1587
The behaviour of natural cohesive soils under dynamic excitations
Le comportement des sols cohérentes naturelles sous excitations dynamiques
Sas W.
Water Centre, Warsaw University of Life Sciences, Warsaw, Poland
Szymański A., Gabryś K.
Department of Geotechnical Engineering, Warsaw University of Life Sciences, Warsaw, Poland
ABSTRACT: In the last forty years, a significant amount of research has been carried out to better understand the mechanical reaction
of soils to dynamic excitations. A variety of laboratory techniques were used for these studies e.g. cyclic torsional shear tests, cyclic
direct simple shear tests, cyclic triaxial tests and resonant column tests. They allowed researchers to measure above all strain
amplitude and frequency of excitation on soil behaviour (Lai et al. 2001). The series of experiments and analysis in the presented
paper were therefore carried out to evaluate dynamic properties of consolidated soils from Warsaw, the capital of Poland, using GDS
Resonant Column Apparatus. Shear modulus (G
0
) determined on the basis of shear wave velocity, measured during resonant tests,
represented here the initial stiffness of the soil. Suitable number of tests showed that the change in initial stiffness was caused by
many factors, among which was mean effective stress (p’). Numerous laboratory experiments confirmed the knowledge from the
literature on growth of the material damping of soil (D) with the strain amplitude. Some considerations concerning the impact of low
and very low shear strain (
) on soil behaviour were also included.
RÉSUMÉ : Au cours des quarante dernières années on a mené un nombre significatif de recherches ayant pour but la meilleure
compréhension des comportements des sols soumis aux charges dynamiques. Pendant les recherches, on a recouru à de différentes
techniques de laboratoire telles que: le cisaillement de torsion en cycle, le cisaillement direct cyclique, les essais triaxiaux cycliques et
les essais à la colonne résonnante. Le recours à ce type de recherches a permis d’obtenir surtout des mesures de l’amplitude et de la
fréquence des vibrations et de leur influence sur les réactions du sol (Lai et al. 2001). Dans l’étude, on a presenté une série
d’experimentations et d’analyses ayant pour but la reconnaissance et l’évaluation des propriétés dynamiques des sols consolidés,
présentes dans la région de Varsovie, capitale de la Pologne, en recourant à la colonne résonnante fabriquée par la société britannique
GDS. Le module de la déformation amorphe (G0), estimé à la base de la vitesse de l’onde transversale mesurée au cours de l’étude,
représente la rigidité initiale du sol. Les recherches menées ont demontré qu’un changement de la rigidité initiale du sol est causée par
de nombreuses facteurs, dont l’un très important est celle de la contrainte effective moyenne (p’). De nombreux essais en laboratoire
ont prouvé l’augmentation de l’amortissement des vibrations dans le sol (D) avec l’augmentation de l’amplitude de la déformation
amorphe et don’t la prevue réside dans la littérature spécifique de référence. Dans l’étude, on a également incorporé des remarques et
des précisions portant sur l’influence de l’ensemble de petites et minuscules déformations sur des propriétés dynamiques du sol.
KEYWORDS: dynamic excitations, resonant column tests, natural cohesive soils
1.
INTRODUCTION
There exist in nature all kinds of vibrations on the ground, such
as for example: earthquakes, water waves, storms, traffic loads,
vibration machinery, construction operations, wind power and
many others. In practical geoengineering, the response of
dynamic characteristics of the subsoil under these oscillations
has become lately a focus of a great interest.. Shear modulus
and damping properties are required for a proper analyzing and
a good understanding the response of the soil influenced by the
dynamic load (Bai 2011).
The initial shear modulus (G
0
) is extensively perceived to be
a fundamental soil stiffness property and is a parameter for
geotechnical researches, both in earthquake engineering as well
as in the prediction of dynamic soil-structure interactions
(Piriyakul and Haegeman 2009). Dobry et al. (1980) employed
the stiffness method for foreseeing the liquefaction potential of
saturated sand provided that the shear modulus at small strains
for soil layers to define the threshold ground acceleration is
known. The initial shear modulus can be measured in laboratory
in the triaxial tests with bender elements technique or in
resonant column apparatus while torsional mode.
Damping ratio (D) is another important parameter in the
investigation of dynamic problems. Small-strain damping ratio
(D
min
) is rather difficult to accurately determine due to a lot of
elements interfering with the experiment, like: equipment
damping, environmental noise, back electromagnetic force. D
min
obtained by resonant column tests is therefore somewhat higher
scatter, but these effects become inessential when researches are
made at high strain amplitude.
To well understand a complex nature of dynamic soil
characteristics, the impact factors should be completely
investigated. Dynamic soil properties are affected by various
factors, among which strain amplitude, confining pressure, void
ratio, overconsolidation ratio, loading frequency, temperature,
anisotropic stress and so forth are the most significant (Hardin
and Drnevich 1972).
As the same importance as small-strain dynamic attributes of
soil, the non-linear dynamic properties play an essential role in
analyzing the dynamic behavior of ground motion during, for
instance, strong earthquake, like Sichuan Earthquake from 12
th
May 2008, with Richter magnitude scale M=8.0, which killed
more than 70 000, injured around 300 000 people and millions
caused homeless. The shear strains triggered in surface deposits
while such big motions may be estimated around 10
-2
% up to
1% (Iwasaki and Tatsuoka 1977). Because of this is very
necessary to search the strain-dependent dynamic characteristics
of soils at the level of 10
-4
till 10
-2
in laboratory.
Based the background and problems mentioned above, this
study aimed at determining the dynamic properties of natural
cohesive soils from Warsaw area by defining the value of shear
