July/August 2021 - 112

rock socket. The top of competent rock
elevation is being determined based on
both the confirmatory borings and
observations made in the field during
excavation. The method proposed to
install the shafts includes stable,
uncased excavation through the glacial
till and weathered rock. During
excavation, the slurry level in the
drilled shaft is being maintained at
least 5 ft (1.5 m) above the water level
surrounding the casing, or within 2 to
3 ft (0.6 to 0.9 m) below the top of the
casing. The referenced " sufficient
casing stick-up " was determined with
tidal fluctuation in mind, to maintain a
positive head of slurry and, therefore,
shaft stability. During the tremie concrete
placement, the addition of concrete
into the shaft naturally displaces
the slurry upward within the casing.
Spoil material removed from the
shaft excavation is discharged onto a
spoil barge that also contains jersey
barrier/filter fabric/polyurethane tarp
spoil bins to prevent any discharge in
waterways. After excavation, and prior
to the installation of the reinforcing steel
and placement of concrete, the bottom
of the drilled shaft is being airlifted and
cleaned of any sediment/debris. To
confirm the cleanliness and soundness
of the rock socket bearing surface, a
specific camera is being lowered into
the hole and used to document
sediment thickness. The center of the
shaft and the bottom four quadrants
(five locations in total) are being
thoroughly inspected to ensure the
sediment thickness does not exceed
specifications' allowable limits. The
crosshole sonic logging (CSL) test
method is also being used for quality
assurance on the integrity of each
installed shaft. The project is particularly
challenging due to the marine
work in a cold climate, in terms of
logistic and safety.
Soil Improvements to Counteract
Melting Permafrost
The Red Dog Mine is just under 621 mi
(1,000 km) northwest of Anchorage and
supplies around 10% of the world's zinc.
Located in the Arctic Circle region, the
entire mine is in a geological permafrost
area, so that the ground is completely
frozen from a certain depth - with the
exception of an active zone near the
surface (that thaws in summer). Based
on an assessment of the permafrost as
well as the soil on site, soil improvements
were identified as a prudent
measure to counteract the effects of a
possible melting permafrost. Bauer
Foundation Corp. was commissioned
by Teck, a Canadian mining company, to
carry out field tests in fall 2019 using jet
grouting and cutter-soil mixing (CSM)
processes. The CSM method was
deemed best suited for the main work of
the project, in combining features of the
diaphragm wall technology and the
mixed-in-place method (MIP). The
existing soil is broken up using a milling
machine, relocated and mixed with an
aggregate. As part of the tests for the Red
Dog Mine, it was also determined to
what extent the existing floors should be
replaced with suitable filling material.
In 2020, 50% of the pilot drilling and
30% of the CSM work were completed
between July and November. In addition,
Bauer was commissioned with
constructing an overlapping pile wall as
an extension of an existing diaphragm
wall. For this purpose, the necessary
pile wall was integrated into the
existing diaphragm wall and the rock
below, using primary and secondary
piles. A total of 93 secondary piles were
constructed. A multi-purpose BAUER
BG 30 drilling rig with special Arctic
equipment was used for the work. One
of the greatest challenges was mobilizing
the equipment within a strict
schedule. Some of the equipment was
transported by air and via the Port of
Seattle by ship across the Bering Sea to a
pier 50 mi (80 km) from the mine. Other
equipment was transported by Hercules
aircraft from Ted Stevens Anchorage
International Airport to the airport
on the mine site. This logistical success
was made possible by the close cooperation
with various Bauer branches and
Teck. Despite such challenges, the
Bauer and customer teams managed to
successfully complete the test work
and the first part of the production
activities by November 2020. The final
production phase, which includes both
CSM work and demobilization, is
expected to end this summer.
112 * DEEP FOUNDATIONS * JULY/AUG 2021
July/August 2021

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