I would use a grade beam. dig down to solid earth and construct a form for a sub-grade footer 18" - 24" wide , 8" - 10" thick. Then form a curb or plinths to set your posts on. 10-12" above grade is recommended. This can be done in one pour or separatly , just be sure to lay in some re-bar or j-bolts to join the two sections if poured separatly. Simpson strong tie TM make several rust resistant base connectors , sold at big box stores in US. Piers could also be used but I would not recommend them for this ap.