I used 4x8x16" concrete blocks for my base on hard ga red clay , put a heavy plastic vapor barrier down and called it a day. Would I use sand...no, it washes. Pea gravel...yes. fyi, 2-3 days and you can drive a car on concrete driveway, a ht should be no different...in fact, the pounds per sq ft of a ht would be less then a mid size car.