The foundation was rebuilt in 1990 to prevent it from collapsing, but it did stand for about 600 years before that! The tower’s foundation was originally built on silt but that silt has clay underneath. It’s why modern construction does soil analysis first. Or, at least they should.

Things still collapse because soil analysis is skipped, like this bridge in Saskatchewan that collapsed in 2018 on the same day it opened to the public and before anyone actually crossed it.