Do earthbags really become hard as rock or is this exaggeration? I did a little experiment on the roundhouse we’re currently building to find out.
The answer depends primarily on the choice of fill material and the degree of compaction. The moisture content and curing process also play a role. We used road base – the material used under roads in many parts of the world – moistened slightly and tamped solid. The video shows the result after about one or two weeks of drying.
Note the ringing ‘ching-ching’ sound when I tap the earthbags with a putty knife. It almost sounds like I’m hitting stone. I tap a compressed earth block (CEB) for comparison. CEBs contain about 6% cement and are rammed in a press at high pressure, so you expect them to be harder. But in reality they’re fairly similar as you can hear for yourself. In both examples, densely compacting correct soil mixtures create very strong building materials. They begin to approach the strength of stone at a fraction of the cost and labor.