Think of why your dog is digging a hole. This is a den. And just when she's nearly got it to the perfect size, you fill the bloody thing in! AGGGGH she must be very frustrated with you.
Digging is a natural behaviour of dogs. Why didnt you get a bird instead? they dont dig.
We leave our dogs to find their own dig spot. Each has 2. One in sun, one in shade. They've dug others, but have given up all but 2 spots each. Once the work out where the best 2 spots are, i fill in the others with their poo.