July/August 2022 - 94

noncohesive soils below the water table.
In cohesive soils water initiates clay
hydration, which typically results in
swelling and softening of the interface
material, causing potential reductions of
shaft capacities or even partial or
complete hole collapses. Once the water
molecules have entered the lattice
structure of the clay particles, the
internal bond between the layers of the
clay matrix are weakened or disrupted
as the ions that provide the bond
between the layers of clay begin to
dissolve and form a solution with the
water. As more water molecules enter
the clay matrix, they begin to electrochemically
repel the lattices from one
another, causing swelling.
At this stage the natural cohesive and
consolidated state of the soil formation is
destroyed and the affected area behaves
like a clay with significantly lower
cohesion; it may also become mobilized
or fluidized. Consequently, sensitive clay
(or shale) formations that are exposed to
water can exhibit significantly reduced
shear strength, which will result in
reduced stability and shaft capacity for
deep foundation elements constructed
in this type of material. It is important to
avoid the contact of free water with
these formations and to use drilling fluids
able to minimize or eliminate clay swelling.
Most of the modern polymer-based
drilling fluids can provide those protective
features to sensitive clays and shales.
The use of water as a drilling fluid is
the cheapest option but bears some
major risks, especially in granular and
cohesive soil conditions. Water levels
within the pile shaft need to be managed
thoroughly by an experienced piling
crew with in-depth knowledge of the
site-specific ground conditions, the
selected excavation methods and the
general environment. Even if the
excavation of bored piles under water
might be possible, trenches cannot be
supported by water. The most proven
and hence safest method for the
stabilization of trenches is excavation
under bentonite fluids.
94 * DEEP FOUNDATIONS * JULY/AUG 2022
Bentonite Fluids
The suitability of bentonite as a drilling
support fluid was discovered in the
early to mid-20th century by oildrilling
companies in the U.S. and
Europe independently (Lam et al. 2018).
Bentonite is known as a swellable
hydroscopic clay with unique electrochemical
properties, allowing the
control of fluid migration into permeable
(granular) soil formations.
Bentoni te platelets can be best
described as rectangular, microscopic
particles carrying a positive electrical
charge at one end and a negative charge
at the other. Hydration is required for
bentonite clay platelets to be fully
dispersed into a slurry, which can take
up to 24 hours. When fully hydrated, the
water-saturated platelets suspended in
the gel-like slurry are transported to the
soil interface inside the excavation.
In permeable ground conditions, the
platelets suspended in the gel-like
slurry will initially migrate into the soil
as a result of the larger pressure head
inside the pile excavation, provided the
bentonite fluid head is kept 1 m (3.2 ft) or
more above the ground water level.
Shortly after starting to penetrate the
borehole wall, the bentonite particles
begin to build up at the interface
between soil and excavation through
filtration. Because of the electromagnetic
charge of the clay particles
they begin to orient in an organized way,
like a chain, with positive charged ends
connected to negatively charged ends.
Within a short time, the bentonite
particles form an impermeable clay
filter cake at the soil-drilling fluid
interface, which hinders and eventually
stops the flow of the bentonite drilling
fluid into the permeable soil formation.
The bentonite filter cake not only
stops fluid migration into the permeable
soil formation but also helps to equalize
the pore water pressure. As a result, the
permeable soil will not be " soaked " by
the bentonite drilling fluid but will keep
most of its natural characteristics,
including its original shear strength.
Furthermore, the filter cake allows for
the bentonite slurry inside the excavation
to apply increased hydrostatic
pressure, because of the increased
pressure head, to the sidewalls of the
excavation, which results in increased
stability of the excavation as long as a
positive pressure head is achieved.
Even though bentonite slurries work
well in permeable soil conditions, they
have some disadvantages in impermeable
formations like clays, shale or
silt where a bentonite filter cake cannot
be created because of the absence of
initial fluid migration through the
soil/excavation interface. Even thick
and fully hydrated bentonite slurries
contain significant amounts of free
water molecules that will react with the
interface of water-sensitive finegrained
soil formations, causing them
to potentially hydrate, resulting in
softening and swelling of the borehole
walls. This softening can drastically
reduce the shaft resistance of deep
foundations and or even cause partial
collapses of the temporary excavation.
Polymer Fluids
The limitations of bentonite-based
support fluid systems in clay and shale
formations stimulated the development
of polymer-based slurries that do
not cause clay swelling, and hence
provide a safe alternative for constructing
deep excavations in cohesive
ground conditions without potentially
weakening and or destabilizing the
borehole or trench walls. Natural polymers
(xanthan, carboxyl methylcellulose
or guar gums) were tested for piling
applications in the 1980s and 1990s but
were found to be too unstable (Jefferis
and Lam 2013) with limited ranges of
properties and significant weaknesses
in fluid loss behavior. Such natural polymers
are generally unsuitable to support
excavations for deep foundations.
Long-chain partially hydrolyzed
polyacrylamide (PHPA) polymers were
introduced in the mid-1980s to the
construction industry and the piling
July/August 2022

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