The Big Dig has extended I-90 (the Massachusetts Turnpike) under Boston's Fort Point Channel into South Boston, where it meets up with the Ted Williams Tunnel. I-90 goes underground approximately where it crosses I-93 (the Southeast Expressway) at the South Bay Interchange. But before it reaches the channel, I-90 also passes beneath nine active railroad tracks carrying commuter and Amtrak trains into South Station, Boston's busiest rail terminal.
Like the rest of Boston's transportation system, railroad operations had to stay in service throughout Central Artery/Tunnel construction. So how do you dig a tunnel big enough for an interstate highway while trains rumble by just a few feet overhead? You don't precisely dig the tunnel - you jack it.
Utility pipes are commonly jacked under active roadways - they're simply pushed through the soil. The Central Artery project used jacking techniques during utility relocation to get ready for tunneling in downtown Boston. But jacking boxes as big as a highway had never been done in North America, until the Big Dig came along.
Three concrete "jacking pits" were dug along the path of I-90 just east of the Southeast Expressway. Two of the pits were responsible for the main line of I-90; the third was a ramp from I-90 westbound to the underground I-93 northbound beneath Atlantic Avenue. Inside these pits, tunnel boxes 80 feet wide and 40 feet high were built, which were then pushed by hydraulic jacks under the railroad tracks.
The jacked sections range from 150 feet long for the I-90 ramp to 260 feet for I-90 westbound to 380 feet for I-90 eastbound. Altogether these three massive jacked tunnels amounted to the largest application of this technique in the world.
Once the jacking pit was built and the tunnel box formed inside, crews broke apart the head end of the concrete pit, scooped away about three feet of soil, pushed the tunnel box ahead, and repeated the procedure again and again. Unfortunately, to do this without stabilizing the soil would cause the tracks to settle, threatening rail operations, so the ground ahead of the tunnel boxes needed to be frozen.
Hundreds of pipes were driven into the ground between the tracks. Inside the steel pipes were smaller plastic pipes. A freezing plant near the railroad tracks pumped a brine mixture that stayed liquid below 32 degrees Fahrenheit into the inner plastic pipes. The brine ran out the bottom of the inner pipe, flowed up to the top inside the outer pipe, and was recirculated to the freezing plant and back again into the ground. Over weeks the circulating brine drew the heat out of the soil little by little, and froze the ground outward from the pipes.
Once frozen, the soil was excavated without settling. Of course the freezing caused the ground to expand, but this movement was predicted and controlled so that the railbed was maintained safely. A special machine called a "road header" excavated the soil ahead of the tunnel box. The road header had a rotating grinder on the end of a moveable arm that chewed out the frozen soil, which was gathered up and removed out the back of the tunnel box into a large bucket and carried out by a crane on the surface. The road header ground away the soil around the freeze pipes, which were cut away.
Two sets of jacks drove the tunnel boxes forward. Two of the boxes were built in two sections and the final box was built in three sections, with a set of jacks in between and another at the rear. As the rear set pushed ahead, heavy pipe-like spacers were placed against the back wall of the jacking pit. In the first jacking operation, more than 50 jacks were used, each with a maximum pushing capacity of 10,000 pounds per square inch. The working limit of the jacks in action was about 6,000 psi. Trains regularly moved overhead as the tunnel sections moved through the ground about 20 feet below the tracks at a rate of between three and six feet per day.
Jacking of the 150-foot-long ramp section began in late summer of 1999 and was finished in early December. The westbound jacked tunnel was completed in August 2000. The eastbound I-90 box was jacked in three pieces and completed in February 2001. With the jacked sections in place, the jacking pits become the path of the highway.