Underground Infrastructure - May 2023 - 21

Microtunneling
fourth day. Breakthrough did occur after
three days. Modern Continental was
willing to risk that the last 60 feet of the
bore forward would be glacial till, consistently,
with no saturated seam leaving
the bore unprotected.
The gamble paid off with the exit
through the diaphragm wall occurring
in a perfectly dry condition. The active
zone was maintained until the trailing
tube was extracted and the pipe penetrated
through the diaphragm wall.
Conventional sealing and waterproofing
were performed from inside the shaft.
Installation of the 1,100-LF 48-inch
cast steel bell and spigot pressure pipe
riding on radiant spacers sliding on plastic
pad was a new challenge in itself. Given
the fragility of three rings per 20-foot pipe
section, Modern had to pull out on a few
occasions to replace damaged spacers.
Annular space backfilling
Following successful pressure testing of
the service pipe, the final challenge was
to grout the annulus between the 72- and
48-inch pipes. Installing grout lines concurrently
with the service pipe would have
been difficult and quality control virtually
impossible, after insertion of the pipe.
Modern proposed and MWRA approved
a novel approach: grouting the entire
length of the 1,100-LF drive by bulkheading
both ends of the annulus at the
shafts. This provided an entry port at the
invert on the launch shaft and a vent port
at the spring line at the receiving shaft,
both connected to a riser to the surface.
Slow-setting, proprietary, non-shrink,
self-hardening slurry was mixed on the
surface and dropped to the bottom of
the shaft through a three-inch riser connected
to the entry port. By flooding the
annulus from one end to the other, the
annulus backfilling was completed with
no possibility of trapping air.
Once the slurry-grout emerged
through the exit riser at the surface, with
®
the same density as when going in, backfilling
was terminated and kept under the
vertical column pressure, until completely
set, in about one week. Although it could
have taken place over a single long shift,
given the slow setting nature of the formulation,
the backfill was completed in two
short shifts. The simplicity of the process
inherently carries assurance of a 100-percent
homogenous annulus backfill. UI
ABOUT THE AUTHOR: Gilbert Tallard is
a civil engineer whose career has been linked
to many aspects of underground construction
and environmental remediation. His specialty is
drilling and support fluids, grouts and selfhardening
slurries. At the time of this project,
he provided Modern Continental with
lubrication and self-hardening slurries advice
and formulations. Also contributing was material
referenced from a paper on geological and
mechanical aspects of the project: Challenges
and Successes in Micro-Tunneling in the Chelsea
River Crossing by Peter J. Tarkoy, Ph.D.
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UndergroundInfrastructure.com | MAY 2023 21

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Underground Infrastructure - May 2023

Table of Contents for the Digital Edition of Underground Infrastructure - May 2023