I haven't been able to reproduce this in a smaller test case. Here's a save:
https://gist.github.com/anonymous/66486404cb7c6cff747e
My colonists refuse to do anything further with the geothermal plant. I had to deconstruct most of the walls to get it as far as it is now. Collapsing the whole building allows them to continue, but is undesirable.
The only problem i see is that you ran out of metal. As soon as i added metal they finished it. Can you check this?
But that might be because the save is missing some stuff.
Could you zip it and add it as a attachement to a post.
You're right.