Three road tunnels forming part of the new
Boston Central Artery/Tunnel System have been constructed under
the railway tracks leading into Boston’s South Station,
which has more than 300 train movements each day.
The absolute requirement from the railway owners not to interrupt any rail services resulted in the use of jacked box tunnelling techniques.
John Ropkins Ltd in association with Edmund
Nuttall Ltd provided box jacking design, planning and
installation services to the Slattery / Interbeton / J.F. White /
Perini joint venture through Edmund Nuttall’s sister company
Interbeton Inc. It was due to Ropkins' expertise in jacked box
tunnelling and Nuttall's extensive general tunnelling experience
that the joint venture was able to secure a US$400m contract
comprising the jacked box tunnels and associated roadworks.
Extensive ground
improvement was necessary to stabilise the weak water bearing
strata. Obstructions included the remains of old wharves,
foundations and railway tracks, the existing sea wall and the
foundation of a triple bascule rail bridge currently in use. It
was decided that the only way to guarantee the integrity of the
railway tracks was to freeze the ground along the alignment of
each tunnel. This involved drilling a grid of vertical freeze
holes to completely freeze the ground to be tunnelled through. A
total of nearly 2,000 vertical freeze pipes was installed. The
large size and length of these tunnels meant that they had to be
split into two or three units, requiring the use of intermediate
jacking stations.
The
patented ‘anti-drag system’ (ADS) was used at the top
and bottom of each tunnel. Using specially designed roadheaders
to excavate the tunnel face, the tunnel units were jacked forward
sequentially in 300mm increments. Because of limited working
space, the first two units of the longest tunnel were constructed
and the lead unit advanced into the ground before the third unit
could be cast and the tunnelling operation completed.
The top
photograph shows the lead and middle units of the longest tunnel
in the jacking pit. Adjacent and to the left is a second jacking
pit with a three unit tunnel under construction. Further to the
left is a third jacking pit from which a two unit tunnel has been
jacked into position under the railway.
An accurate
vertical alignment was achieved on all three tunnels by means of
a long lead unit, a carefully aligned jacking base and the bottom
ADS.
This is believed to be the largest jacked box tunnelling contract ever undertaken world-wide.
This project won the British Construction Industry International Project Award 2002, and was a finalist in The Royal Academy of Engineering MacRobert Award 2002.
Client: Massachusetts Highways Department
System design: John Ropkins Ltd
Contractor: SIWP Joint Venture
Scope of work: Design and construct
Contract period: 48 months
Completion date: January 2001
© John Ropkins Ltd