Either electric, or very strong stock fencing, or a combination of both. We have got away with just stout stock fencing plus a top strand of barbed wire so far (10+ years and we do all our own fencing) but "digginess" depends on the breed of pig. Some are more enthusiastic than others. Within a decent patch of woodland there should be enough to keep them interested. Are you planning just weaners for fattening or permanent residents?