Public Roads
July/Aug 2001
Vol. 65 · No. 1
Learning From the Big Dig
by Daniel C. Wood
Tunnel Jacking
Problem: Construct an underground roadway without disrupting traffic on nine active railroad tracks - including commuter tracks that carry 150,000 people into and out of Boston every workday - right above the roadway.
Solution: Construct the tunnel adjacent to where you want it to go and shove it into place using a technique known as tunnel jacking....
To carry out the tunnel-jacking operation, three concrete jacking pits were dug alongside I-90 just east of I-93. Tunnel boxes 24 meters (80 feet) wide and 12 meters (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.
But the Big Dig tunnel-jacking operation, the largest such operation ever attempted, faced a special problem - the poor quality of the soil. Pushing the tunnel boxes into place without stabilizing the soil could cause the railroad 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 0 degrees Celsius (32 degrees Fahrenheit) was pumped into plastic pipes within the steel pipes by a freezing plant located near the railroad tracks. The brine was circulated back to the freezing plant and returned to the pipes again; the circulated brine over a period of several weeks froze the ground outward from the pipes.
The freezing allowed the ground to be excavated without settling. (It also caused the ground to expand, but allowances had already been made for this movement, and the track operations were unaffected.) The frozen soil ahead of the tunnel box was excavated by a machine called a road header. The soil was chewed up by the machine's rotating grinder, removed out of the back of the tunnel box, and carried to the surface by a crane. The tunnel boxes were then pushed into place by two sets of hydraulic jacks.