September/October 2023 - 92

Several other major utilities crossed
- and in some cases ran parallel to -
the cutoff wall. Among the many
utilities, it is worth mentioning the
following: A high-voltage transmission
line and a gas pipeline, which ran
parallel to the cutoff wall along its
entire length and crossed it at the
upstream end; a conveyor belt that
crossed the cutoff wall to transport
grains from storage silos located on the
land side of the wall to barges stationed
along the Mississippi's shore. Also, a
bridge crossed the river, to connect
Missouri and Illinois, close to the north
end of the cutoff wall.
The project design included the
installation of nine inclinometers in
sets of three. Two sets were installed at
the location of a cutoff wall demonstration
section while the third set was
installed around 3,000 ft (915 m)
downstream. The purpose of the
inclinometers was to monitor any
movement of the surrounding soils that
could signal any potential collapse of
the excavation trench. A series of
increasing movement thresholds and
alarm triggers were set so that the
onsite personnel would be immediately
notified once any of the thresholds
were surpassed, so that appropriate,
and preplanned, emergency actions
could be taken immediately.
Cutoff Wall Construction
Equipment
The contractor determined that the
installation of this cutoff wall required
the use of a hydraulic clamshell grab
(Bauer DHGV) combined with a cutter
(Bauer BC-40), both mounted on Bauer
MC-96 cranes. The cutter was equipped
with a portable desanding unit.
The batching plant, where the selfhardening
slurry was produced, was
equipped with five mixers (three Bauer
CMS-45 dedicated to the preparation of
the self-hardening slurry and two Bauer
SKC-30 for the batching of bentonite
slurry), tanks for water storage and
bentonite hydration, vertical and horizontal
silos for bentonite and binder
92 * DEEP FOUNDATIONS * SEPT/OCT 2023
powders, insulated and heated containers
where additives were stored,
and two trailer mounted 500 kW
generators. Due to the winter conditions
in the area, the batching plant complex
was enclosed in an insulating structure.
Self-Hardening Slurry Mix
Design Program
The self-hardening slurry is a combination
of water, hydrated bentonite slurry,
binder and additives. The slurry has the
property of solidifying after a curing
period that varies based on the ratios
between its ingredients. The contractor
performed an extensive mix design
program to identify a mix that best
suited the geotechnical properties of the
excavated soils and guaranteed the
desired workability life of up to 40 hours,
rendered necessary by the need to
excavate triple-bite panels with a
minimum 2 ft (0.9 m) embedment in the
hard bedrock layer.
Cutoff Wall Installation
The installation of the cutoff wall was
carried out in two stages: a demonstration
section stage and a production
stage. The scope of the demonstration
section phase was to verify that the
construction method provides a cutoff
wall that meets the specified requirements.
This stage was followed by a
quality control testing period at the end
of which, the contractor was required to
prepare and submit a comprehensive
construction report for USACE's review
and approval. Once approved the
demonstration section would become
part of the final cutoff wall.
The contractor received the notice to
proceed in January 2020 and, after a
mobilization that proved unusually
challenging due to the effects of the
COVID-19 pandemic, proceeded with
the installation of the demonstration
section, which was successfully completed
in May of the same year. Upon
USACE's approval of the construction
report, the production stage started at
the end of July 2020. The installation of
the first production panel saw a series of
adverse occurrences manifesting themselves
all at the same time (severe
weather, including violent lightning
storms, and a wall section with extremely
hard bedrock, to mention a few)
to create the proverbial " perfect storm "
that resulted in the cutter getting
trapped at the bottom of the panel at a
depth of approximately 140 ft (43 m). In
response to the issue, and to ensure that
the project schedule was not affected,
the contractor flew in a second cutter
unit that was never utilized since the
original cutter was retrieved from the
excavation 17 days later. In the months
that followed, crews installed the
remaining 260 panels to successfully
complete the cutoff wall construction.
The major issues experienced during the
production stage were the presence of
obstructions, consisting mostly of industrial
remnants; and a high slurry
overconsumption that reached peaks of
almost 130% and averaged at about 90%,
due to the higher than anticipated
permeability of the gravel layer.
Additional Work at the Cutoff
Wall Windows
In early 2021, USACE consulted with
the contractor regarding its intention to
reduce the untreated area at each one of
the two windows. Expressing interest
to pursue USACE's goal and, after a
request for proposal was issued by
USACE, the two teams partnered to
achieve a design that would meet the
necessary hydraulic requirements,
while being constructible. The final
design comprised a combination of
additional single-phase cutoff wall
panels and jet grout columns to be
installed at each one of the two
locations. The cutoff wall was to be
installed above and to the sides of each
utility while the jet grout columns
would provide as much closure below
the utilities as possible. Most of the
columns installed under the utilities
were angled between 15-30 degrees
from the vertical to form a fan-shaped
jet grout curtain. A few vertical columns
were installed to the sides of one of the
September/October 2023

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