Underground Infrastructure - May 2023 - 19

annulus, became a preoccupation for all
involved. Receiving the machine in the
wet was not an option due to subsequent
machine scheduling on another project.
Implementing an annulus controlled
dynamic backfilling, with specially formulated
self-hardening slurry, provided
the solution for an exit in the dry into the
receiving shaft.
Access shafts construction
The general contractor, utility contractor
Modern Continental, undertook specialty
disciplines, such as deep foundations,
inclusive of diaphragm walls, microtunneling
(assisted by Westcon), on top of
regular heavy construction activities.
Challenges included installing a circular
slurry wall, a classic method for
deep shafts sinking in soils, using conventional
clamshells and chisels under
bentonite slurry.
On the North Point side, old deep seashore
obstructions, as well as boulders on
both sides, created trenching difficulties
that were overcome but resulted in substantial
concrete over-pours and required
joint leakage mitigation techniques. Very
hard till soil at subgrade allowed an excavation
in the dry without requiring a
tremie seal, despite the hydrostatic head.
Finally, the shafts performed well, as
intended.
The Soltau RVS 800 was a new slurry
shield type of machine. Water is used
to convey the excavation spoil to the
surface, where a solids separation plant
removes the cuttings and recirculates the
water. Depending upon the soil, bentonite
or polymers are added to the water
to reduce outbound slurry losses and
improve face stability. Permalok joined
jacking pipes represented a technological
advancement and 72-inch was the
largest diameter available.
Further, Modern Continental had
gained substantial experience with microtunneling
around Boston, albeit on
shorter and shallower drives.
As is typical with all new machines,
breakdowns were inevitable.
Eventually, boring issues led to
the replacement of some motors and
The microtunneling machine is lowered into the entry shaft.
UndergroundInfrastructure.com | MAY 2023 19
gearing, immobilizing the machine at
crucial times. The till contained many
more boulders than anticipated by the
DBR, causing slow progress and hardship
for the machine.
During interruptions, lubrication
with bentonite slurry was attempted.
However, with 80 feet of external head
pressure, high lubrication counter-pressures
caused some leakage at the entrance
seal, ultimately allowing brackish
water to appear. A doubling of the
entry seal eliminated this problem and
allowed reconditioning of the annulus.
Lubrication
High lubricant pressure causes thick
mud cakes to form, by filtration at the
interface of the bore with the ground,
reducing the annular space and potentially
increasing future jacking forces.
Delays and brackish groundwater would
have a negative impact on bentonite filtration
performance and could cause
Geologist tunnel expert Dr. Peter J. Tarkoy (PJ@Tarkoy.HU) stands beside the 72-inch Soltau RVS 800
exit in the receiving shaft.

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Underground Infrastructure - May 2023

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