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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
strength ratios from the Bothkennar site ranged between 0.36
and 0.58 with PI range of 28-43. Peak or necking failures were
observed (definition-B) from the samples retrieved by Laval and
Sherbrooke samplers. No apparent trends between K
s
and PI can
be observed in Figure 2(c).
The only anisotropy data from the Middle East are of Iraqi
clays as shown in Figure 2(d). The Iraqi clays in the database
are normally consolidated young or aged clays. The anisotropy
substantially changes (K
s
=0.50-0.89) within a very narrow PI
range (PI=34-36) for the Khor Al-Zubaire clay.
Anisotropic data of Japanese clays are presented in Figure
2(e). It is clear both K
s
and PI values significantly vary even
within a specific site. For example, the Izumo clay has PI range
of 25-100 and K
s
values of 0.70-1.06. From Figure 2(e), one
could suggest K
s
=0.5-0.8 for PI=20-60 and K
s
=0.7 or more for
PI over 60, for Japanese clays. Spatial variation within a site as
well as locality seems to have strong effects on anisotropy than
a single index property, PI.
Anisotropy data from the East Asian countries are shown in
Figure 2(f). All the anisotropy data followed the failure
definition-B. Anisotropic strength ratios typically ranged
between 0.5 and 1.1 for the wide PI range of 14-85. The Namak
clay is similar to the Bothkennar clay in many ways including
stress history, organic contents, laminated features, and estuary
environments. The K
s
range of the Namak clay is 0.45-0.67
with PI=22-41 that is comparable to the Bothkennar clay. Of
special interest is anisotropy data from the Bangkok clay. Berre
and Bjerrum (1973) presented only one data point that was not
from the Scandinavian Peninsula and PI over 35: from Bangkok
east. In fact, the data point (K
s
=0.52 & PI=88) was not selected
in this study because it appeared to be an organic clay. As for
Bangkok clay, published data show a wide range of K
s
=0.74-
1.28 with PI=26-77. As Tanaka et al. (2001a) mentioned,
Southeast Asian clays seem to behave more isotropically despite
the scatter and their moderate PI values.
When Berre and Bjerrum (1973) and Ladd et al. (1977)
reached the conclusion that anisotropy of clays decreases with
plasticity index, very limited test results were available.
Scandinavian low PI clays with failure definition-A that
underestimated S
uE
formed the left lower end and the two data
points from Bangkok organic clay and Atchafalaya clay, USA
(PI=75 and definition-B) formed the right end to conclude the
trend. Once the anisotropy data with consistent criteria in this
study are grouped into their depositional environments, the
trend of K
s
increase with PI can hardly be observed. The
statement “less plastic, and often more sensitive, clays tend to
have higher anisotropy than more plastic clays” by
Jamiolkowski et al. (1985) appears to be appropriate, only if the
less plastic and sensitive clays are Scandinavian and Canadian
low PI clays. This study supports the conclusion by Hanzawa
and Tanaka (1992) that undrained strength anisotropy is not
correlated with plasticity index. Other aspects, such as clay
fraction, clay structure, mineralogy, origin, diatoms, spatial
variation are the major factors that control anisotropy in shear
strength of natural clays. In other terms, depositional and post-
depositional environments, and regional variations are the
governing factors for anisotropy of natural clays, rather than a
single index property like PI. It should be emphasized that index
properties are good indicators of those major governing factors
in a limited sense. For a given local soil, a carefully selected
empirical correlation or a trend based on local data should be
valid and useful. However, a comparison between various
natural clays solely by a single index property such as plasticity
index, without careful consideration of depositional and post-
depositional environments can be misleading.
5 CONCLUSIONS
A large number of anisotropic triaxial test results (CK
o
UC and
CK
o
UE) were collected and analyzed to re-evaluate the
generally accepted trend between anisotropy and plasticity
index. Data selection criteria were established for a consistent
comparison. K
o
-consolidated (recompression) triaxial test
results on geologically normally consolidated, undisturbed
natural clays were selected. Based on the analysis, the well-
known trend that anisotropy decreases with plasticity index
cannot be justified. The trend was developed by limited test
results and different definitions of failure. Anisotropy was
strongly influenced by the definitions of failure in CK
o
UE tests.
When comparing different natural clays, an anisotropy trend
correlated exclusively with plasticity index can be misleading.
Clay structure, clay fraction, mineralogy, origin, diatoms, and
spatial variation are the governing factors for understanding
anisotropy of natural clays. Relationship between anisotropic
strength ratio and plasticity index should be evaluated by
careful consideration of spatial variability, site characteristics,
and depositional and post-depositional environments of an
individual clay of interest.
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