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MacRobert Award
2002 Finalist
Mott MacDonald
 A team led by Mott MacDonald, comprised of Alan Powderham, Stephen Taylor, Christopher Howe, Dr Douglas Allenby and John Ropkins, were responsible for one of the most daring civil engineering projects ever attempted. Three full-size interstate highway tunnels, the largest well over 100 metres long each with sections weighing up to 30,000 tonnes, were jacked under an operating commuter railway in downtown Boston as part of the Central Artery Project to relieve traffic congestion in the city centre.
Innovation
The tunnel jacking scheme
– a crucial component of the overall project, which became known in Boston as the
'Big Dig' faced many challenges but the benefits were huge if the engineers could pull it off. Mott MacDonald proposed jacking, or
'sliding', the tunnels into position rather than the conventional
'cut and cover' tunnelling, which would have meant relocating the railway five times! With seven interconnecting rail tracks carrying over 40,000 people a day, this was a non-starter from both safety and railway operational points of view. This was the world's largest jacked tunnel project with the three tunnels each ten times bigger than any ever built before in the USA. However, it meant that the railway could keep running the whole time, even though it was just two metres above the works in places.
To carry out the tunnel-jacking operation, three concrete jacking pits were dug and tunnel boxes 24 metres (80 feet) wide and 12 metres (40 feet) high were built inside the pits. The plan was to break the head ends of the concrete pits and push the tunnel boxes into place with massive hydraulic jacks. In order to move the tunnel box along once it was inside the jacking pit, crews broke the head end of the pit, removed three feet of soil and pushed the tunnel box ahead.
 However, the soil was of very poor quality and pushing the tunnel boxes into place without stabilizing it could cause the railway tracks to settle, threatening train service. The solution was to freeze the soil ahead of the tunnel boxes, using hundreds of steel pipes that were driven into the ground between the tracks. A brine mixture that stayed liquid below zero degrees Celsius was pumped into plastic pipes within the steel pipes by a freezing plant located near the rail tracks. The brine was circulated back to the freezing plant and returned to the pipes again. The ground became frozen after several weeks of this treatment and could be excavated.
The soil ahead of the tunnel box was excavated using a road header, a machine with a rotating grinder at the end of a movable arm. The grinder broke the frozen soil apart, this was gathered and removed through the back of the tunnel box. When the soil around the freeze pipes was ground, the pipes were then cut away.
Two sets of hydraulic jacks were used to drive the tunnel boxes forward at a rate of three to six feet per day. The team also developed a new anti-drag system above and beneath the tunnel sections using a system of steel ropes to help them slide into place more easily.
The tunnel jacking was begun in late summer 1999 and the final tunnel was finished in February 2001. The Central Artery Project will be completed in 2005. Designed to take most of
Boston's highway traffic underground, it is the biggest, most demanding infrastructure project the USA has ever undertaken.
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