The man was working inside the trench in the village of Letterston when the sides caved in. A specialist rescue crew is at the scene, along with firemen and paramedics.
More top news
Low cloud will bring murky conditions with extensive hill fog.
The team, which is manned by volunteers, will operate out of Pontardawe Fire Station in partnership with Mid and West Wales Fire Service.
Dan Biggar has no doubt that Wales remain flrmly in the Six Nations title mix ahead of next Saturday's clash against Scotland.