Underground Infrastructure - May 2023 - 18

MICROTUNNELING
Challenges Become Firsts
in Chelsea River Crossing Project
Untold Story of a Key 'Big Dig' Project and Fundamental Role of the Contractor
What attracts many contractors to underground
construction are the challenges
- geological, mechanical and/or technological
- they face. More than in other
fields of construction, contractors have
the leading role in deciding on the course
of events that they, alone, can control. As
a result of overcoming challenges, technological
firsts can be established.
This was definitely the case with
Massa chusetts Water Resources Authority's
(MWRA) project to replace a pressurized
water syphon under the Chelsea
River channel at the North Point Fort
area of East Boston.
It involved installation, by means of
a 1,100-linear-foot (LF) microtunnel
under the river, with a 72-inch diameter
drive allowing insertion of a 48-inch
diameter cast steel pressure water line.
Diaphragm walls were used for the
construction of both launch and receiving
shafts, with an invert 85 feet down
from the ground surface. In view of occasional
boulder obstacles in the till, a
new Soltau RVS 800 machine, powered
by three planetary motors, was selected
to allow access to the face through the
bulkhead to eventually break down the
obstacle under compressed air.
A hyperbaric chamber was available
onsite but was never used. A 72-inch
diameter, one-inch-thick Permalok pipe
was used as the jacked pipe, the largest
diameter available at the time. Although
an exit seal was present at the launch
shaft, no re-entry seal was planned at
the receiving shaft, given the uncertainty
about accurately reaching the target.
The geological profile along the
alignment was through glacial till with a
coarse alluvium passage in the middle of
the bore corresponding to the Chelsea
River bed original thalweg. The ground
cover between the bore and the river bed
was approximately 35 feet.
Firsts achieved
In the course of this project, numerous
firsts were performed that contributed
substantially to the progress of microtunneling
technology that is of great use
in today's ever-intensifying conquest of
underground space:
* Tri-motor U.S. cutting head drive
micro-tunneller slurry shield
* Over 1,000 LF of 72-Permalok
jacked pipe drive
* Dry receiving shaft entry with 80 feet
of head and dynamic annulus grouting
* Annulus backfilling by gravity
flooding with slow-setting slurry-grout
The project was completed during the
summer of 2000.
Once underway, jacking forces became
a concern even before reaching the
alluvium. Machine interruptions were
caused by breakdowns, due mainly to
the abundance of boulders, and the high
alluvium permeability required lubrication
improvements that were implemented
successfully.
Project Site Location
18 MAY 2023 | UndergroundInfrastructure.com
Despite mechanical problems with
the equipment, the question of how to arrive
at the receiving shaft in the dry, with
possibly an 80-foot water head on the
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