I want to build an in ground buried cold frame out of earth bag construction. It will be 4 feet deep and will have straight sides 15 feet long. Is that fine, or do I need buttresses? For clarification, the bottom two feet will usually be filled with compost, with a foot of soil on top of that. But every year I will shovel the stuff out, so I don't want it to collapse on me.
Can I fill the bags with either a. concrete rubble or b. clay soil? I know neither are idea, but then it is a cold frame, not a house.
I assume that only polypropylene will work, not burlap? (I'd be plastering the inside, but not the outside against the soil, of course.
How long will this thing last under good conditions?