Construction of jacked tunnel
Requiring a quantum leap in scale – over ten times the size of any
jacked tunnels attempted in the US before – the Boston Central
Artery/Tunnel has been the largest US Federal Works engineering
project ever undertaken. The project has cost approximately $14.8
billion to design and construct, spanning 20 years from project
inception to completion.
Development involved the replacement of the steel elevated
Interstate 93 highway through downtown Boston with a new subsurface
highway built directly beneath the existing elevated structure. In
addition, there was construction of a new highway, much of which is
through tunnel, from the interchange of Interstates 90/93 in
downtown Boston to Logan International Airport, to the east in
Boston Harbour. Overall, the project entailed construction of 7.5
miles of new highway with approximately half running underground.
The project dramatically reduced congestion and the environmental
and socio-economic consequences of such a huge construction project
in the heart of a major American city were of paramount importance
to both the Federal and State governments.
Mott MacDonald was appointed as foundation engineering
sub-consultant for Contract 9A4. Contract 9A4 was one of around 50
contracts on the CA/T project. It was one of the most challenging
elements of the project and involved the design and construction of
a variety of complex permanent and temporary works required for the
new interchange between the I-90 and I-93 highways. The works
entailed construction of bridge viaducts, cut and cover tunnels and
jacked tunnels. The tunnel jacking project involved the design,
construction and installation, in difficult ground conditions, of
three large jacked tunnels and jacking pits (deep excavations) plus
ground stabilisation by ground freezing beneath a fully operational
multi-track railway leading into Boston’s South Station. The size
of the three jacked box sections was approximately 24m wide by 11m
high with a maximum length of 115m. Ground cover to the top of the
jacked boxes was as little as 1.8m off-track and 2.5m below track.
The jacked tunnels provide dual lane highways beneath the railway
as part of the new interchange of Interstates 90 and 93. All
railway operations had to continue as normal, with over 400 train
movements in and out of South Station per day.
Our role
Mott MacDonald and US sister company Hatch Mott MacDonald proposed,
developed and undertook the design and site supervision of the
three reinforced concrete jacked box tunnels, jacking pits, cut and
cover tunnel and associated works. This presented many challenges,
as the works were carried out in poor ground conditions, with tidal
groundwater and with many buried obstructions resulting from over
200 years of industrial use and land reclamation in the area. The
general geological stratigraphy comprised superficial deposits of
variable fill and organic material overlying significant thickness
(10-30m) of Boston Blue Clay. The upper layer of the clay was
stiffer than the rest of the deposit as a result of earlier
oxidization. Thin layers of fine sand are also found throughout the
deposit. Glacial till underlies the clay, and the bedrock (at 30m+)
was Cambridge Argillite.
Project highlights:
Artificial ground freezing in the track near
Boston South StationExcavation adjacent to and
tunnelling beneath overhead electrified railway, while maintaining
fully operational railway service- Development of an innovative tunnel jacking system
- Three full cross-section jacked tunnels, forming one of the
largest and most comprehensive schemes of this type ever
undertaken
- Size of jacked sections: 24m wide x 11m high x 115m (max) long
with cover in the range of 1.8m-8m
- Major foundation engineering challenge presented by the scale
and complexity of the sub-surface construction combined with the
variable and difficult ground conditions
- Major artificial ground freezing operation to stabilise ground
and limit ground and railway movements
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