Jessica Morrison, writing for NOVA Next:
The top of the tunnel would be just 10 feet below the bottom of the subway tunnel. Ground freezing pipes were laid horizontally and uncomfortably close to the subway’s support piles. The proximity meant that engineers had to be aware not only of where they froze, but how the frozen soil would behave. Because the density of ice is lower than the density of liquid water, as soil freezes, it expands or swells. That expansion can cause cracks and create large, air-filled spaces. If this happens, the functionality of the frozen soil could be lost.
“During the freeze periods, we were very careful not to have to much heaving. When water freezes, it expands, and there’s a rail above,” Horodniceanu says. Avoiding complications meant testing and modeling the characteristics of the soil long before freezing began. “We knew the limits, and we were constantly making sure that we stayed below the limits,” Horodniceanu says. “It sounds exotic, but this is done every day.”
Very cool. (Hah!)
(via boing boing)