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.
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.
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.
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.
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.