Mix the sliced apples with sugar & the spices you'll be using & let them sit for a while. They'll give up their excess juices which you can drain off, simmer to thicken & then add back to the apples. Then pile the apples in the crust & top with the remaining crust. You shouldn't have this problem then since it's caused by the apples shrinking & giving up their juices during baking.

Here is a place to look for an interesting read:

Yes, I'll agree that the apples do collapse during cooking, but I think a lot also depends on your crust recipe. The ratio between the flour and shortening (fat) has a lot to do with it, too. Seems like more flour makes the crust fix quickly, but the shortening will help to keep the crust pliable long enough so that it can settle down on the filling before completely baking. Perhaps you could try another recipe for the dough.

Here is a link that might be useful: Discussion Of The Gap Problem