Underground Infrastructure - October 2023 - 24

Tunnel
adjusted to a more suitable soil layer,
the running sands would unavoidably be
encountered as part of the shaft excavations
on both the launch and receiving
sides. Due to the depth and expected
soil conditions, the contractor, Harper
Brothers Construction LLC, selected a
concrete secant pile shaft construction,
which would create a water-tight shaft,
similar to a coffer dam.
With the circular shafts being 44-foot
outside diameter on each side of the tunnel,
the inside diameter of 31 feet would
provide the necessary working area
needed to set and launch the tunnel boring
machine. With a secant-pile shaft, the
primary secant piles were initially driven,
then the secondary secant piles were
driven in an overlapping fashion among
the primary secant piles for a completely
integral and water-tight shaft. After the
piles were in place, the contractor began
excavating from the top.
A closer look at the earth pressure balance machine supplied by Akkerman that was used for the
96-inch-diameter tunnel excavation.
" The highlight of the project was
the secant pile shafts. The drilled secant
shafts were 99 feet and 104 feet,
which was pretty deep relative to other
projects. It was an incredible tunneling
project, " according to Corey Garnett,
project manager at Harper Brothers. " A
true partnering spirit was taken on between
the contractor, the owner and the
engineer, which was one reason why the
project was successful. "
EPBM goes to work
A view of the staging area for the 41-ft diameter secant pile shaft where tunneling operations
were launched.
After the shafts were completed and the
bottom seal slab set in place, Harper
Brothers used an earth-pressure balance
machine (EPBM) with a soft ground
cutter head designed for fat clay to excavate
the tunnel under US59. The 96in
EPBM could work in open mode or
closed mode by changing the primary
conveyor type - which is either a belt
conveyor (for non-pressurized conditions/open
mode) or a screw conveyor
(for the closed mode).
A view inside the 96-inch-diameter tunnel from within the launch shaft at 33 feet below sea level.
24 OCTOBER 2023 | UndergroundInfrastructure.com
The face of the machine had foam
injection ports for the injection of foam
or slurry for muck conditioning that aids
in controlled soil removal. The operator
could choose the advance rate or the
spoil removal rate, as necessary, to keep
the earth pressure balance within the
acceptable range. This prevents the soil
and groundwater from entering the face
of the tunnel in an uncontrolled manner.
As tunnel excavation progressed, the
contractor used a steel ring beam and
steel lagging support system designed to

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Underground Infrastructure - October 2023

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