[Above and below: ground freezing of the Hallandsås rail tunnel in Sweden. Source.]
As many discussions have recently noted, there are no shortage of expenses and ‘unsustainable luxuries’ entailed in forcing public transit underground. The notable expenditures and construction delays of these projects invite speculation into the details and processes of building tubes through the substrata of cities. Accordingly, images of 22 foot+ diameter drill bits have become relatively commonplace, whether for subway construction or the boring of very deep sewer lines. One interesting fissure in the tunneling process we stumbled upon are procedures applied when the tunnel’s path encounters discontinuities in the city’s extant geology. Enter ground freezing.
[Extension of the 7 line, NYC. Liquid nitrogen is pumped through a dense matrix of tubes in a cubed volume of ground in order to freeze it. Source]
Along the route of Manhattan’s new Second Avenue subway line, the geology varies, “passing through rock and soft ground, consisting of sands, silts, and clays over Manhattan schist, [where] there are faults and shear zones as well as fractured rock” (*).
[Map of Manhattan’s pre-anthropogenic Geology]
Around 91st St., the 2nd Ave. subway path intersects with one of these areas of fragmented rock and water seeps. A tunneled airspace through this material would likely result in the roof caving in upon it. So rather than shoring up the tunnel (the older method), this entire section of fault and fissure will be frozen. To do this, ninety foot pipes extending vertically into the earth will be filled with brine. The re-circulating brine solution will be chilled by large refrigeration units placed at the surface. This process will take about ten to twelve weeks to solidly freeze the ground at a temperature of -20 degrees F. (…which can’t be cheap), and once solidified, processes within the subterranean landscape are also frozen. Groundwater flows cease, pipes no longer transfer liquids. Ironically, frozen soil (depending on the amount of water in the interstitial spaces) is about twice as strong as concrete, through which the über mechanized incarnation of a Dune worm can easily drill through.
[Ground freezing techniques. Source.]
In burrowing through previous formative eras, these works create their own localized ice ages of relatively brief tenure. And once cored through, thawing the earthen slabs comes with its own set of challenges – a process which must be performed in a ‘very careful and orchestrated way‘ * to avoid compromising foundations of adjacent structures. After these fault zones have been frozen, they tend to have even more interstitial space, cracks and fissures due to the expansion/contraction induced by the industrial freeze. You have to wonder if any of this deep ground was ever frozen prior to now, even in an ice age. We are interested in these type of moments where singularities in the environment are inadvertently amplified through strategies to appropriate them.
[Ground freezing in the construction of Budapest’s underground]