Hello,
I think the people who pour large slabs really want to make sure of ground drift, but even with the largest of foundations you can only slow the drift. Unfortunately by disturbing the clay layers you allow water to get in and that will swell the surrounding clay. Give yourself two plumb line marks 90degree apart on the pillar for an easy check of movement over time.
If it looks like your doing a lot of bags, do yourself a favour and get a concrete mixer 2nd off gumtree. The mixer can always be resold for a $50+ lose after you have finished.
Alex's suggestion with the radials is very good at limiting drift with limited infrastructure.
Steve
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