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Degradation of clay due to cyclic loadings and deformations
La dégradation de l'argile due à des chargements et des déformations cycliques
Åhnberg H., Larsson R., Holmén M.
Swedish geotechnical institute, Linköping, Sweden
ABSTRACT: Cyclic loading and large deformations may significantly reduce the undrained shear strength of clay. The stability of
constructions and clay slopes can be significantly affected if subjected to disturbances from large local loads and different kinds of
construction works such as piling, blasting, etc. The risk for widespread failures may be particularly high when quick clay is present
in the area. At the Swedish Geotechnical Institute, a study has been performed on the susceptibility to disturbance from cyclic stresses
and strains of different clays. The influence of different factors on the strength degradation has been examined in series of laboratory
tests on different types of sensitive clays. The laboratory tests on the soils have included basic characterisation tests, CRS oedometer
tests and active static and cyclic triaxial tests. The triaxial tests have been performed at various overconsolidation ratios with varying
cyclic shear stress levels and deformation levels. In the field, trials with cyclic T-bar tests in situ have also been performed. The
results of the study have given an increased insight into the effect of cyclic stresses and deformations on the degradation of different
types of clay.
RÉSUMÉ : Le chargement et les déformations cycliques peuvent réduire de manière significative la résistance non drainée au
cisaillement de l'argile. La stabilité des constructions et des pentes d'argile peut être considérablement affectée si elles sont soumis à
des perturbations causées par de grandes charges locales et différents types de travaux de construction tels que l’enfoncement de
pieux, le dynamitage, etc. Le risque de rupture généralisée peut être particulièrement élevé lorsque l'argile sensible est présente dans
la région. Une étude a été réalisée à l'institut suédois de géotechnique, SGI, sur la sensibilité de différentes argiles sensibles aux
perturbations par des contraintes cycliques. L'influence de différents facteurs sur la dégradation de la résistance a été étudiée dans une
série de tests en laboratoire. Les tests de laboratoire ont inclus entre autres des essais oedométriques CRS et des essais triaxiaux actifs
statiques et cycliques réalisés à différents rapports de surconsolidation avec différents niveaux de contrainte de cisaillement cyclique
et différents niveaux de déformation. Sur le terrain, des essais « T-bar » cycliques ont également été réalisés. Les résultats de l'étude
ont accru notre connaissance de l'effet des contraintes et des déformations cycliques sur la dégradation de différents types d'argile.
KEYWORDS: cyclic loading, sensitive clay, shear strength,
1 INTRODUCTION
Failures in soft clay sometimes occur in connection to
construction work. Occasionally, these and other local failures
also develop into large scale landslides. The parameter
commonly used for estimation of risks in connection with
construction work and local failures is the sensitivity of the soil
and quick clay areas are considered as special risks. However,
the sensitivity alone does not describe how much disturbance is
required to break down the soil structure and strength. Available
charts and methods to estimate risks in this aspect are normally
connected to earthquakes and do not include highly sensitive
clays. Other parameters to describe this property in clays and
methods to measure it has therefore been sought for a long time,
(e.g. Söderblom 1969, Larsson and Jansson 1982, Tavenas et al.
1983). However, no generally established methods and rules
have been brought forward.
An investigation has recently been performed at the Swedish
Geotechnical Institute (SGI) with the aim of finding guidelines
for which types of sensitive clay in Sweden can have their
strengths easily broken down and thereby constitute special
risks (Åhnberg and Larsson 2012). This paper presents some of
the results in this project.
2 SCOPE OF THE INVESTIGATIONS
2.1
Types of soil tested
Twelve different test sites were involved in the
investigations. Four of these are located in the eastern part of
Sweden - two in the Stockholm-Mälardalen area and two in the
Linköping-Norrköping area. The clays here have been deposited
in alternating lake and brackish sea water. The other sites are
located in the western part of Sweden, five along the Göta river
valley and three further to the northwest in the middle part of
the Bohuslän province. The clays at these sites are marine clays
deposited in sea water. The environment with salt seawater at
deposition generally resulted in heavy flocculation of the clay
particles and a more open structure compared to the clays
deposited in brackish water in eastern Sweden. Leaching of the
salt and possibly other processes have later led to various
degrees of increased sensitivities, (e.g. Rankka et al. 2004). The
sites were chosen to cover clays with different conditions at
deposition, with low, medium and high plasticity and with
sensitivities varying from medium to quick clays. The
investigations have been made on clays with the typical
environments at deposition
, mineral compositions and types of
organic matter in this region. A special emphasis has also been
made on highly sensitive and quick clays which are common
and constitute special problems here.
