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Construction Technology - Restoring the Leaning Tower of Pisa
(www.youtube.com)
This is a most excellent place for technology news and articles.
They did so with temporary weights or other methods like cables to make sure it didn't get worse or collapse.
It's not a very long video :/ it's mentioned in there.
Cables were just there in case the tower tipped over.
It was more a combination of temporary weights and freezing the ground.