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Microstructural changes leading to chemically enhanced drainage
Modifications de microstructure entraînant un drainage chimiquement amélioré
Minder P., Puzrin A.M.
ETH Zurich, Institute for Geotechnical Engineering, Zurich, Switzerland
ABSTRACT: The hydration state of clay mineral surfaces is a key influence factor on the mechanical and hydraulic behaviour of
clays. Small changes of the cation occupancy of these surfaces can lead to pronounced changes in macroscopic material parameters.
The sensitivity of the material response allows for designing chemical soil improvement by selectively exchanging the cations. In this
study we explore the effect of a targeted cation exchange in smectite clays to modify soil properties in situ. The highly selective and
strongly exchanging organic cation guanidinium was used to stabilise the interlayer distance between clay platelets.
On the particle scale the cation exchange led to the formation of stable aggregates. Mercury intrusion porosimetry and oedometric
tests confirmed the stability of these aggregates and of an opened pore structure also under high stresses. Macroscopically, the
modification resulted in a permanently increased permeability. The magnitude of the improvement is such, that after infiltration of the
chemical into a clayey soil around an injection pipe, the modified soil zone could act as a drainage conduit. Potential application of
such flexible drainage systems are in creeping landslides, where continuing displacements cause failure of conventional drains.
RÉSUMÉ : Un facteur d’influence prédominant pour le comportement mécanique et hydraulique des argiles est l’état d’hydratation
des surfaces des minéraux argileux. La sensibilité accentuée de la réponse du matériau permet de concevoir des méthodes
d’amélioration chimiques pour sols au moyen d’un échange ciblé de cations. Dans cette étude, nous explorons les effets d’un tel
échange dans le contexte d’un argile smectique pour modifier les caractéristiques d’un sol in situ. Le cation organique guanidinium a
été utilisé pour stabiliser l’espace entre les couches des lamelles d’argile. Au niveau particulaire, l’échange de cations a engendré des
agrégats stables. Les essais oedometriques et de porosimétrie par intrusion de mercure ont confirmé la stabilité de ces agrégats et une
structure de pore ouverte, même sous forte sollicitation. Au niveau macroscopique, la modification se manifesta par une augmentation
permanente de la conductivité hydraulique. L’amélioration est telle qu’après l’infiltration du produit chimique dans un sol argileux
entourant une conduite d’injection, le sol affecté peut être utilisé comme drain. Les applications de tels drainages se trouvent par
exemple dans les glissements de terrains en état de fluage, où les déplacements continuels entravent le fonctionnement de drainages
conventionnels.
KEYWORDS: Soil improvement, chemical modification, hydraulic conductivity, clay minerals, drainage.
1 INTRODUCTION
Seepage of groundwater in creeping landslides is a key
parameter for the creep velocity. A reduction of the water table
by means of drainage could increase the overall safety of critical
slopes. Conventional drainage systems based on rigid drainage
pipes are prone to failure due to the on-going deformation in
such unstable areas. The development of alternative drainage
techniques with increased operating life in creeping landslides
has therefore a high potential in commercial application.
For fine grained soils, both theoretical and experimental
studies have pointed out a high dependency of the permeability
on the pore fluid. Major influence factors are the type of liquid
(mainly via its dielectric constant, Fernandez and Quigley
1985), type of dissolved salts and ionic strength of the solutes
(e.g. Madsen and Mitchell 1989, Lagaly et al. 2006). These
studies were often carried out in the context of hydraulic barrier
design and containment of nuclear wastes, where the increase
was an unintended and dangerous effect caused by contaminants
and leachates.
With the goal of purposely creating zones of higher
permeability in-situ as part of a drainage system, this paper
focuses on the fundamental aspects required in the development
of an innovative soil improvement technique. Enhancing soil
permeability with chemicals may be accompanied with
unexpected side effects such as deterioration of stiffness or
strength. In field applications such mechanical consequences
could result in excessive deformation or failure. The effects on
other geotechnical parameters than hydraulic conductivity
therefore need to be addressed as well.
2 MATERIALS AND METHODS
Mineralogical investigations indicated that guanidinium cations
affect directly the interlayer distance of the stacked sheet-
silicate structure of montmorillonite (Plötze and Kahr 2008).
This strongly binding cation is therefore chosen as a possible
chemical agent to increase permeability by inhibition of
interlayer swelling.
In order to assess the mechanisms behind the increase in
permeability caused by guanidinium, experiments on different
scales were performed. To quantify the changes in soil fabric
and structure due to the cation exchange, a closer look on the
resulting changes on particle and aggregate scale was taken. The
macroscopic stability of the new features was subsequently
determined in standard geotechnical tests.
2.1
Materials
The laboratory tests were carried out on soil samples
reconstituted with commercially available, standardised
constituents. A commercial Ca-bentonite (Calcigel, Südchemie,
Germany) with a total montmorillonite content of 65% was used
as fine grained component. Other occurring mineral phases in
this bentonite were quartz, feldspar, kaolinite and mica. Where
