Our building has a very damp unfinished basement with a dirt (mostly clay) floor.  It would be impractical for us to fully finish this basement for a number of reasons (i.e. egress, headroom, etc) but our goal is to make it suitable for climate‑controlled storage, i.e. dehumidifier, lights, and some concrete slab flooring.  Withing the basement, we can install some perforated pipe to a sump pit, a layer of ¾” stone for drainage, and pour a concrete slab on top of that. 

Question: Should we include a layer of polyethylene plastic, or not?  If so, ABOVE or BELOW the stone layer?  What’s standard practice?  Just trying to minimize the water vapor transmission.  

My nightmare concern is: if the plastic is below the stone layer, could a plumbing leak or unusually high seasonal groundwater event cause the whole thing to become buoyant and crack the slab?  I suppose the perf pipes embedded in the stone layer would prevent that, and so the plastic goes above the stone layer, right?!

Thanks for all advice.
Theodore