Boston's Central Artery/Tunnel Project, known as the Big Dig, was one of the most complex highway construction endeavors in U. S. history.
One of its greatest engineering challenges occurred at the Jacking Shield Pit near South Station, where massive concrete tunnel sections needed to be pushed directly beneath ground supporting busy commuter and Amtrak railway lines. The soil in this area consisted of soft, water-saturated clay and fill. Traditional excavation risked triggering major ground settling, which could have derailed trains or forced the shut down of vital northeast corridor rail lines.
To solve this, engineers deployed artificial ground freezing. They installed a grid of vertical pipes directly into the soil and circulated a liquid brine cooled to minus 20 degrees Fahrenheit (-29 Celsius) via a centralized refrigeration plant. Over several months, the subterranean water froze solid, turning loose soil and mud into a structurally sound wall of frozen earth with the compressive strength of soft concrete.
With the ground stabilized, massive hydraulic jacks pushed giant, hollow concrete tunnel boxes through the frozen earth while excavation equipment scooped out the interior material. Sensors continuously monitored soil temperature and track elevation to ensure zero settling occurred overhead. Once the tunnel sections were locked in place, the refrigeration was turned off, allowing the ground to thaw naturally without interrupting a single train schedule.