Yea, unsupported block walls would blow out. I built a bin using PT 4x4 posts set in concrete on 3 ft centers, planked with PT 2x6 lumber. Back corners are interlocked. 12'x28'x6' high, tarp on the ground and stapled to the walls. Left some slack in the tarp so the staples wouldn't pull out when the truck backed in. When the tractor trailer dumped 24 ton nut size, the coal spread the entire 28' length and the walls splayed out a good bit. I wanted to backfill around the outside with dirt before the delivery to shore up the walls, but didn't get it done. I used my loader to pile up the coal in the back, then used some cables and turnbuckles across the top to keep it from spreading further. The bin is big enough to hold two loads if I were to close off the end and load it with the tractor. No roof on the bin, I bring the coal inside with 4.5 gallon buckets. A few times we had to move a couple shovels of snow to get to the coal, not a big deal here because snow doesn't usually last too long. In a colder area I would run some 2x4 lumber across the top for support and tarp it. Hang plastic bottles filled with water off the grommets to keep the tarp taught. Like using rubber tarp straps but way cheaper, plus they won't lose their tension over time like the rubber straps do. Safer too, lots of people have lost an eye to the rubber straps. Hang the bottles with rope and use u shaped nails over the rope to allow the water bottle to move vertically but not be blown around by the wind.