Spec's Garden - I am thinking of a complete change

I am putting a 6in soil base on the bottom then the wood, I know it is diffrent but the soil of the bed I am removing just now was bone dry when I started and now after over a week or more of heavy rain 8 to 10 inches down is still powder dry, so placing the bed on concrete will stop any tree roots getting in and drying out the soil, the pile of logs I hope should eventually act as a resourvar for water, I will also be drilling holes in the slabs for drainage, hopefully this will prevent any flooding within the bed.