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Polymer support fluids: use and misuse of innovative fluids in geotechnical works
Les polymères: l'utilisation de nouveaux fluides de forage en travaux géotechnique
Jefferis S.A.
Environmental Geotechnics Ltd. and University of Oxford, United Kingdom
Lam C.
The University of Manchester, United Kingdom
ABSTRACT: Bentonite slurries have been used for over sixty years for the temporary support of excavations such as bored piles and
diaphragm walls. At intervals over this time polymer products have been tried in place of bentonite but not always successfully.
Recently it has become clear that, if used properly, polymer fluids offer many advantages over their bentonite counterparts, including
improved foundation performance, lower environmental impacts, smaller site footprint and also simpler preparation, mixing and final
disposal as they are used at much lower concentrations. They are also more easily managed than bentonite. However, successful use
requires that the some specific characteristics of polymers are respected, in particular, it must be recognised that they are sorbed onto
soils so that the polymer concentration in solution drops during use.
RÉSUMÉ: Les coulis de bentonite ont été utilisés depuis plus de soixante ans pour la mise en œuvre des pieux forés et parois
moulées. Des boues polymères ont été testées pour remplacer ces suspensions d’argile mais les résultats n’ont pas toujours été
conclusifs. Récemment, il est devenu évident que, si utilises correctement, les polymères offrent de nombreux avantages, entre autre
une amélioration de la performance de fondation et une réduction de l’impact sur l’environnement; les procédés de préparation et de
mélange sont facilités ainsi que la disposition de déchets car la quantité de polymères utilisée est plus petite que la quantité de
bentonite nécessaire dans les coulis de bentonite. Cependant le succès de l’utilisation des polymères est limité par certaines de leurs
propriétés – en particulier, le fait qu’ils s’adsorbent aux sols diminue leur efficacité durant le forage
.
KEYWORDS: bentonite, diaphragm walls, piles, polymer, support fluids.
1 INTRODUCTION
1.1
Background
Bentonite slurries have been used for over sixty years for the
temporary support of excavations for slurry trench cut-offs,
bored piles and structural diaphragm walls and somewhat more
recently for slurry tunnelling. At intervals over this time
polymer alternatives have been tried but not always
successfully, so that for some the word ‘polymer’ has become
an anathema. However, recent developments have shown that, if
used properly, polymer fluids offer many advantages over their
bentonite counterparts, including improved foundation
performance, smaller site footprint, reduced environmental
impact and simpler mixing and final disposal as they are used at
much lower concentrations than bentonite.
The many advantages polymer solutions offer can be
achieved only if specifiers and users have a proper
understanding of their properties and their in-situ behaviour and
recognise that not all polymers are the same – the properties of
the various polymers used in excavation works can vary very
substantially. Unfortunately, it is still not unusual for users
and/or specifiers to treat excavation support polymers as if they
were a single material similar to bentonite. Polymer solutions
are fundamentally different fluids to bentonite slurries and each
type of polymer has distinct physical and chemical properties
which must be respected to avoid misuse.
1.2
Natural and synthetic polymers
Early polymer fluids tended to be based on naturally derived
products such as carboxymethyl cellulose, xanthan and guar
gums but they had a limited range of properties, were easily
biodegraded and thus short-lived unless treated with biocides
which can have negative environmental impacts. Furthermore,
like bentonite they could not inhibit the dispersion of fine soils
such as clays into the excavation fluid and thus required
cleaning before re-use.
In recent years, the advent of synthetic polymers has allowed
the development of fluid systems with tailored properties.
Systems can be designed to be bio-stable, environmentally
benign and to inhibit clay dispersion so enabling repeated use
without specialised soil-slurry separation plant such as
hydrocyclones, dewatering screens and centrifuges. Today, with
these benefits, synthetic polymers account for the vast majority
of polymers used for foundation construction and in oil-well
drilling (where bentonite free muds are regularly used). Natural
polymers continue to be used for excavation projects where
rapid biodegradation is useful such as the construction of
permeable reactive barriers and deep drainage walls.
1.3
Objectives
To promote best practice in the use of polymer support fluids
for the construction of deep foundations, this paper sets out the
latest understanding of the behaviour of polymer fluids and also
presents experience drawn from recent research and case
histories from around the world.
2 SUCCESS THROUGH PROPER USE OF POLYMERS
2.1
Operational benefits
The operational benefits offered by polymer fluids traditionally
have been one of the main reasons for contractors to switch
from bentonite to polymers. For example, Lennon et al. (2006)
note that the size and cost of the ancillary plant required for
bentonite slurries make them relatively uneconomic for urban
sites with restricted space and access such as those in city
