You may find that digging hole and filling with concrete is no better than bolting steel to existing slab if it is reinforced correctly and is on good footings.
I have a steel pier on a slab on filled and reinforced concrete block work, again on footings and piers in clay. I can walk around while imaging.
The problem is the additional weight you are adding with a tall concrete pier unless it is above existing concrete footings of a reasonable standard. And then , in clay , it would require deep drilled concrete piers to keep stable over time and moisture levels depending on slope and clay depth. Only an engineer with a drill log report could answer these questions more exactly.
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