So how is that working out in the game? Does it look weird?Digging simply adds another direction that zombies can move in. Upwards, they climb or jump. Sideways, they move or smash. Down, they drop off ledges or dig.
Digging down into dirt is no harder than digging sideways into concrete.
Digging does not effect sensing you. Sensing works in all directions and is distance based. The game does not model sound absorption by blocks (maybe someday we can add that), so hearing you up, down or sideways is the same. Digging does not just apply to ground. If a zombie is stuck in a 2nd story room and you are below him in a 1st story room making loud noises they hear, they can dig down through the floor.
Is it harder for a zombie to dig though concrete than soil? It should.