Our instructions for our buttress free oval said they could be left out completely on the round ends. We didn't put them in. I have read there is no downward pressure on the walls, only outward, but we did had some bulging of the metal side in between the uprights where the dirt was too high, so something's moving somewhere. That was simply resolved by removing a tiny bit if dirt with a pocket knife (I'm still amazed).

Shelley