3321
Suffusion in compacted loessial silts. Interaction with granular filters.
Suffusion dans les limons lœssique compactés. Interaction avec les filtres granulaires
Terzariol R.E., Rocca R.J., Zeballos M.E.
Universidad Nacional de Córdoba, Argentina
ABSTRACT: Loess soils, which occupy much of central Argentina, are characterized by their high sensitivity to change in moisture.
This condition categorized loess structure as internally unstable soil. Despite this natural condition, silts derived from loess are
frequently used as material in roads and hydraulics constructions. In filtration processes, the soil should be shielded from erosion
phenomena. The filter must have a particle size consistent with the ground to protect. In particular, sandy silts can be affected by
phenomena of suffusion, or internal instability of the soil structure. The filter must control the loss of material and ensure the stability
of flow. The present study shows the experimental results of flow applied to loessial silts, with low compaction. Tests have been
conducted using filtration columns that simulate the soil-filter-drain. The test methodology applied aims to follow the research lines of
similar studies worldwide. Studies have focused on analyzing the influence of variables such as degree of compaction, hydraulic
gradient and composition of the filter material. The results obtained allow recommendations with reference to construction systems to
be applied and the composition of the filter in order to properly control the suffusion.
RÉSUMÉ : Les lœss, qui occupent une grande partie de l'Argentine centrale, se caractérisent par une grande sensibilité au changement
de teneur en eau qui permettent de classer ces sols dans la catégorie des sols à structure interne instable. En dépit de cet état naturel,
les limons provenant des lœss sont fréquemment utilisés comme matériau pour les routes et les constructions hydrauliques. Dans les
processus de filtration, le sol doit être à l'abri des phénomènes d'érosion. Le filtre doit avoir une granulométrie compatible avec le sol
à protéger. En particulier, les limons sablonneux peuvent être affectés par des phénomènes de suffusion ou d'instabilité interne de la
structure du sol. Le filtre doit contrôler la perte de matière et assurer la stabilité de l'écoulement. La présente étude montre les résultats
expérimentaux de débit appliqués à des limons lœssique faiblement compactés. Des tests ont été effectués en utilisant des colonnes de
filtration qui simulent le sol filtre de vidange. La méthodologie de test appliquée vise à suivre les lignes de recherche d'études
similaires dans le monde entier. Les études ont porté sur l'analyse de l'influence de variables telles que le degré de compactage, le
gradient hydraulique et la composition du matériau filtrant. Les résultats obtenus permettent recommandations par rapport à des
systèmes de construction qui doivent être appliquées et la composition du filtre afin de bien contrôler la suffusion.
KEYWORDS: loess, sandy silts, filters, suffusion
1 INTRODUCTION.
In Argentina there are large deposits of sandy loam soils. These
soil called loess, have been transported by wind. They have a
high sensitivity to moisture changes, so they are considered
unstable. One of the features of most interest is the hydraulic
behavior of these soils. These silts can be used in the
construction of cores in dams and embankments. On the road
works in the central plains of Argentina, they are used in the
formation of embankments. In these cases, the compacted soil is
affected by infiltration processes and filtration through a porous
medium. These processes, in some structures, generate erosions,
which produce construction damage, or even in total failure.
This publication shows the characteristics of a soil protection
system in order to avoid phenomena of erosion by filtration.
The soil protection actions can be several. A classic action is
granular filter construction. Other possible actions include
changes in the internal structure of the soil, including the use of
components that increase their internal resistance to erosion, or
the construction of "barriers" formed by geosynthetics.
The behavior of natural filters has been one of the main
subjects of study in geotechnical engineering. Terzaghi's
original studies, since 1929, have been supplemented by
Sherard and Dunnigan (1989), who made the most significant
advances in the study of the behavior of filters.
Similarly, significant advances have been made by Khor and
Woo (1989), Foster and Fell (2000) and Delgado and Locke
(2008), Semar et al (2010). These authors have contributed to a
better description of the behavior of soils in filtration problems,
and have also generated recommendations on controlling
erosion.
Erosion by filtration may start from the modification of the
physico-chemical composition of the soil, resulting in
dispersion phenomena. When soil structural organization, i.e.
the accommodation of the particles, can lead to instability and
erosion, the phenomenon is called suffusion, erodibility or
internal erosion. Indraratna (1997, 2006, 2007), Wan and Fell
(2004, 2008), Bonelli (2006), Bendahmane (2008), Indraratma
et al (2008, 2011) and Benamar et al (2010), among others, have
done important research in this field. These authors have
developed new criteria for evaluating the potential of internal
erosion, or suffusion in granular soils.
To understand the importance of this problem, we must
analyze historical cases of dams whose cores have been
constructed with silt. The most notable example is the Nurek
dam in Tajikistan. A much discussed case is the Teton Dam in
Idaho, USA failed in 1976 during the first fill. The failure is
caused by internal erosion. In some cases of failure, it is
considered that the material used in the construction of the core
has a large influence. Smalley I.J. and Dijkstra T.A. (1991)
indicated that the loess should not be used in cores of dams,
because they lack the mineralogy and structural properties
required for this application. Compacting the slime still remain
