#monitoring

Frost heave is not water expanding by nine per cent. It is ice lenses growing by sucking water towards the freezing front, which is why heave can be hundreds of millimetres and why silt is far more dangerous than clay. Under a cryogenic tank the freezing front never goes away.

A tank can go down a long way without complaint, and tilt without much trouble, but it cannot take being bent. The survey exists to separate those three things, because only one of them actually threatens the shell.