same as the rest...more detail. stretch it out. maybe even some false alarms. generate more interest in the girl (for instance, give her a life (or nonlife) outside of digging so we can care if she dies). Perhaps a friendship or other interest in her companions would enliven the story. Basically, you have a good enough idea to turn it into anything from a short story to a whole freakin' novel if you please.

And there's two kinds of horror: gore horror and suspense horror. Methinks you want suspense, so go ahead, drown us in detail while keeping lots of things secret. If it helps, try to think of the scene as "play-by-play"; don't get ahead of yourself.