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There could be rock ..it may not be bed rock so the test is to drive a crow bar onto it...bounce it up and down..if it is a floater it sounds dull, if bed rock it has a sharp sound..after probably half a dozen holes and checking rock you will tell the difference easy ...however if the rock is so big that you can not tell if it is a floater or not it really does not matter just key into it...
The star pickets top protrudes into the area that you will pour your concrete and so they project out into your pad enclosures but not beyond the final pad pour...have an angle grinder on hand in case you can't drive them all the way in ...you can cut 5hem off rather than banging away forever..if you choose to pour your pier using a pvc tube which is in my view the best way you can place four star pickets in the tube before you pour and even drive them into the ground.
I would be happy to have a floor with air under rather than a pad on the ground as a pad on the ground can hold heat whereas a floor space allows thermal equilibrium to be reached easily.. so make sure you have ventilation.
Alex
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