What is almost certainly the most expensive and complicated interchange in the history of world highways is beginning to take shape in the centre of Boston. A three dimensional nightmare of tunnels and viaducts, the South Bay interchange, has to be completed within three years, at a cost of $397M (£244M), while keeping 190,000 vehicles a day flowing freely through the centre of the site.
This alone would be challenge enough, but the interchange team also has to cope with nine main surface level railway lines, Boston's Red Line metro below, and the nearby US Post Office main distribution centre.
On one key section, tunnelling expertise in poor ground conditions is making a major contribution to the project's success. This is where the new link to the Ted Williams tunnel has to find a way eastward beneath the busy main rail lines and over the Red Line metro. A concrete immersed tube tunnel was chosen for the highly skewed river crossing, but to get under the rail lines British-Canadian consultant Hatch Mott MacDonald went for a jacked tunnel solution, one of the largest and most complex ever attempted in the US. This has to cope with the legacy of Boston's unique history.
Back in Victorian times the city went in for a gigantic land reclamation programme along the shores of the many creeks and rivers that flowed into the harbour - a decades-long exercise known as 'cutting down the hills to fill the valleys' which eventually more than quadrupled the city's land area.
These reclaimed areas provided natural transport corridors, used by both rail and road builders. Logan Airport itself is built on one of the last areas to be reclaimed. But the route taken by the Central Artery crosses one of the earliest reclamation sites, where at least two generations of docks and jetties, and a few sunken vessels, have been abandoned and buried in their turn. The first 5m is a highly unpredictable mixture of gravel, rubble and timber, overlaying some 3m of soft organic material. Below that is Boston Blue Clay, a more amenable material despite its tendency to develop a stiff crust. All in all, these are not ideal conditions in which to build 14 viaducts with a combined length of 3km and more than 1.75km of tunnels, even though most of the tunnels will be relatively straightforward cut and cover.
Hatch Mott MacDonald project manager Steve Taylor says tunnel jacking was by far the best way of getting under the rail lines, which have to be kept live throughout. 'At the closest point the tunnels are only 2m below the tracks, but cut and cover construction is not practical because track diversion is not an option.'
Hatch Mott MacDonald originally went for soil stabilisation via a combination of permeation grouting in the fill, jet grouting in the organic stratum, and steel reinforced soil nailing in the clay. But when the Dutch-American joint venture SIWP, made up of Slattery, Interbeton, JF White and Perini, was awarded the £244M contract for the interchange in January this year, plans began to change.
'The contractor preferred to use soil freezing, even though this makes excavation more difficult, because of the greater predictability,' reports Taylor. 'With no free water in the ground, risk is obviously lower.'
In all, the contractor will have to freeze a layer of ground 1.5m thick around three 35m wide tunnels up to 110m long, and keep it frozen until February 2000. This task has been eased by the decision in June to delete one of the original off ramps. A neighbouring ramp was diverted into the main westbound tunnel, allowing one of the original four tunnels to be dispensed with.
SIWP is planning minor changes to the details of the jacked tunnel sections as well, which would result in the abandonment of the projected central guidance tunnel.
Taylor explains: 'We went for shorter sections up to 21m long, which are easier to steer, and need lower jacking forces.
'The contractor preferred longer sections around 30m, which meant fewer joints. Longer sections are harder to steer, but they are less likely to be deflected off-line.'
Another likely change is the dropping of the steel drag sheet above the tunnel sections, which was designed to reduce friction and disturbance of the ground immediately above the tunnel. Instead, Dutch-based Interbeton drew on the experience of its sister HBG company Edmund Nuttall and opted for drag-ropes top and bottom.
This system relies on a layer of longitudinal 19mm diameter steel cables at 38mm centres on the upper and lower surfaces of the tunnel units to reduce friction and aid steering. It is more familiar to the contractor, says Taylor. 'It is always a good idea to take advantage of a contractor's experience on projects of this kind.'
First pours on the tunnel sections are not expected until May next year. Jacking is due to begin in late 1998, and take 14 months. Jacking pits are now under construction to the north of the rail lines, and the first freezing pipes were installed in November 1997.