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The Big Dig
Boston, Massachusetts, faced a road congestion problem; namely, an elevated 6-lane freeway, called the city’s Central Artery (I-93), ran through the city center and caused persistent traffic congestion. In addition, the given road was beginning to fall. The problem was unavoidable since initially, in 1959, it was projected to pass 75,000 vehicles a day. In the early 90s, this number increased to 200,000, making the freeway one of the most congested in the USA.
As a result, the traffic flowed at a low speed for over 10 hours a day, which led to a deteriorated environmental situation additionally. Furthermore, accidents on the crumbling elevated freeway were four times more often than the national average for city and federal highways. The Big Dig was projected to replace the 6-lane elevated motorway with an 8/10-lane underground freeway that runs directly below the existing one and becomes 14-lane at its northern ends.
The Ted Williams
To connect the Ted Williams tunnel to interstate 90, engineers were required to build a 10-lane highway beneath Boston’s Fort Point channel. That meant crossing over a subway and passing the city’s largest factory and its commuter rail lines. Furthermore, the location had the weakest soil in the entire highway alignment to the greatest depth. If engineers could not discover a solution to harden the water-saturated soil, the whole construction would be impossible. The solution was borrowed from Japanese engineers who developed efficient and economic reclamation of weak lands utilizing a technique called soil mixing. The method implied mixing existing soil in place with cement. Therefore, engineers decided to strengthen 750 thousand cubic yards of land; enormous blades penetrated 30 feet deep into the channels and mixed earth with cement. It needed to wait for three years before the soil would harden.
While constructing the last missing link of the highway to connect the tunnel beneath the channel to interstate 90, the engineers encountered two critical problems, namely, the daily train moves and the soft soil beneath the tracks. Concerning the latter problem, the soft soil made the tunnel building impossible. Moreover, there was another challenge – how to strengthen the soil without disrupting New England’s train service.
The solution was found by the contractor who offered to freeze the soil. Pipes were inserted around the tracks and filled with saltwater chilled to minus 30 degrees. After several months, with a constant circulation of coolant, the ground became solid enough to support construction. The plan implied building sections and then pressing them beneath the tracks using an innovative technique known as tunnel Jackie.
Jackie was applied to push the sections beneath the tracks of the tunnel, which have pipes with the cooled liquid. To resist the friction and keep the ground beneath the train tracks, engineers decided to pull steel cables in the opposite direction of the jacking. Coal mining machines ground a passage beneath the tracks, while tractors quickly scooped up the frozen dirt and rushed it to the end of a pit where giant cranes hauled it out. Once the machines had dug at the cavity, Jack, with the pushing capacity of 10,000 pounds per square inch, forced the 35,000-tonne box forward at a very slow rate, about 3 feet per day. It took three years to squeeze three tunnel sections beneath the tracks.