Pigs are strong, even small pigs, and will quite happily batter their way through standard fences. In fact, escaping pigs are one of the reasons given by people looking to rehome. So you ideally need a stock proof fence (plenty of advice and sources online), or a stone wall is good so long as it’s high enough. Not that pigs are known for jumping but some have been known to climb, or at least stand tall enough to push themselves over. Any fencing must also be secure into the ground so they don’t bury under. Do not underestimate a pig’s drive to go after that yummy smell that has drifted to them on the breeze! They are smart, surprisingly quick movers and have the solid force of a battering ram. Likewise, electric fencing is only successful if pigs have been trained to respect it, usually as youngsters when they can't just barge through it at speed. 

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And once you have pigs, daily inspections of the perimeter are a good idea, to make sure there is no damage.