Aerial view of site 9A4 showing thrust pits adjacent
to railwayProject Description
The I-90/I-93 interchange is one of the most complex sections of
the Boston Central Artery Tunnel project, linking the north-south
Central Artery to the Ted Williams Tunnel leading to Logan Airport.
Part of this multi-level interchange includes three tunnelled
crossings beneath eight active railway tracks providing MBTA and
Amtrak service into Boston’s South Station. Interruption of rail
traffic was inevitable under the preliminary engineering design.
Hatch Mott MacDonald (HMM), Mott MacDonald’s North American sister
company, proposed an alternative tunnel jacking solution that
allowed construction of the tunnel without interrupting train
service, saving millions of dollars of railway operating revenues.
The primary features of the tunnel jacking proposal included:
- Three jacked tunnels which are the largest and most complex
ever constructed in the world. The largest tunnel section measures
79-feet wide by 36-feet high by 370-feet long
- Three 80 feet deep jacking pits, to facilitate tunnel
construction and propulsion constructed using a combination of
cantilevered and post-tensioned and conventionally braced diaphragm
walls.
- 1200-ft cut-and-cover tunnel as approaches to the jacked tunnel
segments
- A combination of dewatering and ground treatment comprising jet
grouting and ground freezing to control and minimize track
settlement during excavation of the jacking pits, and subsequent
tunnel installation.
As part of this an extensive geotechnical instrumentation program
was established to enable continuous monitoring of ground and track
movements associated with the tunneling effort.
Roadheaders excavate the frozen ground tunnel
faceMott MacDonald Role
As well as providing design and construction services for the
tunnel sections and jacking pits Mott MacDonald provided extensive
design and construction phase services associated with the
geotechnical instrumentation for the project. In conjunction with
the project management consultant and the MBTA, HMMestablished and
specified tolerable rail movement criteria for the contract,
establishing thresholds for remedial response actions. During
construction HMM provided full and complete interpretation of
observed movements and reporting of the impacts. The specified
instrumentation included:
- Inclinometers in slurry walls to monitor lateral wall
movements
- Inclinometers and magnetic probe extensometers to monitor
horizontal and vertical ground movements associated with jacking
pit and tunnel construction induced by effects of jet grouting,
excavation, ground freezing and tunnel jacking.
- Vibrating wire piezometers and observation wells monitored
ground water pressures.
Freeze pipes on railway lineGround
temperatures during freezing were monitored using temperature
sensors located between freeze pipes.- Precise manual leveling of top of the rails was used for
vertical control
- Horizontal track monitoring was achieved by optical survey
using total station
- A project wide Oracle database was established to manage
monitoring data and was accessible to all project staff. The
database allowed real-time monitoring of track movements, and
contained alarms to warn as threshold and limiting movement values
were approached
- The intranet site RAILMON was developed for fast processing of
the track data
Project Highlights
I-90 Westbound thrust pitThe jacked
tunnels are the largest and most complex ever constructed in the
world- Deepest tunnel has 24-feet of cover
- Shallowest tunnel has 6-feet of cover
- Major foundation engineering challenge presented by the scale
and complexity of the sub-surface construction combined with the
variable and difficult ground conditions. Mostly reclaimed land
consisting of Boston Blue Clay, thin layers of fine sand, glacial
till. The bedrock is Cambridge Argillite
- Combination of dewatering and ground treatment to control and
minimize settlement during excavation
- Comprehensive geotechnical instrumentation strategy implemented
to monitor track and ground movements
- All tunnel boxes were successfully jacked without interruption
to rail service
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