1611
Cyclic Loading Behavior of Saturated Sand with Different Fabrics
Comportement du sable saturé avec des structures différentes sous chargement cyclique
Sze H.Y., Yang J.
Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong
ABSTRACT: The undrained response of saturated sand under cyclic loading has been a subject of long-standing interest. Although it
has been recognized for long, the effect of fabric remians a critical problem that is not yet well understood. In this paper, cyclic
triaxial test results from a strategically designed experimental program are presented to demonstrate how significant the effect of
fabric can be on the undrained cyclic behavior of sand under different combinations of initial state and initial stress conditions. A
significant finding of the study is that loose sand specimens, prepared by different reconstitution methods and thus having different
initial fabrics, exhibit similar failure modes under the conventional symmetrical loading condition, but they show distinct failure
patterns under the non-symmetrical cyclic loading condition. A qualitative explanation of the macroscopic observations is also
proposed from a microscopic perspective, which sheds light on the mechanisms involved.
RÉSUMÉ : Le sujet de la réponse du sable saturé sous chargement cyclique non drainé a suscité de l’intérêt depuis longtemps.
Cependant il est reconnu que l’effet de la structure reste un problème clef qui n’est pas encore bien compris. Dans cet article, les
résultats d’essais triaxiaux cycliques faits dans le cadre d’un programme expérimental conçu spécialement sont présentés afin de
démontrer la signifiance de l’effet de structure sur le comportement non drainé du sable soumis à différentes combinaisons d’état
initial et contraintes initiales. L’un des résultats les plus importants est que les spécimens de sable lâche préparés par des méthodes
différentes de reconstitution, et donc avec des structures initiales différentes, montrent les mêmes modes de rupture sous chargement
symétrique, mais différents schémas de rupture sous chargement non symétrique. Une explication qualitative de ces observations
faites à l’échelle macroscopique est donnée d’un point de vue microscopique, éclairant les mécanismes impliqués.
KEYWORDS: cyclic loading; fabric; failure; liquefaction; sand; soil behavior
1 INTRODUCTION
The understanding of cyclic failure behavior of soils plays a
pivotal role in geotechnical earthquake engineering design
against liquefaction and cyclic mobility types of damages.
Complication of this subject lies in the fact that cyclic behavior
is under the inter-related influences of various factors.
Dominant ones such as soil density, overburden pressure and
initial static shear stress have been assessed and discussed (e.g.
Vaid et al. 2001, Yang & Sze 2011a & b). An important one,
the fabric effect, has, nevertheless, been largely neglected.
Fabric is defined as the spatial arrangement of particles and
voids. Since sand is a discrete granular material, how it behaves
macroscopically is a result of the microscopic interactions
between grains and voids. So far, most studies focused on the
effect of fabric anisotropy on the monotonic behavior (e.g. Oda
1972a). Regarding the cyclic behavior, only cyclic strength is
concerned (e.g. Mulilis et al. 1977). No particular effort has
been placed on investigating how fabric takes effect on the
cyclic failure pattern, which is of fundamental importance for
understanding of cyclic loading behavior of soil.
In terms of the cyclic failure pattern, several studies (e.g.
Yamashita & Toki 1993, Oda et al. 2001) have offered some
valuable data. While these studies did shed light on this subject,
they focused only on very dense sand with relative density over
80%, which is of less practical interest. Also, no concern was
given on how the fabric effect might change as the initial state,
in terms of soil density and effective confining pressure, shifts.
Moreover, no attempt has ever been placed on relating it with
the initial shear impact. The complicated inter-related effects of
initial state and initial shear have already been demonstrated by
Yang & Sze (2011a & b).
For long, fabric anisotropy of sand has been recognized but
not being adequately taken into account in cyclic behavior
study. A fundamental difficulty is the lack of well-organized
laboratory test data to reveal more comprehensively the fabric
effect on the cyclic failure response and how it possibly relates
with other key factors. This study is thus prompted, by means of
cyclic triaxial testing, to investigate thoroughly the fabric effect
on the cyclic failure pattern of sand as well as its dependence on
initial state and initial shear. The focus is on loose soil becasue
its susceptibility to liquefaction is of the most concern.
In this paper: (1) details of the cyclic triaxial test program
and sample reconstituion methods are described; (2) the
observed cyclic failure patterns exhibited under various
combinations of initial fabric, state and shear conditions are
discussed; and (3) a qualitative explanation of the observed
fabric effects is given from the micro-mechanical perspective
aiming to shed light on the fundamental understanding of the
role of fabric on the cyclic behavior of sand.
2 EXPERIMENTATION
All cyclic triaxial tests were carried out on Toyoura sand. The
material properties are summarized in Table 1.
Table 1. Physical properties of the test material.
Mean
grain size
Coeff. of
Uniformity
Max. void
ratio
Min. void
ratio
Fines
Content
0.175mm
1.5
0.977
0.605
0%
2.1
Sample Reconstitution Methods
Different initial fabric can be simulated by means of changing
the sample reconstitution method in the laboratory (Mulilis et
al. 1977). Depending on the methods of reconstitution, it is
possible to mimic the fabric possession of in-situ soil deposit
having different formation histories (Miura & Toki 1984).
Two common sample reconstitution methods, as moist
tamping (MT) and dry deposition (DD), were strategically
selected for this study. It takes advantages of their known
different fabric possession and that they tend to mimic different
in-situ fabric - MT simulates construction fill compaction; DD
