Conditions MAJOR HIGHWAYS
A $6.5 billion feat of engineering
By Boston.com Staff
From the Turnpike's current endpoint at the South Bay interchange, just south of Chinatown, engineers faced the daunting task of driving a nine-lane highway underneath South Boston, the Fort Point Channel, and below nine active railroad tracks carrying 40,000 people a day into South Station. These tracks would have to remain in service during construction. The tunnel would also pass within inches of the 81-year-old Red Line subway tunnels and skirt the massive US Post Office annex and Gillette's main razor factory.
Tunneling beneath the tracks
To push beneath the busy South Station rail yard, Big Dig engineers employed a process known as "tunnel-jacking," a technique which had never before been implemented on such a large scale anywhere in the world.
Tunnel jacking was familiar to engineers in England and Western Europe, but had rarely if ever been used for highway construction in the United States. In this country, the technique had primarily been used for small-scale projects like sewer and utility pipe installation.
First, three huge pits were excavated between the tracks and the existing Turnpike terminus. Inside these trenches, known as "jacking pits," workers constructed three massive sections of tunnel, each 80 feet wide and 40 feet tall, and ranging in length from 150 to 380 feet.
Powerful hydraulic jacks were then used to push these three "tunnel boxes," inch by inch, into the ground beneath the railroad tracks. A special machine known as a "road header" mounted inside each tunnel box excavated the soil ahead, making room for the box to move forward at a rate of between three and six feet per day. Meanwhile a water-clay mixture was sprayed around the edges of the concrete sections to act as a crude lubricant. In order to stabilize the soil before the digging, a chilled brine mixture was pumped through hundreds of pipes driven into the ground between the railroad tracks.
The tunnel jacking process lasted a year and a half, beginning in the late summer of 1999 and ending in February 2001. After the tunnel boxes were in place, the jacking pits were covered over and transformed into part of the highway tunnel.
Crossing the channel
The crossing of the Fort Point Channel presented another unique challenge. The easiest way to construct an underwater tunnel is to prefabricate the tunnel in massive steel sections, float the pieces to the proper location, then sink them to the bottom of the waterway and join them together. This is the method that was employed to lay the Ted Williams Tunnel across Boston Harbor. However, this technique wouldn't work for the Fort Point Channel tunnel because there wasn't enough room to float the huge tunnel sections beneath the low bridges that cross the channel at Summer Street, Congress Street, and Northern Avenue. The Fort Point Channel tunnel would have to be constructed on site.
To do this, Big Dig engineers built the "casting basin," a massive pit cut into the earth on the south side of the channel, next to the Gillette factory. The basin, in effect a huge dry dock, was 1,000 feet long, 300 hundred feet wide, and 60 feet deep -- large enough to hold an aircraft carrier or three Titanics. A temporary steel bridge was built to carry A Street over the gaping hole. The basin was sealed off from the channel by a giant steel wall, and inside of it workers constructed six concrete tunnel sections, the longest one 414 feet long and the widest 174 feet wide.
When the tunnel sections were ready to be put in place, workers sealed them up watertight at either end, then removed the steel wall, flooding the casting basin. The tunnel sections were floated out into the channel and positioned, then sunk to the channel floor and joined together. An engineer compared the finished structure to a covered bridge under water. At low tide, there would be as little as 8 feet of water clearance above the tunnel tubes. Because the Fort Point Channel is no longer used for shipping, this didn't pose a problem.
The casting basin was not large enough to accommodate all six tunnel sections, so the float-out process had to be repeated. The first four tunnel sections were built and floated out in January 2000. Then the casting basin was resealed and pumped dry and the two remaining tunnel sections were constructed. After the last two sections were floated out in June 2001, the casting basin was converted into part of the highway tunnel.
Sinking the tunnel sections was a delicate process that left little room for error. The sections could not be moved once they were placed, so they had to line up precisely. And there was another challenge: The Red Line subway tunnel passes just a few feet beneath the Fort Point Channel, right under the path of the new highway. Laying the heavy highway tunnel sections on the channel bottom would damage the subway tunnel unless something was done to reinforce the ground beneath the channel. To do this, the Big Dig drilled 110 shafts as deep as 145 feet into the bottom of the channel on either side of the subway tunnel, then filled the shafts with concrete. These concrete shafts, each six-feet wide, fit into sockets built into the bottom of the highway tunnel sections.
Story based on information from the Globe and the Central Artery/Tunnel project