445
Anisotropic strength ratio and plasticity index of natural clays
Étude de la relation entre l’anisotropie et l’indice de plasticité des argiles naturelles
Won J.Y.
Technip
ABSTRACT: A number of K
o
-consolidated triaxial compression and extension test results were collected to re-evaluate the
relationship between anisotropic strength ratio and plasticity index of natural clays. The database covers 203 pairs of triaxial tests
performed on normally consolidated natural clays from 14 countries, published between 1972 and 2007. Data selection criteria were
established for a consistent comparison. The anisotropy was strongly influenced by the definitions of failure in extension tests. Once
the anisotropy data are grouped into their depositional environments, no general trend of anisotropy with plasticity index can be
observed. The well-known trend that anisotropy decreases with plasticity index cannot be justified. A trend of anisotropy correlated
exclusively with plasticity index can be misleading. Anisotropic characteristics of a natural clay should be evaluated by careful
consideration of site specific characteristics, spatial variability, depositional and post-depositional environments of the clays.
RÉSUMÉ : Des résultats d’essais triaxiaux K
o
-consolidés en compression et en extension ont été collectés pour réexaminer la relation
entre l’anisotropie et l’indice de plasticité des argiles naturelles. Les données consistent en 203 séries d’essais sur des argiles
normalement consolidées de 14 pays, et publiés entre 1972 et 2007. Un critère de sélection des données a été établi pour faire des
comparaisons cohérentes. L’anisotropie est fortement corrélée avec le type de rupture en extension. En groupant les données en
fonction de leurs conditions de dépôts, aucune corrélation entre l’anisotropie et l’indice de plasticité n’est observée. La diminution
bien connue de l’anisotropie en fonction de l’indice de plasticité reste inexpliquée. Considérer que l’anisotropie ne dépend que de
l’indice de plasticité serait erroné. L’anisotropie d’une argile naturelle doit être évaluée en tenant compte soigneusement des
caractéristiques spécifiques du site, de la variabilité spatiale, des environnements de dépôts et post-dépôts de l’agiles.
KEYWORDS: Clay, Undrained Shear Strength, Anisotropy, Plasticity, Triaxial Test, Extension Test.
1 INTRODUCTION
Undrained shear strength of natural clays shows anisotropy, i.e.
it displays different undrained shear strengths under different
shear stress conditions and directions. Anisotropy in stiffness,
permeability, and shear strength comes from stress-induced and
inherent anisotropy. Anisotropic characteristics of natural clays
play a significant role in many geotechnical applications, such
as slope stability, and bearing capacity of shallow and deep
foundations both in onshore and offshore applications.
Anisotropy is routinely investigated for offshore foundations
(driven piles and suction anchors). In onshore projects,
however, the anisotropy is seldom investigated because of many
reasons. In many cases, thus, geotechnical engineers resort to
empirical trends to evaluate anisotropy of the soil of interest.
It is generally known that the anisotropy decreases with
plasticity index, i.e. high plastic clays are more isotropic than
low plastic clays. The trend was reported by Berre and Bjerrum
(1973) and Ladd et al. (1977) decades ago, based on a limited
number of test results. Besides the debates whether or not
plasticity index alone can represent soil characteristics, there
have been numerous anisotropy test results published since
then. Therefore, it is worthwhile to collect and analyze the
available anisotropy information to re-evaluate the anisotropic
undrained shear strength characteristics of natural clays.
A number of published K
o
-consolidated triaxial compression
and extension tests performed on normally consolidated natural
clays were collected. A total of 203 pairs of K
o
-consolidated
triaxial compression and extension test results from 14 countries
were analyzed. Different test conditions and methods of the data
necessitated consistent data selection criteria. The data were
grouped into their regions and/or depositional environments.
This paper presents review on depositional environments,
discussion on test conditions, and re-evaluation of the generally
accepted anisotropy trend.
2 METHODOLOGY
From an extensive literature review, K
o
-consolidated anisotropy
test results were collected. The published data and test
conditions were carefully reviewed to select acceptable data.
The database was, subsequently broken down into respective
regions or depositional environments including Scandinavia,
Canada, Europe, Middle East, Japan, and East Asia.
2.1
Anisotropy
In this study, the anisotropy was evaluated by K
o
-consolidated
triaxial compression (CK
o
UC) and extension (CK
o
UE) tests.
Anisotropic strength ratio (K
s
=S
uE
/S
uC
) was defined as the
undrained shear strength ratio of extension strength (S
uE
) to
compression strength (S
uC
). The higher the anisotropic strength
ratio is, the lower the anisotropy is.
2.2
Data selection criteria
Anisotropy can be evaluated by various test methods such as
plane strain, hollow cylinder apparatus, triaxial compression
and extension tests as well as field vane tests with different
shape and length of blades. Each method measures different
aspects of anisotropy. Only CK
o
UC and CK
o
UE test results on
undisturbed natural clays were selected and analyzed. For
consistency, data from other test methods were excluded. It was
found that many published data did not include detailed test
conditions. To avoid unnecessary discussion on the effects of
consolidation methods for anisotropic triaxial tests, only the
results that followed the recompression concept i.e., consolidate
the specimen to an estimated in-situ overburden stress (
’
vc
)
before the undrained shear, were selected. Many test data for
USA clays were excluded because the majority of them were
obtained from the SHANSEP (Stress History and Normalized
Soil Engineering Parameters) approach. The following data
selection criteria were established for a consistent comparison.
Geologically normally consolidated clays
Apparently (lightly) overconsolidated clays
K
o
(or anisotropically)-consolidated undrained triaxial
Undisturbed natural clays
Consolidation method: Recompression
