1503
Effect of stress anisotropy on cyclic behavior of dense sand with dynamic hollow cylinder apparatus
Effet de l'anisotropie de contrainte sur le comportement cyclique du sable dense avec dynamique
appareil cylindre creux
Jafarzadeh F., Zamanian M.
Sharif University of Technology, Tehran, Iran
ABSTRACT: It is well known that soils have an anisotropic response and collapse can be induced by imposing different modes of
shearing. Changing the inclination and magnitude of the major principal stress with respect to the depositional direction in most cases
will increase the collapse potential and brittleness as well as reduce the shear strength and shear stiffness. In this study the effect of
stress path with changing in direction of the major principal stresses on the cyclic behavior of Babolsar sand is studied. A series of
cyclic undrained tests on dense sand samples with induced anisotropy were conducted with an automatic dynamic hollow cylinder
apparatus. Results showed that inclination of the major principal stress with respect to the depositional direction has significant effect
on samples responses to the cyclic load. Because of the differences of the sand particles interlock in various directions, the sand
strength parameters changes with loading direction. Changes in loading direction would change the specimens stress status and would
cause different deformation pattern. This kind of stress anisotropy had not significant effect on shear modulus and damping ratio.
RÉSUMÉ: Il est bien connu que les sols ont une réponse anisotrope et que l’effondrement peut être induit en imposant différents
modes de cisaillement. La modification de l’inclinaison et de l'intensité de la contrainte principale majeure par rapport à la direction
de sédimentation va, dans la plupart des cas, augmenter le potentiel d’effondrement et la fragilité ainsi que réduire la résistance au
cisaillement et la rigidité de cisaillement. Dans cette étude, l’effet du chemin de contrainte avec évolution dans le sens de la contrainte
principale majeure sur le comportement cyclique du sable Babolsar est étudié. Une série d’essais non drainés cycliques sur les sables
denses avec anisotropie induite a été réalisée avec un appareil automatique dynamique cylindre creux. Les résultats ont montré que
l’inclinaison de la contrainte principale majeure par rapport à la direction de dépôt a un effet significatif sur les réponses des
échantillons à la charge cyclique. La direction de chargement change les paramètres de la résistance du sable, en raison de la variation
des connexions entre des particules dans les différentes directions. Le changement de la direction du chargement peut changer l’état
de la contrainte qui peut produire différents chemins de déformation. Ce type de contrainte anisotrope n’avait pas d’effet significatif
sur le module de cisaillement et sur le taux d’amortissement.
KEYWORDS: stress anisotropy, major principal stress direction, sand, cyclic loading, hollow cylinder tests.
1 INTRODUCTION
Soils are anisotropic materials. Environmental and geological
conditions during the deposition of the soils, along with the
particle shapes, sizes, and void structures are some factors
constituting the natural anisotropy of the soil. The fabric of the
soil may later be disturbed (further anisotropy) with application
of loads and, thus, plastic strains. Casagrande and Carillo (1944)
distinguished these two sources of anisotropy as inherent and
induced anisotropies; respectively. Inherent anisotropy is
impacted by the particle shapes and depositional conditions and
is independent from strains. Induced anisotropy is the
reconfiguration of the soil fabric to withstand the applied loads.
Oda (1972) assessed the initial fabric (inherent anisotropy) of
sands and its effect to the mechanical behavior. He used four
different sands with different roundness and concluded that the
orientation depends on the particle shape and the method of
compaction.
Many researchers have used the Hollow Cylinder (HC)
apparatus to investigate the cyclic behavior of sandy soils.
However, the boundary conditions in these studies varied
greatly and the primary focus was on regenerating simple shear
conditions rather than systematically investigating the effect of
principal stress rotation and intermediate principal stress on the
cyclic behavior of sands.
Tatsuoka et al. (1986) designed a torsional hollow cylinder
testing apparatus that could cyclically shear the specimens
under undrained simple shear conditions by preventing any
axial strain development and inner cell volume change. These
boundary conditions eliminated changes in the inner and outer
radii during shearing. The effect of continuous principal stress
rotation was observed only at small strains below 0.2 %. Above
this strain level, the effect of continuous principal stress rotation
was negligible.
Yamashita and Toki (1993) conducted undrained cyclic
triaxial and torsional HC tests on sand specimens. The major
principal stress rotation was not varied in a controlled manner
and was somewhere between 0° or 90° from the vertical. They
found that the cyclic strengths obtained from cyclic triaxial tests
and torsional HC tests were not equal and the difference may be
more pronounced depending on the sample preparation
technique.
Shibuya et al. (2003) investigated the effect of inclination of
the major principal stress form vertical, α; and intermediate
principal stress ratio, b=(σ
2
-σ
3
)/(σ
1
-σ
3
); on the monotonic and
cyclic behavior of sands. The pore pressure response of the sand
subjected to these stress paths showed that large pore pressures
are generated due to continuous principal stress rotation at
constant deviator stress, (σ
1
-σ
3
)/2. Changes in b also caused
changes in excess pore pressures, but the changes were not as
significant as the ones created by the continuous principal stress
rotation.
Altun et al. (2005) used a cyclic torsional simple shear
apparatus similar to the one used by Towhata and Ishihara
(1985). They investigated the cyclic undrained behavior of
sandy and silty soils. Their testing program did not include
investigation of the effect of either α or b on the cyclic behavior
of sandy or silty soils.
Many other researchers have shown that the undrained shear
strength of the sandy soils decreases with the increase in α
