Underground Infrastructure - May 2023 - 20

Microtunneling
even thicker mud cakes than in a fresh
water environment.
For the 20-foot long Permalok pipe
sections, lubrication ports were initially
provided every other pipe, or at a 40-foot
gap. This was an optimistic approach to
lubrication that proved workable, until
mechanical breakdowns interrupted the
drive. Further on, getting into the pervious
gravelly alluvium of the Chelsea River's
thalweg, the loss of lubrication efficiency
increased jacking forces to a level
that compromised the ultimate goal.
An intermediate jacking station was
built and installed in record time, thus
making drive completion plausible. High
alluvium permeability allowed the bentonite
slurry to escape through the gravel,
possibly breaching the river bottom and
preventing the maintenance of sufficient
counter pressure to maintain the annulus'
integrity. Jacking pressures increased
significantly, in view of being only at midpoint
of the drive, using more than twothirds
of the available jacking force.
When a crab surprisingly appeared on
the shaker screen of the solids separation
plant, it was considered a prank or a sign
of a breakthrough to the river - which
could be possible, under the circumstances.
The mystery was never solved, but
induced serious efforts in controlling the
annulus, including increasing the number
of lubrication ports to each pipe section.
Another approach was formulating a
much thicker lubricating slurry, with very
low filtration, using more bentonite, polymers
and other gelling additives. This was
implemented all along the 240-LF alluvium
on the alignment, before reverting to
a straight bentonite/polymer lubrication
at each pipe and once regaining control
of the annulus past the river thalweg.
In addition, it was decided that the
lubrication around the machine should
be achieved independently of the jacked
pipes. A direct line to the machine
shield's ports was installed and dubbed
the " Bentonite Express. "
With the addition of the intermediate
jacking station, boring progressed slowly,
but regularly, and removed any doubt
about the outcome. Friction forces of
the section forward of the intermediate
jacking station were maintained at a
normal level, thanks in part to Bentonite
Express saturating the annulus around
the trailing pipe.
The section behind the intermediate
jacking station flirted with maximum
jacking frame capability available from
the launch shaft but did crawl forward.
Exit
It is not known whether the accuracy of
the guidance system, or the contractor's
lack of confidence with such a long drive
for the times, removed the possibility of
reaching the receiving shaft on-target
through a receiving seal.
Typically, in such a situation, the response
is to exit in a flooded shaft and
retrieve the machine with the help of divers.
This was not an option, because the
machine was committed to another project
and the current job was well behind
schedule, leaving no time to recondition
the flooded machine.
The problem was how to exit in the
dry with an annulus under 80 feet of
head, with a potential communication in
the Chelsea River. An imaginative solution
had to be devised without delaying
the progress of the machine.
Since the bore was back in the glacial
till, the ground was considered as essentially
impervious, albeit erodible soil.
The challenge was then to impede any
groundwater flow forward from the Chelsea
River alluvium bed through the annulus
into the receiving shaft. With concern
for not increasing the jacking forces in
any significant way, limited options were
available to control the annulus.
The final plan was to create an active
annulus sealing zone 40 feet long and
about 60 feet away from the receiving
shaft. This involved filling the annulus
with a slow-setting, self-hardening slurry-grout
that would pack the annulus
with a mud cake capable of resisting the
hydrostatic head, while remaining plastic
enough not to noticeably impact the
jacking force, until the machine could
eventually break through.
Injection pressures were maintained
at all times, even during jacking pipe
addition breaks. Injection ports were
created for the train of pipes that would
travel through the treatment zone, and
connections were made, accordingly, to
keep the process dynamic until breakout
and machine extraction.
Boring is about to begin on the Chelsea River Crossing project.
20 MAY 2023 | UndergroundInfrastructure.com
The fact that the injected formulation
was, nevertheless, cementitious required
close coordination between all trades.
Time was of the essence, with activities
taking place around the clock.
Based on rates of progress, the active
sealing zone would start setting after the
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