)
s
1
3
x
10
9
Ma10
s
1
s
1
s
1
s
1
3
x
10
8
3
x
10
7
3
x
10
6
3
x
10
5
Visco-plastic strain
vp
Infinitesimal strain rate
(maximum settlement curve)
Visco-plastic strain ε
vp
Effective vertical stress
p'
σ
'
v0
p'
1
Lab. test
A
C B
cL
c0
0
c
FD
log
1
p
p
e
C
In situ
EOP
2 1
cL
c0
0
c
ED
ln
exp 1
1
log
1
c c
p
p
e
C
1.0
×
10
–5
s
–1
1.0
×
10
–6
s
–1
D
F
from 24 h oedometer
1.0
×
10
–7
s
–1
from
24 h oedom ter
1.0
×
10
–8
s
–1
E
3.3
×
10
–11
s
–1
1.0
×
10
–9
s
–1
Figure 9. Isotache compression curves in the laboratory and prediction
of long-term consolidation settlement.
