Tunnel jacking
An innovative underground contruction technique in which tunnels are formed as pre-fab boxes, then gradually pushed under an active railroad or highway while excavators remove soil ahead of the tunnel box
By Peter J. Howe, Globe Staff, 3/8/1999
As work gears up this spring to extend the Massachusetts Turnpike to the new Ted Williams Tunnel, one of the more amazing construction feats planned involves shoving concrete boxes as big as basketball arenas under the busy railroad tracks outside South Station.
And to help the 30-million-pound tunnel sections move along, giant swaths of mucky ground will be frozen solid in the middle of summer to ease the job of mining out the soil.
As Big Dig engineers have geared up for completing one of the most difficult links in the $8 billion project, the stretch from the end of the turnpike towards the new tunnel to Logan Airport, they have faced a complex conundrum.
At the same time that the project is urging commuters to leave their cars at home to keep downtown traffic moving, traditional techniques for tunneling under the railroad tracks would have wrought chaos for more than 43,000 rail riders coming into South Station every day.
So, borrowing a technique that has been used extensively in England, Germany and Japan but almost never on such a vast scale, a team of contractors including the Dutch firm Interbeton have planned what will be North America's largest attempt at "tunnel jacking."
"If you put it all together, what they're doing in Boston is really the top end of the technology," said project engineer Stephen Taylor of Hatch Mott MacDonald, a California- and London-based firm, who is one of the world's leading experts on tunnel jacking. "This is the largest job in the US, period, and it ranks among the largest in the world."
Three sections of highway tunnel must be built between the current end of the turnpike and the Fort Point Channel: one each for the main east and west barrels of Interstate 90, and a third carrying a westbound off-ramp from I-90 to the Southeast Expressway, Interstate 93.
Inside gigantic underground vaults called jacking pits that are nearly complete, the tunnel sections will be built in the form of eight concrete boxes, three stories high, nearly 80 feet wide and 65 to 140 feet long.
More than 1,600 vertical pipes have been drilled in and around the railroad tracks to help freeze the soil -- most of which is soft, watery clay -- down to three feet above the bottom of the tunnel boxes.
The pipes are fed by a chilling plant that will take a brine mixture of water and calcium chloride and chill it to minus 22 degrees Fahrenheit. Pumping operations, which began last week at the first of the three jacking sites, will freeze the ground solid over a period of several weeks.
"What you get with the freezing is more control over the behavior of the ground," said Philip M. Rice, an assistant resident engineer for project manager Bechtel-Parsons Brinckerhoff overseeing the tunnel jacking.
The first section to be jacked, starting as early as June, will carry part of the off-ramp for vehicles headed west from the Ted Williams Tunnel down the new Mass Pike extension to Interstate 93 north and south.
As mining machines called roadheaders gouge out the frozen soil ahead for payloaders to carry off, the tunnel boxes over the course of a year will be steadily pushed through the earth by about 60 to 90 immense hydraulic jacks. Each jack can exert up to 891 tons of force, but in practice they will be operated at well below their maximum capacity.
Waist-high guide walls inside the jacking pit will help keep the boxes moving along the proper alignment at first. Once the boxes are in the ground, engineers can actually steer the box horizontally and vertically by applying more force on one side or the other.
As excavation crews encounter the freeze pipes, they will simply snip and dispose of them. Based on the area's past history as a waterfront before it was filled in during the 19th century, engineers also expect to encounter old wharves, pilings and probably granite blocks in the tunnels' path.
Hundreds of greased steel cables on the top and bottom of each box will serve as a lubricating device to help ease the boxes -- weighing 24 to 36 million pounds apiece -- through the ground without dragging or shearing off soil above and below. The tunnels will sit about 8 to 30 feet below the railroad tracks.
The work is taking place amid one of the Big Dig's biggest beehives of activity, a sea of cranes that can be seen east of the end of the Turnpike, with the old Broadway Bridge and US Postal Service mail processing plant nearby.
While the term "tunnel jacking" may conjure images of structures being rammed through the earth, a top Big Dig civil engineer overseeing the project, Paul Goguen, said, "It's not really that they're shoving this thing through the soil. It's like they're cutting a foot or two ahead and then jacking forward to catch up."
