Looking for reasons for the dog's digging habits I have noticed a number of things going on in the garden which the dogs avoid and don't dig close to while other attract the dog and cause it to dig.
Once you have identified the causes its time to act on them.
Our dogs have chosen one end of the garden to use as a toilet and in this area they do not dig. Is it the dog poop that stops them digging? I think so. To test my theory I buried some dog poop in the holes dug by the dogs and lo and behold the digging in that area stopped. Now its just a case of filling the other holes with the dog do and my problem should be over. An added trick is to feed the dogs pineapple and use the pineapple induced poop, it apparently works better than the normal.
If your dog is a hunter like my sheep dog appears to be she chases anything that moves in the garden. i.e moles, rats and mice, with mole hills being her favourite. If you have these rodents in the garden the dog will dig to find them. The trick is to rid your garden of small animals that attract the attention of the dog and cause it too dig. For mice and rats it is necessary to put out poisons but
set the baits in areas in your garden where the dogs will not get to. You don't need your dog eating rat poison. For moles place crushed garlic in the burrows and this should send them on their way.
Our labrador although not an avid digger still digs occasionally when she has food, especially bread that she wants to store. She normally finds a spot where the sand is soft , drops the bread and covers it with sand using her nose. Our sheep dog is a glutton, and as soon as the labrador moves off she sniffs out the bread and digs it up. I suppose the way to stop this behaviour is not to feed them too much so that they will not resort to burying their food. The same applies to dogs who bury bones. Just stop feeding them bones and then they won't bury them.
Some other methods one can apply to a digging dog is to bury a balloon filled with water in the holes the dog has dug. When the dog digs it up and bursts the balloon she will get wet and avoid that spot in future. Another method to use if you can catch your dog in the act of digging is to spray it with a hosepipe and wet it. A dog will hate getting wet and will soon learn that digging gets them sprayed so will stop digging. Our dogs although they have not been sprayed with the hose for digging immediately make for their kennels when they see me reach for a hosepipe, so this method is bound to work for digging.
For areas such as along a walls or fences where the dog can eventually dig its way out a good idea is to bury chicken wire under the sand and when the dog digs in this area and feels the wire it is likely to stop digging.
Of course there are a number of harsher methods one can use to stop the dog digging. One is to clip the dogs nails short so that its feet get sore when it digs. Another is to confine the dog when you are not with it to stop its digging.
For people who have the space in their garden it might be a good idea to invest in a sandpit in which you encourange your dog to dig. In this pit you can bury treats for the dog which will attract it to the pit.
A bored dog is also likely to dig so if you can get your dog out on a walk a couple of times per day it will go a long way to solving the digging problem.
In our case both dogs have too much time on their own so utilise their energies by being destructive.
Above were just a few suggestions that might help in stopping your dog from digging.
I wish you success with your endeavours to stop your dog digging.
Happy gardening