We call them trench silos, my dad had 2 of them. One filled with corn silage and the other high moisture that would feed about 250 head of feeder cattle all year long. The corn silage trench silo was huge, probably +150 yards long, 40 yards wide and 15 feet deep, it took hundreds of truck loads to fill.
Brings back lots of memories... The funnest job was to push the loads down the hill with a loader tractor, got a little hairy once in a while, silage is awfully soft when not compacted. Also remember me popping a brake line as I was backing up to the edge to dump, thankfully the silo was not empty and I just went over the edge and got hung up and stuck. Could have been a bad ride...