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.