building tire walls in the garden, not for a house

I'm thinking about building an earth-sheltered greenhouse in my garden. I have a 5-6' high bank that curves around a corner in a good aspect. I'm wondering about using tires to build the walls against the bank, and like two course of tires at the open side, and then building up with cobwood. So the tire walls would be load-bearing, but not as much weight as most earthship homes. I am thinking about a round shape, with a reciprocal roof, half-glazed and half green roof. I'm wary of building with tires because of the ramming, I don't think I could do it the way I've seen it done - too much hard work (bad back). But if it's for a lighter load, would they have to be rammed as full? And what about just using tires to build low retaining walls in the garden, not structures - could these be more loosely packed (requiring less hard work with the sledge?)

i believe with any loadbearing tire wall, the tires must be packed as tight as possible.
because you are basically using the tire to form a brick, the brick must be solid if it is to support a load.

for a garden wall, i dont think they need to be pounded as full. the weight of the earth/tire should hold back the wall.

im not professional engineer, but those are my thoughts.