1439
An experimental approach to evaluate shear modulus and damping ratio of granular
material
Une approche expérimentale pour évaluer le module de cisaillement et le taux d'amortissement du
matériau granulaire
Bolouri Bazaz J.
Civil Engineering Department, Ferdowsi University of Mashhad, Mashhad, Iran
Bolouri Bazaz H.R.
Civil and Environmental Engineering Department, Tehran University, Tehran, Iran,
ABSTRACT: During the last few decades enormous research has been published to clarify the response of sand under cyclic loading
conditions. This can be attributed to the growing concern with safety of structures subjected to similar cyclic loading patterns. The
aim of the present experimental work is firstly to investigate the responses of granular materials when subjected to cyclic stressing
under plane strain conditions. In particular it is to ascertain if soil eventually reaches a preferred state. In order to assess whether such
a state can develop a series of tests were conducted that covered a wide range of cyclic loading conditions. It is shown that granular
materials, when subjected to a large number of stressing cycles, behave elastically with a linear stress-strain relationship. Two main
dynamic mechanical properties of sand, namely shear modulus and damping ratio have been evaluated.
RÉSUMÉ : Au cours des dernières décennies, énorme de recherche a été publié afin de clarifier la réponse de sable sous des
conditions de chargement cycliques. Cela peut être attribué à la préoccupation croissante de la sécurité des structures soumises aux
mêmes modèles de chargement cycliques. Le but de ce travail expérimental est d'abord d'étudier les réactions des matériaux
granulaires lorsqu'ils sont soumis à des contraintes cycliques dans des conditions de déformations planes. En particulier, il est de
savoir si le sol finit par atteindre un état préféré. Afin de déterminer si un tel état peut se développer, une série de tests ont été
effectués avec une large gamme de conditions de chargement cycliques. Il est montré que les matériaux granulaires, lorsqu'elle est
soumise à un grand nombre de cycles de contrainte, se comportent de façon élastique avec une relation linéaire entre contrainte et
déformation.
KEYWORDS: granular, material, cyclic loading, shear modulus, damping ratio.
1 INTRODUCTION
Interest in the effects of cyclic loading on soil strength has led
to increased efforts at simulation in the laboratories. The main
object of almost all laboratory tests has been to study the
behavior of a given material under cyclic loading (e.g. Huang
Y., Huang et al. 2004). The ideal purpose of laboratory element
tests is to study the behavior of soil under conditions similar to
those encountered in practical situations (Springman et al.
1995), and to determine the basic mechanical properties of soil,
preferably as a set of constitutive equations which can be used
for design or analysis. The response of many geotechnical
structures to vibration is mostly dominated by the soil dynamic
properties (Sitharam et al. 2004), among which shear modulus
and damping ratio are the most important. Liquefaction, as the
most catastrophic failure phenomenon, is onset of instability
and is governed by the stress level and shear modulus (Abdul
Lahi et al. 2010). In the presents research the strength and
deformation behavior of dense and loose sand samples when
subjected to cyclic stressing under plane strain condition are
investigated. It is shown that when sand is subjected to cyclic
stressing between two equal peak stress ratios, the sand particles
arrange themselves to resist the directional loading and this
results in the creation of a cross anisotropic structure.
2 MATERIAL
Standard Leighton Buzzard sand (BS sieve sizes 18-25; 850-
600
m) with a specific gravity of 2.66 was used for the present
investigation. Two different methods of preparing sand samples,
slow raining through air and releasing a large quantity of sand
from a height of 750 mm above the ground level, resulted in
dense and loose samples with initial voids ratio of e
0
=0.52
1
and e
0
=0.72
1 respectively. The 100 mm sand cubic samples
were formed in a cubic rubber membrane, supported by a
perspex box. Applying a negative pressure of around 50 kPa
enabled to remove and position the samples in the apparatus.
Sand samples to be considered were dry and subsequently all
stresses were effective stress.
3 APPARATUS
The Biaxial Tester which has been fully described elsewhere
(Ogunbekun, 1988) shears soil under plane strain conditions.
The boundaries of the apparatus apply principal stresses with no
application of shear stress. Contact sensors detect any change in
position of the centers of the stretched pressure rubber bags
through which the major and minor principal stresses are
applied to the sample boundaries. Boundary movement of the
Biaxial Tester is achieved with the aid of a hydraulic system.
Four driving pistons provide inward or outward boundary
movement during deformation of a sample to just maintain or
just break contacts in the automatic mode. The main novel
feature of the Biaxial Tester is the changing dimensions of the
flexible boundary, through which the stresses are applied.
4 STRESS PATHS
Cyclic stress-controlled tests were performed under plane strain
condition (
z
=0). After positioning in the Biaxial Tester, the
specimen was biaxially loaded with
x
=
y
=50 kPa. The stress
orthogonal to the plane of strain,
z
was initially applied at 50
kPa via the water filled plane strain boundary. At this stage the
tap of the top plane strain boundary was closed in order to
record the variation of
z
during the test. The negative pressure
on the sample was then released and the test commenced by
