2465
Design of Deep Soil Mix Structures: considerations on the UCS characteristic value
Dimensionnement des structures en soil mix : considérations sur la valeur caractéristique UCS
Denies N., Van Lysebetten G., Huybrechts N.
Belgian Building Research Institute, BBRI, Belgium
De Cock F.
Geotechnical Expert Office Geo.be, Belgium
B. Lameire
Belgian Association of Foundation Contractors ABEF, Belgium
Maertens J.
Jan Maertens bvba & KU Leuven, Belgium
Vervoort A.
KU Leuven, Belgium
ABSTRACT: Since several decades, the deep soil mix (DSM) technique has been used for ground improvement works. But in recent
years, this technique has been increasingly used for structural applications. Standardized guidelines for the execution and the design
of this kind of applications are not currently available. For the purpose of developing such guidelines, mechanical characteristics of
DSM material were investigated. Within the framework of a Flemish regional research program (IWT 080736), DSM material from
38 Belgian construction sites, with various soil conditions and for different execution processes, has been tested. Internationally
QA/QC activities are commonly related to tests on core samples for the determination of the Unconfined Compressive Strength
(UCS) and the modulus of elasticity (E) of the material. Both values allow an approach of the design which takes into account the
bending characteristics (EI), the deformation (E), the arching effect (UCS) and the structural resistance (UCS) of the element. For the
semi-probabilistic design approach presented in Eurocode 7, a “characteristic value” of the UCS has to be defined as part of the
design of DSM structures. The present paper discusses the definition of this value.
RÉSUMÉ : Depuis plusieurs décennies, la technique du soil mix est utilisée comme procédé d’amélioration du sol. Mais ces dernières
années, elle est de plus en plus utilisée pour des applications structurelles. Aucune directive n’est actuellement disponible pour
l’exécution et le dimensionnement de telles applications. De manière à développer de telles directives, les caractéristiques mécaniques
du matériau soil mix ont été investiguées. Dans le cadre d’un programme de recherche financé par l’IWT, l’agence gouvernementale
flamande pour l’innovation, des échantillons de soil mix de 38 sites de construction ont été testés pour différents types de sol et
différents systèmes. La qualité du matériau soil mix est généralement contrôlée à l’aide d’essais, réalisés sur des échantillons carottés
in situ, par lesquels sont déterminés la résistance à la compression simple (UCS) et le module d’élasticité (E) du matériau. Ces deux
grandeurs permettent une approche du dimensionnement tenant compte de la rigidité flexionnelle (EI), des déformations (E), de l’effet
de voûte (UCS) et de la résistance structurelle (UCS) de l’élément. Au vue de l’approche semi-probabiliste de l’Eurocode 7, il est
important de définir la valeur caractéristique de la résistance du soil mix (UCS) à prendre en compte dans le dimensionnement. Le
présent article discute de la définition de cette valeur caractéristique.
KEYWORDS: Deep soil mix wall, structural design, ucs characteristic value
1 INTRODUCTION
The Deep Soil Mix (DSM) process was introduced in the 70’s
in Japan and in the Scandinavian countries. Since several
decennia, DSM has been known as a ground improvement (GI)
technique. According to the classification of GI methods
adopted by the ISSMGE TC 211, DSM can be classified as
ground improvement with grouting type admixtures. Numerous
reviews and recent progresses of the DSM technique are
referred in Denies and Van Lysebetten (2012). The results of
national and European research programs have also been
published in multiple interesting reports (such as Eurosoilstab
2002), while the European standard for the execution of deep
mixing “Execution of special geotechnical works – Deep
Mixing” (EN 14679) was published in 2005. Most of these
research projects focused on the global stabilization of soft
cohesive soils such as clay, silt, peat and gyttja (result of the
digestion of the peat by bacteria). More recently, DSM is
increasingly being used for structural applications such as soil
mix walls (SMW) for the retaining of soil and water in the case
of excavations.
In the DSM process, the ground is mechanically mixed in
place, while a binder, based on cement, is injected. For SMW
applications, the DSM cylindrical columns or the rectangular
panels are placed next to each other, in a secant way. By
overlapping the different soil mix elements, a continuous SMW
is realized. Steel profiles are inserted into the DSM fresh
material to resist the shear forces and bending moments. The
main structural difference between SMW and the more
traditional secant pile walls is the constitutive DSM material
which consists of a soil – cement mixture instead of concrete.
Elements such as piles or diaphragm walls only comprise
standardized components and their characteristic strength can be
defined by the strength class of concrete. The design approach
for the DSM material is very different since the existing soil is
used as an essential component of the final product. Moreover,
the DSM strength depends not only on the soil type, but also on
the DSM technique, the amount and the type of binder, etc.
Within the framework of the BBRI “Soil Mix” project
initiated in 2009 in collaboration with the KU Leuven and the
Belgian Association of Foundation Contractors (ABEF),
numerous tests on in situ DSM material have been performed. A
good insight has been acquired with regard to mechanical
characteristics that can be obtained with the CVR C-mix
®
, the
TSM and the CSM systems in several Belgian soils as reported
in Denies et al. (2012). BBRI information sheets (BBRI, 2012a
and b) have been published for the purpose of helping
contractors to improve the quality control (QC) of their finished
Lameire B.
