Boston's Central Artery/Tunnel Project, famously known as the Big Dig, was one of the most complex urban engineering feats in modern history. The most delicate challenge sat directly beneath South Station, where ten active railroad tracks carried thousands of commuters daily. Engineers needed to construct a massive tunnel for Interstate 90 right underneath these tracks.
However, the ground beneath South Station was a nightmare of soft clay, loose fill, and high groundwater. Standard digging methods threatened to sink the tracks, derailing trains and shutting down the city transit network. The audacious solution?
Artificial Ground Freezing. Engineers drilled more than 2,000 freeze pipes horizontally and vertically into the moist ground, forming a dense grid beneath the station tracks. They hooked these pipes to a giant temporary refrigeration plant that continuously circulated a calcium chloride brine solution chilled to minus 15 degrees Fahrenheit (-26°C).
Over several months, the extreme cold drew heat out of the soil, freezing the surrounding groundwater and fusing the mud into a solid, impenetrable block of frozen soil as strong as concrete. With the ground stabilized into a temporary subterranean ice fortress, workers used a method called tunnel jacking to push huge concrete tunnel sections directly through the frozen matrix. Giant hydraulic jacks exerted thousands of tons of force to slide the tunnel boxes into place beneath the tracks.
Throughout the entire process, trains continued to roll into South Station on schedule, oblivious to the sub-zero engineering miracle happening just feet beneath their wheels. Once the concrete tunnel structure was fully secured, the refrigeration units were turned off, allowing the ice block to slowly thaw naturally back into ordinary dirt.