Sinking Boston's elevated I-93 highway underground and extending the I-90 interstate highway from central Boston to Logan Airport comprises 109 construction contracts. The project, which is now two thirds complete, will provide 260km of new highway lanesand is due to be competed by 2004.
Four tasks pose the greatest challenges for the programme managers and the rest of the construction team.
1) Constructing northbound and southbound tunnels on the I-93 state highway to New Hampshire.
Capacity of the elevated artery is 75,000 vehicles a day, but an estimated 190,000 vehicles now use the route every 24 hours. An 8km tunnel with 8-10 lanes will replace the six lane artery providing a new capacity of 245,000 vehicles a day. The northbound tunnel is set to complete in late 2002 and the southbound tunnel by early 2003. Demolition of the elevated highway and the creation of parks in its place will begin in 2002.
The engineering challenges on the job include creating an estimated 8,000 linear metres of steel reinforced concrete tunnel walls in the USA's biggest ever slurry wall construction operation. A cut and cover tunnel 37m deep is being created while traffic moves overhead.
2) The Charles River Crossing
The widest cable stayed bridge in the world, providing six lanes northbound and four lanes southbound, is expected to complete by late 2001.
The cable stayed bridge is the first to use an asymmetrical design, and to comprise a hybrid design of steel in the main span and concrete in the back span.
Alongside the cable stayed crossing, the four lane Leverate Circle Connector bridge will link the busy Storrow Drive from the western Boston suburbs with the north of the city. The connector was finished on time last October.
3) The South Bay Interchange and Ted Williams Tunnel
Traffic for the airport or travelling north of Boston, which is currently forced to go through the centre of the city, will be able to come off the I-93 and join the new I-90 connection at the South Bay Interchange when it is completed in 2003. The estimated 85,000 vehicles a day will instead use the new Ted Williams Tunnel, which was finished on time in 1995. The underwater approach to the Ted Williams Tunnel through the Fort Port Channel is expected to be completed in late 2001.
Part of the tunnel is being built in the widest and deepest circular cofferdam in the USA, on the doorstep of a major US Post Office building and the Gillette factory. The tunnel project also involves building the largest highway tunnel ventilation system in the world.
Ground on the approach to the Fort Point Channel has been frozen to stabilise the rail tracks above, while the tunnels are jacked.Weak ground underneath the immersed tube tunnels in Fort Point Channel is being mixed with shotcrete.
4) Traffic mitigation.
An enormous traffic and environmental mitigation programme accounts for one third of the Big Dig budget. The operation includes a computerised tracking system to monitor all mitigation commitments. An army of community liaison officers field neighbourhood and business concerns; a monitoring centre provides 24 hour telephoneaccess for the public to forward complaints and a night time
noise patrol team stalks the construction sites.