2915
General Report of TC 214
Soft soils
Rapport général du TC 214
Sol mous
Ovando-Shelley E.
Universidad Nacional Autónoma de México
Rangel-Núñez J.L.
Universidad Autónoma Metropolitana
ABSTRACT: The papers in the Soft Soils session cover almost comprehensively the main topics related to soft soil engineering. The
15 papers that were submitted came from countries located in at least four continents, thus demonstrating that soft soils are widely
spread and that research into their properties and characteristics is still of paramount importance for an increasing large number of
geotechnical engineers worldwide. Theses papers will provide readers an opportunity to learn many aspects of soil engineering from
the experiences shared by other colleagues around the world. In almost all of the papers the importance of characterizing soft soils
properly is highlighted. It is also evident that thorough soil exploration as well as field and laboratory testing are necessary to achieve
this purpose. The papers also show the extensive and extended use of exploration techniques that were rare only a few decades ago. It
is also notable that most contributions refer to case histories in which basic soil mechanics concepts have been successfully applied
and in which instrumentation was used judiciously to monitor and interpret soil behavior. Also, improved field instrumentation
systems have had a positive influence on the development of many projects.
RÉSUMÉ : Les articles de cette session sur les sols mous couvrent presque complètement les principaux sujets liés à la géotechnique
des sols mous. Les 15 communications qui ont été présentées proviennent de pays situés dans au moins quatre continents, démontrant
ainsi que les sols mous sont largement répandus et que la recherche sur leurs propriétés et caractéristiques est toujours d'une
importance capitale pour un grand nombre croissant d'ingénieurs géotechniques dans le monde entier. Ces documents donneront aux
lecteurs l'occasion de découvrir de nombreux aspects de l'ingénierie des sols à partir des expériences partagées par d'autres collègues à
travers le monde. Dans la plupart des articles est mise en lumière l'importance de la caractérisation des sols mous, et il est donc
nécessaire d'avoir une reconnaissance et des essais sur le terrain et en laboratoire d'excellente qualité. Les documents montrent
également l'utilisation intensive et prolongée de techniques d'exploration qui étaient rares il y a seulement quelques décennies. Il faut
aussi noter que la plupart des contributions se réfèrent à des études de cas dans lesquels les concepts de base de mécanique des sols
ont été appliqués avec succès et dans lequel l'instrumentation a été utilisée à bon escient pour contrôler et interpréter le comportement
du sol. En outre, l'amélioration des systèmes d'instrumentation sur le terrain ont eu une influence positive sur le développement de
nombreux projets.
KEYWORDS: Soft soils, soil improvement, historical cases, characterization of soft soils, analysis, numerical modeling & fracturing.
1 INTRODUCTION.
Urban expansion in many cities around the world as well as the
construction of large industrial facilities and associated
infrastructure have often made it necessary to build large
projects in very soft soils where complex foundation solutions
may be are required. Proper characterization of these soils is a
crucial first step in applying specific analysis and design
methods.
Geotechnical problems related to the presence of soft soils
can appear in every other country around the world and in some
regions geotechnical engineers face them on a day to day basis.
Specialists now make reference to soft, very soft and even ultra-
soft soils, depending on their specific properties. This is why
characterization of these materials may also require the use of
innovative means and even terminology to describe and identify
them. Constitutive models for these soils have continued to
appear over the last years as well as new conceptual solutions
for foundation systems. New constitutive methods and new
methods for improving the characteristics of these soils are also
available.
The fifteen papers submitted to the Session deal with these
topics and cover almost totally the subjects related to soft soil
engineering. Three of the papers deal with analytical studies,
one of them describes construction procedures, studies related
to the determination of mechanical properties are discussed and
described in two papers and one paper focused on discussing
soil fracturing and fissuring within the context of regional
subsidence. The eight remaining papers describe case histories
in which characterization, analysis, design and construction
related topics are dealt with.
2 METHODS OF ANALYSIS
Espinoza and Li (2013) present a hybrid drained-undrained
model to design prefabricated vertical drains to improve the
drainage characteristics of soft soil when surcharge loads are
applied (Espinoza et al 2011). The model is based on the
concept of virtual sand piles, where the soils located closer to
prefabricated vertical drains dissipate excess pore pressures
generated during construction faster than the soils located
farther away. The authors show the influence of the construction
rate on the maximum generated excess pore pressure. Their
model considers the radial variation of excess pore pressure
between drains. In their analyses the authors selected the
magnitude of excess pore pressure that would have a negligible
effect on the berm stability and then back-calculated the
separation between drains necessary to yield this value. Soil
columns around each prefabricated vertical drain mobilize their
drained shear strength during loading, whereas the soil outside
the virtual sand piles develops an undrained shear strength
response during loading. This methodology was applied during
construction of a high mechanically stabilized earth berm over
very soft, low permeability and extremely compressible soil in
