891
An experimental study on the consolidation of soft clayey soils using
electrochemical methods
Étude expérimentale de la consolidation des argiles molles avec des méthodes électrochimiques
Cardoso R., Nogueira Santos J.
ICIST, Instituto Superior Técnico, Lisbon Technical University, Portugal
ABSTRACT: An experimental study was performed where the acceleration of the consolidation using electroosmosis of normally
consolidated saturated white Kaolin was investigated. The speed of consolidation was measured through the consolidation coefficient
computed using the results of oedometer tests in which DC voltage was applied during the entire loading period. The inclusion of
drains and electrodes in the drains was also studied in an experimental apparatus developed for this purpose. The results are
interpreted and data is analyzed considering the application of this technique in practice.
RÉSUMÉ : Une étude expérimentale a été réalisée où la consolidation est accélérée en utilisant l'électro-osmose dans du Kaolin blanc
saturé normalement consolidé. La vitesse de consolidation a été mesurée par le coefficient de consolidation calculé en utilisant les
résultats des essais oedométriques dans lesquels la tension continue était appliquée au cours du chargement. L'inclusion des drains et
des électrodes dans les drains a été également étudiée dans un appareil expérimental développé à cet effet. Les résultats sont
interprétés et les données sont analysées en examinant l'utilisation de cette technique pour la pratique dans plusieurs conditions
naturelles.
KEYWORDS: Electrokinetic, consolidation, water percolation, electrical technique, electroosmosis.
1 INTRODUCTION
Soil treatment with binders or other cementing materials, or the
adoption of speed up consolidation techniques are necessary to
treat soft clayey soils to make them appropriate to be used as
foundation soils. Several techniques are available nowadays,
such as pre-charge embankments, radial drainage, drainage with
vacuum, electrochemical methods, etc, which have specific
advantages/ disadvantages, variable cost and different
implementation challenges. This paper focuses on the use of
electrochemical methods, such as electroosmosis. Some field
cases are described in the literature (see Glendinning et al.,
2005, for example) and concern stabilization of slopes,
excavations and embankments. Several aspects must be
analyzed concerning the use of dewatering techniques based on
electrochemical effects, mainly related with design,
cost/efficiency and possible collateral effects.
The working principle of electrochemical methods is to
apply a difference in electrical potential between electrodes
placed in the soil to move the positive ions (cations) towards the
negative electrode (cathode) and the negative ions (anions)
towards the positive electrode (anode). These techniques are
used to drain saturated soils and therefore to improve their
mechanical properties, as well as for decontamination purposes.
For the particular case of clay minerals, due to their negatively
charged surface, they attract positive ions and immobilize them
on the double layer to neutralize the electrical forces involved.
Thus, a movement of cations will occur, which will carry the
pore water in the same direction. The water is carried in this
flow of ions in a viscous manner and, if water is drained, this
will results in dewatering from the positive electrode (anode) to
the negative electrode (cathode).
The work presented in this paper describes an experimental
study where normally consolidated saturated white Kaolin
specimens were investigated in order to understand the
advantages of using electroosmosis to accelerate consolidation
when compared with adopting a mesh of drains to ensure radial
drainage. The last method was also included in the study
because radial drainage is one of the most used methods for
accelerating the consolidation of clayey soils.
The speed of consolidation was measured through the
consolidation coefficient and the consolidation time necessary
for a given settlement to occur. The electroosmosis method was
studied in tests performed in oedometer conditions, in which
DC voltage influence on settlements was analyzed. The study of
radial drainage was performed in an experimental apparatus
developed for this purpose. The inclusion of electrodes in the
drains was also studied.
2 ELECTROOSMOSIS
As a technique for accelerating consolidation, electroosmosis
can be studied has if it was a case of pore pressure increment,
which can not cause undrained failure in the soil. Esrig (1968)
studied the different types of pore pressures that can be
developed in the soil mass as function of the drainage
conditions. The more usual condition on consolidation by
electroosmosis is assuming that drainage is performed only
through the cathode electrode.
Water flow
q
h
is given by Equation 1, where
k
e
is the coefficient
of permeability measured when water percolates only due to
electroosmotic effects,
k
h
is the coefficient of hydraulic
permeability,
u
is the excess of pore water pressure caused by
electroosmosis (and/or by the increment of vertical stress) and
V
is the voltage. This equation shows the proportionality between
the voltage in the soil and the pore water pressure developed
and is used to find the distribution of the pore water pressure.
x
V k
x
u
k
k
q
e
e
h
h
(1)
Equation 2 comes from introducing Equation 1 in Darcy’s
equation and both in the equation that governs the one-
dimensional consolidation (Terzaghi et al. 1996). In Equation 2,
m
v
is the compressibility index of the soil,
w
is the volumetric
weight of water,
t
is time and
c
v
is the coefficient of
consolidation. Distance
x
is measured along the flow path, one-
dimensional in this case.
t
u
c x
V
k
k
x
u
v
w
h
e
1
2
2
2
2
(2)
The solution of Equation 2 is Equation 3, which is used to
compute the excess of pore water pressure developed during the
