National Grid's energy cable tunnels in Snowdonia will be built by Hochtief.

National Grid’s Visual Impact Provision (VIP) project, will see the removal of ten pylons and a roughly 3km stretch of existing overhead line, which runs from Minffordd, across the Dwyryd Estuary, and continues eastwards to just beyond Cilfor. 

New Civil Engineer writes that the tunnel will be bored between two shafts and will have a diameter up to 4.4meters. Tunnel Boring Machine method will be used.

https://www.newcivilengineer.com/latest/hochtief-awarded-contract-to-build-national-grids-snowdonia-tunnel-24-01-2022/