207
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Stress-path effects on the grading of an artificial material with crushable grains
Stress-trajectoire effets sur le granulométrie d'un matériau artificiel avec des grains déformables
Casini F.
Universitat Politecnica de Catalunya, Barcelona, Spain
Leu J., Low B., Wanninger F., Zimmermann A., Zwicker P., Springman S.M.
Institute for Geotechnical Engineering, ETHZ, Zurich, Switzerland
Viggiani G.M.B.
Università degli Studi di Roma Tor Vergata, Roma, Italy
ABSTRACT: Granular materials forming natural slopes, embankments, foundations, pavement structures, and rail track structures are
subjected to static and dynamic loads, which may cause particle breakage to occur. Results are presented of an experimental
investigation into the mechanical response of an artificial granular material, consisting of crushed expanded clay pellets,
commercially known under the brand name LECA (Light Expanded Clay Aggregate) to various stress path tests. The material was
reconstituted, with a maximum particle size of 2 mm, to obtain grading curves with the same mean diameter
d
50
, and different
coefficients of uniformity,
U
(= 3.5, 7, 14, 28) or the same
U
and a different
d
50
(= 0.5, 1 mm). The constant volume friction angle and
the minimum and maximum densities corresponding to each grading were determined before stress path testing in one dimensional
and triaxial compression at different stress levels. Changes in the LECA grading after the stress path tests were described using two
parameters defined respectively as the mean diameter and coefficient of uniformity of the final distribution over the value of the initial
distributions, both of which were assumed to be consistent with self-similar grading with varying fractal dimension.
RÉSUMÉ: Les matériaux granulaires formant des pentes naturelles, des talus, des fondations, des structures de chaussées et des
structures de voie ferrée se sont soumis à des charges statiques et dynamiques, qui peuvent causer la rupture des particules de se
produire. Les résultats sont présentés sur une recherche expérimentale sur le comportement mécanique d'un matériau granulaire
artificielle, composée de boulettes d'argile expansée concassées, commercialement connu sous le nom de marque LECA (Light
agrégat d'argile expansée) à différents tests chemin de stress. Le matériau a été reconstituée, avec une granulométrie maximale de 2
mm, pour obtenir des courbes de gradation avec le même diamètre moyen
d
50
, et différents coefficients d'uniformité,
U
(= 3,5, 7, 14,
28) ou même
U
et une autre
d
50
(= 0,5, 1 mm). L'angle de volume constant de friction et les densités minimale et maximale
correspondant à chaque classement ont été déterminés avant le test dans un chemin de contrainte de compression triaxiale dimensions
et à différents niveaux de contrainte. Les changements dans le classement LECA après les essais de chemin de stress ont été décrits en
utilisant deux paramètres définis respectivement comme le diamètre moyen et le coefficient d'uniformité de la distribution finale au-
dessus de la valeur des distributions initiales, qui étaient tous deux censés être compatibles avec auto-similaire classement avec plus
ou moins la dimension fractale.
KEYWORDS: artificial material, grain crushing, grain size distribution, breakage, stress path testing.
INTRODUCTION
Particle breakage describes the response to loading in which soil
particles become smaller, while other mechanisms of
deformation, such as slippage, dilation and creep occur.
Degradation processes associated with loading-induced grain
crushing affect the macroscopic mechanical behaviour of
granular materials. For a given material, breakage is affected by
both stress level and stress path direction and can cause volume
loss leading to settlements and a reduction in the hydraulic
conductivity, as finer particle fractions fill voids. Moreover,
elastic and frictional properties of the soil are modified due to
changes in grain size distribution. Understanding the
mechanisms of grain crushing is therefore crucial, as this affects
the stress-strain response of the soil under loading.
Different measures have been suggested to quantify the
amount of particle breakage in a sample of granular material.
Hardin (1985) introduced the relative breakage,
B
r
, based on the
relative position of the current cumulative particle size
distribution from the initial cumulative distribution and a cut-off
value of ‘silt’ particle size (of 0.074 mm). The use of the latter
implied that all particles would eventually become finer than the
(arbitrary) cut-off value in the fragmentation process. This
denies the growing understanding that the grain size distribution
of an aggregate of any initial grading, under large confining
pressure and extensive shear strains, tends to become self-
similar (fractal) (Turcotte, 1986; McDowell and Bolton, 1998).
Several studies (
e.g
.: Sammis
et al
., 1987; Tsoungui
et al
.;
1999) have shown that the main effect of particle crushing is to
increase the proportion of fine material without significantly
changing the size of the largest particles. Larger particles are
cushioned by surrounding smaller particles (Imre
et al.,
2010;
2011), making them more resistant to crushing and giving them
a higher coordination number, which is defined as the number
of the particle’s nearest neighbours
. Smaller particles, with
smaller coordination numbers, are more likely to be crushed in
the fragmentation process. In other words, the cushioning effect
due to the large coordination number for larger particles
outweighs the effect of reducing strength with increasing
particle size (Casini and Viggiani, 2011; Casini
et al.,
2013).
Methods and the results are presented for an experimental
investigation on an artificial granular material under different
loading conditions. In particular, the paper explores the
evolution of the main physical properties, such as the angle of
friction and the range of voids ratio together with the grading,
sphericity and angularity of the particles under different loading
conditions.
For practical reasons, the experimental programme was
carried out on an artificial granular material, which was
reconstituted at different initial grading with grains that crush at
relatively low stress. Samples of the artificial,
‘crushable’,
granular soil, have been subjected to different effective stress
paths in one dimensional and triaxial compression, in order to
understand the mechanisms of grain crushing better.
