[Retric]

Making an existing tunnel wider is vastly cheaper than building an equivalent tunnel from scratch. Simply knowing the soil composition makes a huge difference.

In hard rock you can use drilling and blasting across long sections rather than just the leading edge. Alternatively, a TBM needs to remove significantly less material.

[the-dude]

Are you sure about that? Because I have never heard about a tunnel project which has been 'upgraded' this way ever.

And if I start to think about the practical problems, like breaking up part of the tunnel before widening it, it quickly seems like building a new one might be more efficient.

[Retric]

Assuming the tunnel is not in use then yes.

Here the are filling the tunnel and using a TBM. https://www.newcivilengineer.com/farnworth-tunnel-widening/1...

Widening an existing tunnel without interrupting traffic is still possible like: https://www.rocksoil.com/pdf/233_r.pdf. Though with increased costs.

However, this changes for under sea tunnels which are segments placed down and thus can’t really be widened.

[NeedMoreTea]

Almost every tunnel has a lining that makes widening prohibitive. Which is why the London Underground is forever stuck with small tunnels on the original lines.

[Retric]

A larger issue is the underground is in use forcing you to do something like: www.rocksoil.com/pdf/233_r.pdf

That and the cost vs benefit is not considered worth it.

[jcranmer]

But more expensive than building the tunnel the right size to begin with, when you take into account the cost to build the too-small tunnel in the first place.

[the-dude]

It is common courtesy to indicate you have editted your post.