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Glen,
As you say its all about the soil type !
You mention clay - and that is one of the worst types !
Clay absorbs water in Winter and expands and dries out in Summer and contracts.
Is your ground hard just because its Summer or is it always like that ?
If its only because its presently dried up clay, then you will need to keep digging - ideally (if at all possible) until you have broken through the clay layer.
A typical stable pier footing in ordinary soil would be between 0.5 and 1.0 cubic metre - whereas what you have described is 0.15 cubic metres.
If its in rock or stable conglomerate then I guess you could be smaller, but key to a piers design is mass, the other is stability - such that when the ground moves due to moisture your pier does not.
I assume you have tried filling your hole with water to soften up the ground ?
Rally
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