[BasilAwad]

"To keep the soft ground from collapsing, engineers snaked coils of coolant through the soil to form a protective arch of frozen earth. That let crews work safely while traffic rumbled overhead. Cost: $1 million per foot."

[EvanKelly]

I wonder if this is the cost of actually applying this technique, or also includes the cost of researching and testing the safety of this method prior to using it on this project.

[Anechoic]

It likely includes all of the preliminary engineering geotech, and environmental work that came before implementation. You can't just freeze the ground like that, there are a lot of potential adverse effects (ground) shifts being the biggest) that have to be accounted for and mitigated.

[vkou]

I interpreted that to be the cost of building a foot of tunnel, not the technique.