I doubt anyone has taken measurements but we will see.
All pigs root, it is part of being a pig. Some say Berkshires and Middle Whites dig less than other breeds and Kune's dig least of all. In my experience BK and MW dug no less than any other breed. I have never kept Kune's. Kune's are said to graze and do grow slowly compared to other breeds, so maybe if they have lots of good grass they aren't as predisposed to rooting for goodies, I don't know. I suspect if they haven't enough grazing they will dig.
Pigs will turn over virgin ground more than ground that has been dug by pigs and reseeded. They will dig more at certain times such as autumn and after rain.
If you don't want to "damage" your land or do anything to repair damage, don't keep pigs. Depending on how much land you give them, soil type and the good, old English weather your ground will range from being turned over to extremely muddy with any variation in between.