A deep grave

[lost man]

Hi,

Could you please tell why the playwright says 'deep grave' and not just 'grave'?

"You'd do right to leave that rope, Bartley, hanging by the boards . It will be wanting in this place, I'm telling you, if Michael is washed up to-morrow morning, or the next morning, or any morning in the week, for it's a [B]deep grave we'll make him by the grace of God."
Riders to the sea, John Millington.

Thanks.

[Delta40]

No idea except to say a shallow grave often suggested a hasty burial. Deep graves of at least 6ft were required as a result of the plague fo health reasons so I imagine the extensions coined from these explanations could inspire a writer.