Current plans call for the first tunnel section, the westbound turnpike exit known as Ramp D, to be jacked into place over 6 weeks in June and July.
Then the first of three sections of eastbound turnpike tunnel will be jacked into place in November, followed by the westbound lanes between December and February, with the rest of the eastbound sections jacked into place between next April and June. (All estimated dates could change.)
Normally, highway tunnels would have been built by taking one track at a time out of service, building a cut-and-cover tunnel under it, replacing the track, then moving on to the next track and repeating the process.
However, the area where the tunnels must pass contains a complicated nexus of "ladder tracks" and "interlockings" that must stay in service to allow dispatchers to direct trains to available empty tracks at the station. Taking out even one track at a time could snarl the entire terminal.
On an average weekday, 40,635 people ride MBTA commuter trains into South Station, plus 2,000 to 3,000 Amtrak intercity passengers headed to New York, Chicago and Washington, according to railroad officials.
"It just became a logistical nightmare to do it the conventional way and almost impossible, truly," Goguen said.
Amtrak officials have insisted on a regimen of daily inspections to ensure railroad tracks do not get pushed up more than 7 inches by the freezing and heaving of the soil underneath.
Some vertical movement of the tracks is inevitable. But as long as it remains within a few inches, Amtrak crews can use ballasting machines that raise the tracks and remove crushed rock the ties sit on to put them back into lower alignment.
And Taylor is confident the project will produce minimal aggravation for rail passengers. "Effectively, the trains don't know there's a tunnel being built under them," Taylor said.
Both technologies -- soil freezing and tunnel jacking -- have roots more than a century old.
The first soil freezing patent was issued in 1883 to a German inventor, and it has been used regularly in coal and other mining operations, such as to temporarily shore up a weak seam. However, Taylor and other specialists believe the Big Dig may represent the largest application of soil freezing to date for underground construction.
Tunnel jacking, likewise, dates conceptually back to the 1890s, when the Northern Pacific railroad began building culverts for brooks and creeks under its tracks by mining out earth and jacking pipes through, instead of taking busy lines out of service.
By the early 1960s, European engineers began adapting the technique to push much larger, rectangular-shaped tunnel boxes under railroad tracks and other sites they wanted to avoid disturbing.
Around the world, scores of tunnel jacking projects have been carried out, such as pedestrian and street underpasses beneath major British Rail lines, and projects in Germany, India, Japan and South Africa.
In Taiwan, Taipei authorities are using tunnel jacking techniques to complete two 300-foot-plus stretches of a sorely needed crosstown highway connection under a runway and taxiway at its airport.
In North America, however, examples are rarer. About 10 years ago Ontario officials used the technology to build a 115-foot-long tunnel for a two-lane ramp for the Don Valley Parkway under Canadian Pacific Railroad tracks near Toronto.
Michigan highway officials estimate they saved $15 million in reconstructing the 58-year-old Davison Freeway in Detroit by using tunnel jacking to construct an 11-foot-diameter drainage line under the road. California has carried out about a half-dozen tunnel jacking projects for flood relief culverts, none wider than 30 feet, according to Taylor.
In Boston, the tunnel jacking is part of a $397 million contract to build major portions of the fantastically complicated new interchange between the turnpike and the Southeast Expressway, which must be finished even as some 300,000 vehicles a day continue to pass through the area.
The contract, awarded to a joint venture of New York-based Slattery Skanska, Interbeton, J.F. White of Newton and Framingham's Perini Corp., represents one of the Big Dig's most successful uses of "value engineering," where contractors take plans, figure out ways to complete them more cheaply and share the savings with the state.
Overall, according to Goguen, who oversees six contracts in the interchange area, the joint venture has come up with seven ways to save at least $4 million.
For example, as initially designed, plans called for the ground to be stabilized by injecting concrete grout around and under the tunnel-jacking zone to create a sort of underground bathtub, then dewatering the soil contained inside the tub and mining through it.
But the contractors concluded they could get the job done faster and for $2 million less by freezing the soil instead, Goguen said. They also determined they could build the tunnels in fewer, larger sections to save money and time.
This story ran on page C1 of the Boston Globe on 3/8/1999.
© Copyright 1999 Globe Newspaper Company.
');
//-->
