Geosynthetics October/November 2022 - 15

direction so that the masts of the wick
drain installation rigs could be adjusted
to vertical. Figure 2 shows a typical wick
drain installation orientation plan. The
grade changes at boundaries between
segments were normally rounded out
over an area due to prior and ongoing
trafficking over the drainage blanket.
Where a wick rig traversed a grade break
between segments, there was a zone over
roughly the length of the equipment
tracks where vertical installations could
not be maintained, and so it was necessary
to provide additional " fill-in " wick
drains in these zones.
Wick drain materials
In many locations, the confining pressures
that will be applied to the installed
wick drains are higher than those normally
experienced in wick drain ground
improvement. These high pressures
result from the depth of wick drain
installation (up to 131 feet [40 m]) combined
with more than 131 feet (40 m)
height of stockpile that will eventually
be placed on top of the current ground
surface. The concern was that the wick
drains currently available in the market
might not tolerate the high confining
pressures that could affect the drainage
performance of the installed wick
drains. Wick drain materials available
in the market were considered and compared.
The selected wick drain material
was tested prior to beginning the project
to confirm that it would meet the
design requirements. The wick drain
testing carried out included grab tensile
strength, puncture strength, trapezoidal
tear strength, permittivity, apparent
opening size, tensile strength, discharge
capacity and low-temperature brittleness.
An important design consideration
was the wick drains' discharge capacity
under the normal and clogged conditions
in the short and long term to
ensure that the wick drains will perform
as expected. Laboratory discharge capacity
testing was carried out on wick drains
for confining pressures up to 145 psi
(1,000 kPa). Additional testing was conducted
on samples obtained by construction
quality assurance (CQA) and CQC
during construction.
Wick drains discharge capacity may
be reduced due to the foundation clay
consolidation and the resulting vertical
and horizontal deformations within the
ground. It is important for the settlement
and horizontal deformations within
the clay foundation to be less than the
maximum wick drain material allowable
strain. In this project, the clay consolidation
and deformation analyses were carried
out using the Terzaghi method and
finite element modeling to assess the
foundation deformations during and at
the end of the stockpile operation.
The effect of a dense layer
Given the complicated surficial geology
of the site, it is possible for a dense
layer, cobbles or even boulders to occur
within the clay till layer. Such layers
could result in an early refusal of the
wick drain mandrel. Where such early
refusal occurs, the wick drains will not
penetrate through the entire depth of
the foundation clay layers and into the
underlying sand till. Early refusal of
a single wick drain is of little consequence;
however, this issue could substantially
affect the performance of wick
drains if the dense layer is extensive. If
a large area of foundation clay remains
untreated due to the presence of an
overlying dense layer, the clay without
wick drains will not experience strength
gain in the time required to raise the
stockpile, allowing potential failure
surfaces to develop at depth, passing
below the zone of wick drain foundation
improvement. In this project a few
dense layer areas were suspected based
on wick drain termination depths that
Due to the large
size of the wick
drain installation
area, it was not
possible to provide
a single uniformly
sloped surface for
the entire wick
drain area.
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Geosynthetics October/November 2022

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