What diameter and height of tank?
Also, how embedded in the ground is the ringwall?
I've always assumed that the ringwall acts as one continuous unit; and that you would engage massive passive earth pressure to offset the lateral wind load.
This would be conservative, but you could also take a 1ft long segment of the ringwall and treat it like a footing. Then figure out what the wind related overturning moment would be; and from there get a bearing presure. I bet you would be outside the kern though, so that might not work.