I don’t think that’s unique for construction of tunnels in loose ground. Such projects typically go over budget because one offs are hard to cost estimate, and

I don’t think that’s unique for construction of tunnels in loose ground. Such projects typically go over budget because one offs are hard to cost estimate, and politically, you can’t sell a project that will cost, say “between $X and 5 × $X billion”. Saying “X billion” is a way easier sell a project.