Cheap stem wall ideas or waterproofing underground rammed earth???

I want to build a rammed earth greenhouse/milking barn/pottery shed/rabbitry--no building permit needed for farm buildings . It is approximately 3 foot down to bedrock. I want to just pin into the bedrock and ram walls up from there as a 4' concrete stem wall is cost prohibitive. I will have a good perimeter drain to daylight. Any ideas on how to waterproof the outside for an affordable price?

6 mil or better plastic sheeting, it as been used by code since invention on block walla , M Oehler of $50 And Up Housebook uses it an i use it , great stuff if kept out of sunlight and oceans

a structure underground that is round developes less ststic pressure than a straight wall when exposed to prolonged rainfall