Have you considered just pouring a foundation wall set deep in the ground on a footing, then bolting the columns to the knee wall? That's how we built ours. Visited a few other wood buildings in N IL that were put up in recent years in the same manner. Just like everyone's 100+ yr old barns that are still standing, a wood frame shed set on concrete walls should be also. Concrete is better & deeper than it was 100+ yrs ago also. Nothing in the dirt except concrete & re-bar. We chose that over the perma-columns. These will not heave. Also....absolutely NO varmints dig under....none.