Geosynthetics October/November 2022 - 16

Construction considerations for wick drain ground improvements-Part 2
did not match expectations. Boreholes
were drilled in these areas to confirm
the presence of a dense layer and associated
early termination of wick drains, as
shown in Figure 3. For the confirmed
locations and areas where zones of foundation
clay were not penetrated by wick
drains, and thus would not have the
same degree of strength gain as zones
with wick drains, 2D or 3D limit equilibrium
computer stability analyses were
carried out to verify that the factor of
safety for stockpile geotechnical stability
still met the design requirement.
FIGURE 3 Drilling investigation to
check the presence of a dense layer
Wick drain mandrel bending
During the second wick drain installation
campaign, a significant variation in
penetration depth was reported between
adjacent wick drains at certain areas (e.g.,
in the order of 49 feet [15 m] differences
in depth over the 7-foot [2-m] horizontal
wick drain spacing). Boreholes were
drilled at these locations to identify the
correct refusal depth. The boreholes indicated
that the shallower wick drain penetration
depth was often correct. Based
on the findings of the field program, it
was inferred that the wick drain mandrels
were bending and following the contact
with a steeply sloped surface on the bottom
sand till layer or bedrock, rather
than penetrating these layers to reach
refusal. Bending of the mandrel resulted
in incorrect reported penetration depths
of wick drains.
As part of the CQC and CQA, the rig
mandrels were checked to confirm they
remained straight, and bent mandrels, if
observed, were straightened or replaced.
Cold weather
For this project, water was used to lubricate
the mandrel, and as a result, wick
drains could not be installed when site
temperature fell below 32°F (0°C). An
attempt was made to install wick drains
in freezing conditions and without the
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Geosynthetics | October November 2022
use of water; however, due to the high
plasticity (stickiness) of clay, the rig was
not able to push the wick drains to the
design depth. While it may be possible to
advance a mandrel through shallow frost
depths, freezing air temperatures must be
considered if it is necessary to use water
as a lubricant to achieve the required
penetration depths.
Sufficiently covering the wick drains
with stockpile material prior to ground
freeze is another cold weather consideration.
If the cover cannot be placed on
the drainage blanket before the ground
starts to freeze, fill placement on the
wick drains must be delayed until the
foundation soil and the drainage blanket
are fully thawed the following spring. If
this procedure is not followed, a multiyear
frozen layer could be buried at and
below the drainage blanket, preventing
the drainage of water through the wick
drains and drainage blanket, and resulting
in wick drainage not performing as
the design intended. In this project, no
rockfill was allowed to be placed on top
of wick drains or the drainage blanket
while they were frozen.
As-built data collection
and data processing
Due to the large quantity of daily wick
drain installations, data management
was an important part of this project.
CQC installation data were collected
daily using the data acquisition systems,
reviewed and processed by the
contractor, and then provided daily to
the CQA team to review. CQA review
of the CQC data included spatial data
review of installation details using CAD
software. A color-coded plot showing
the refusal depth of each wick drain
was particularly useful in spotting nonconforming
installations.
The automated data acquisition system
and spatial data review techniques
implemented in this project made the
Geosynthetics October/November 2022

Table of Contents for the Digital Edition of Geosynthetics October/November 2022
