When we raised chickens, we buried the fence 12" deep and at the 6" mark, we added a second piece coming out at an angle that was 18" and went to the 12" mark. So, from the fense, dig a 12" trench 18" wide, the original fence would be 12" deep and at the 6" spot, added a 18" piece and angle it toward the other side of the trench width, backfill and you now have a problem that most digging animals can not solve.