2481
Standardization of the molding procedures for stabilized soil specimens as used for
QC/QA in Deep Mixing application
Normalisation des procédures pour la production d’éprouvettes de sols stabilisés utilisées dans les
processus de QC/QA pour des applications de « Deep Mixing »
Grisolia M., Leder E., Marzano I.P.
Department of Civil and Environmental Engineering (DICEA), “Sapienza” University of Rome
ABSTRACT: An international collaborative research has been undertaken to establish common understanding of the key issues
involved in Quality Control/Quality Assurance (QC/QA) of Deep Mixing technique and propose international standards on design,
execution and execution control. The aim of the study is to investigate the influence of the laboratory procedures on the mechanical
properties of stabilised soil specimens and develop an innovative method to select the appropriate molding technique. A large
laboratory testing program was carried out on seven types of heterogeneous natural soils, as found in Rome, and on Kawasaki clay
stabilised with Portland cement. Thirty soil-binder mixtures with different workability were prepared using five different molding
techniques, varying initial water content of the soils, water to cement ratio and binder amount. Unconfined compression tests have
been carried out systematically on over 800 specimens. The applicability of different molding techniques in function of the
workability of the mixture has been investigated and from the results it was possible to define an “applicability index” and therefore
the range of applicability for each technique in function of the mixture’s workability.
RÉSUMÉ: Une étude internationale a été entreprise dans le but de définir des orientations communes pour les procédures QC/QA liés
aux travaux effectués par « Deep Mixing » et proposer des normes internationales relatives à la conception, l'exécution et le contrôle
des opérations. Le but de cette étude est d'étudier l'effet des procédures de laboratoire pour la réalisation des éprouvettes de sols
stabilisés et de développer une méthode innovante pour sélectionner à chaque fois la technique de réalisation appropriée. Un vaste
programme d'essais en laboratoire a été réalisé en analysant plus de trente mélanges différents de ciments et sols à partir de huit sols
naturels de Rome et Tokyo. Cinq techniques de réalisation ont été utilisées pour la confection d’éprouvettes testées avec des essais de
compression simple.. L'applicabilité des différentes techniques de réalisation a été étudiée selon l'usinabilité du mélange. A partir des
résultats, il a été possible de définir un index d’applicabilité et donc un champ d'application de chaque technique en fonction de
l’usinabilité du mélange..
KEYWORDS: Deep mixing, workability, laboratory procedures, operational abaci.
1 INTRODUCTION
The Deep Mixing Method is a widely spread in situ ground
improvement technique using different kind of binders to
enhance mechanical and physical properties of soils (Terashi
1997; CDIT 2002).
Laboratory mixing tests are essential to QC/QA processes
and performed to obtain the mechanical and physical properties
of stabilized soil samples. The laboratory test results provide
crucial information for the estimation of the mix design and in-
situ properties to utilize in the geotechnical design. (Bruce et al.
2000; Larsson 2005; Marzano et al. 2009; Terashi and Kitazume
2011; Filz et al. 2012). At the moment many laboratories
produce and test soil-binder specimens without a standard
procedure, therefore the results for the same soil-binder mixture
could be very different and not usefully compared. In fact
molding techniques have a great influence on the mechanical
and physical properties of the stabilized soil specimens (Grisolia
et al. 2012; Marzano et al. 2012). This influence is strictly
correlated to the workability of the soil-binder mixture defined
as the property of the mixture of being easily mixed in the bowl
and placed in the mold. High workability refers to liquid type
mixtures (easier to place and handle), while low workability to
sticky and stiff type ones.
Workability represents diverse characteristics of fresh
mixture that are difficult to measure quantitatively, because a
soil-binder mixture is a complex material with a wide range of
particle sizes and time-dependent properties. The definition of a
parameter representative of the mixture’s workability and an
univocal method for the evaluation of the mixture’s workability
are currently not available (Koehler and Fowler, 2003) even if
such parameter could be well related to consistency when
considering mixtures made up of cohesive soils.
A new method for the evaluation of the mixture’s
workability was introduced and applied in the study. It is based
on the measure of the torque required to turn an impeller in soil-
binder mixture through a commercial device which is applied
directly on the mixer. This method has the advantage to provide
the possibility of measuring the workability for each mixable
mixture, independently on the type of the materials used.
Furthermore, the study develops a procedure to select,
through an “applicability index” function of the initial mixture
workability, the molding technique that provides densest
specimens with highest strength and results repetitiveness in
order to obtain very useful reference values to set specification
limits to be achieved in field applications (ratio between
laboratory and field target strength is reported for instance by
JGS 0821-2000 and EuroSoilStab 2002).
2 MATERIALS AND METHODS
The experimental work consisted in a laboratory investigation
on the effect of different molding techniques on the unconfined
compressive strength,
UCS
(measured according to the JIS A
1216:2009) and wet density,
γ
(defined as the specimen’s
weight divided by the volume of the mold) of cement stabilised
soil specimens under various mixing conditions.
2.1 Materials
Eight types of natural soils stabilised with Portland cement
added in wet or dry form were used. The tests were performed
on: Kawasaki Clay (
KC
), manmade Silty Deposit (
SD
), Silty
