Metaphor
As you may know after reading the story, of course the misfit was not buried alive. What this phrase is taken from is a portion of the tale where he is recanting his experience and in an effort to paint a picture of just how horrible his time was in the penitentiary. He uses a metaphor by comparing his time there to being buried alive. One thing to keep in mind as you read and analyze future works of literature is that any time comparisons are being made that are not technically/literally true, it’s a safe bet what you’ll have on your hands is figurative language—a metaphor.