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Assessment of embankment stability on organic soils using Eurocode 7
Évaluation de la stabilité des remblais sur sols organiques en utilisant l'Eurocode 7
Lechowicz Z., Wrzesiński G.
Department of Geotechnical Engineering, Warsaw University of Life Sciences - SGGW, Poland
ABSTRACT: A problem of the stability assessment of stage-constructed embankment on soft organic subsoil is discussed.
Calculations were performed for the first and the third stage of embankment construction at the Antoniny site and for the failure test.
The analysis contains measured and corrected shear strength values of organic soils obtained in the virgin and consolidated organic
subsoil by staged construction. The statistical analysis of the field vane test results was performed to obtain mean values, standard
deviations and probability distribution. Derived, characteristic and design values of undrained shear strength of organic soils were
determined. The stability analysis according to design approaches of Eurocode 7 was carried out using mean values of undrained
shear strength of organic soils. Additionally, the stability analysis was carried out using undrained shear strength reduced by 0.5 and
1.0 standard deviation. Based on probabilistic stability analyses the safety factor and the reliability index
were calculated.
RÉSUMÉ: Le problème présenté est celui de l'évaluation de la stabilité d’un remblai experimental construit par étapes sur un sous-sol
organique. Les calculs ont été effectués pour la première et la troisième étape de la construction du remblai sur le site d’Antoniny et
pour l’essai de rupture. L'analyse contient les valeurs mesurées et corrigées des valeurs de résistance au cisaillement des sols
organiques obtenus dans le sous-sol vierge et consolidé par les étapes de construction. L'analyse statistique des résultats d'essais sur le
terrain a été réalisée pour obtenir des valeurs moyennes, l’écarts-type et la loi statistique de distribution. Les valeurs, caractéristiques
de la résistance au cisaillement non drainé des sols organique ont été déterminées pour la conception. L'analyse de stabilité selon les
approches de l'Eurocode 7 a été réalisée à l'aide des valeurs moyennes de la résistance au cisaillement des sols organiques. L'analyse
de stabilité a été réalisée également en utilisant la résistance au cisaillement réduite de 0.5 et 1.0 écart-type., Les acteurs de sécurité et
de fiabilité de l’indice b ont été calculés sur la base de l’analyse probabilistique de la stabilité.
KEYWORDS: organic soils, undrained shear strength, embankment stability, Eurocode 7, characteristic and design values.
1
INTRODUCTION
Geotechnical problems associated with embankments located on
organic soils are more difficult than on soft mineral soils mainly
due to their higher compressibility and very low virgin shear
strength (Hartlen and Wolski 1996). The variability in the value
of soil properties is a major contributor to the uncertainty in the
stability of an embankment located on soft and highly
compressible subsoil. This means that even construction of an
embankment of only several meters height may lead to failure.
As a result, the methods of design and construction that are
normally used in other soft soils, may not be adequate in
organic soils.
However, in many countries there are large areas with
organic soils where different kinds of embankments have to be
constructed. In such cases, the prediction of soil behaviour and
the selection of a proper design method become an important as
well as a difficult engineering task. In practice, embankment
construction by stages utilizing the change in shear strength due
to consolidation process is often selected solution. For such a
stage constructed embankment, time and money can be saved if
the embankment stability is accurately predicted.
Stability analysis according to Eurocode 7 can be performed
using following Design Approaches: DA1 (Combination 1),
DA1 (Combination 2), DA2 or DA3. Many countries as well as
Poland accepted in 2010 Design Approach DA3 for design of
slopes. In particular Design Approaches are applied different
partial factors to characteristic values of parameters (Frank et al.
2004, Bagdahl 2005, Frank 2007, Bond and Harris 2008, Van
Seters and Janses 2011, Orr 2012).
To assess the variability in the value of soil properties the
statistical analysis should be performed. Statistical analysis
allows to obtain mean values, standard deviations and
probability distribution. Thus, characteristic values of
geotechnical parameters used in calculations can be obtained.
The characteristic value X
k
used in geotechnical
calculations corresponding to a 95% confidence level that the
actual mean value, X
m
is greater than this value, is given by
Schneider (1999):
X
k
= X
m
(1 - k
n
V)
(1)
where:
k
n
- the factor, depending on the type of statistical distribution
and the number of test results,
V - variation coefficient.
Schneider (1999), on the basis of calculations, has shown
that a good approximation to X
k
is obtained when k
n
= 0.5; i.e.
if the characteristic value is chosen as one half a standard
deviation below the mean value, as in the following equation:
X
k
= X
m
- 0.5SD
(2)
In this paper a problem of the assessment of embankment
stability on soft organic subsoil is discussed based on the
Antoniny test site. This is an adequate example to perform
stability
analysis and research of organic subsoil under stage constructed
embankment because each case of embankment construction,
from loading of virgin subsoil to failure test can be considered.
2
DESCRIPTION OF THE ANTONINY SITE
The Antoniny test site is located in north-western Poland in the
Noteć river valley. In 1980s the Department of Geotechnical
Engineering of Warsaw University of Life Sciences in co-
operation with the Swedish Geotechnical Institute performed
extensive field and laboratory investigations.
