Box turtles often don't dig down very deep. In fact some hibernate with the top of their carapace or shell exposed without any problems. Even so, if your in a northern area, and you plan to hibernate a box turtle outside, I recommend constructing a box of moist crushed leaves, mulch, sand, and dirt that is at least a foot deep. Box turtles will dig down under this and be much better off than exposed. You may be past the point where the turtle will do much of anything for the remainder of this year, (too cold to move).
Some people hibernate their box turtles inside their garage in a box containing moist crushed leaves, mulch, sand, and dirt. They take the turtle out once a month and soak in cold water. If your garage gets above 50F or below 20F, be careful of the turtle coming out of hibernation or getting too cold.
Different types of box turtles can take different temperature extremes. Not hibernating at all is usually the safest bet. If yours is half buried in hard clay soil in a northern state right now, I'd move him.
Answering these may help my make a specific recommendation.
What state are you in?
Are you at a high elevation in moutains?
What type of box turtle do you have?
What is the ground that the turtle is currently in like, (loose, hard, grass area, moist mulch/leaves)?
jbly