Here’s Patti Stouter’s report on her earthbag shear test.
The quick way to find out how to build strong enough for earthquakes? Hundreds of thousands of dollars for earthbag testing to university engineering departments. I’m helping one US university seeking grants just for earthbag, and I’m looking myself as well. Email me at email@example.com to get involved.
The slow way? Wait for individual students to chose some facet to research, and hope their tests reflect building techniques accurately. Sand bag construction has received some excellent testing recently. This should pave the way for more knowledge of bags filled with cohesive soil containing clay. But what do I say today to emails from Haitians who need to build there now?
Recently I tested a 4’ long x 4’ high (1.2 x 1.2 m) wall portion of plastered earthbags filled with a rather low strength soil containing clay. This 15” (38 cm) thick wall with 2 strands of low-tensile strength barbed wire between each course resisted more than 2200 pounds (10 kN) shear force before the plaster was seriously damaged. It resisted 2700 pounds (12 kN) maximum before it began to deform.
My test wall is still sitting there, holding itself up, unlike adobe that cracks all its mortar joints and begins to fall apart. When pushed, my wall is still stiff. Also unlike the recent sand bag tests, earthbags did not move apart under stress.
But without reinforcing steel or mesh or cement stucco my wall was at least as strong as unreinforced adobe. For my wall’s square face area it seems it withstood at least twice as much force in a static test as the unreinforced adobe for the New Zealand earthen building standards withstood in cyclic (pulsed) dynamic testing.
I’m no engineer. My test is not highly accurate, or repeated three times to confirm results. And it almost broke our simple equipment.
But with stronger earthen fill, reinforcing mesh in the plaster and vertical rebar, it looks to me like earthbag can cheaply be much stronger than reinforced adobe. My test was much stronger than sand bags, and about as strong as 11” (28 cm) wide sand bags with chicken wire and cement stucco.
I used diagonal compression to mimic the shear force of a perpendicular wall moving in an earthquake. As happened in the sand bag tests, the wall end far from the pressure compressed a little (mine 4%). The edges of openings seem more vulnerable than continuous walls. That’s why the NZ standards doesn’t let you use short walls for bracing – 4’ or 5’ 10” minimum depending on the wall height.
Why did my bags start to shift? My earthbags formed firm, solid blocks under tamping that easily held up a man’s weight spanning 10 inches. They held a 3 inch nail securely. They didn’t crumble when jumped on.
But I played the devil’s advocate and used a soil that when settled in water showed about 8% clay and more than half silt. And the clay wasn’t a terribly strong one. This fill might be strong enough for non-hazardous areas. My cured bags remained solid blocks in the wall after testing. But they crack at the corners when kicked, and crack badly when dropped from about 3’ high (a test used on adobe blocks). So in the test, the barbed wire points flaked a conical depression in the soil loose, and the wall shifted under sideways pressure.
If someone can offer me stronger testing equipment I’d be happy to build a test wall with stronger fill. I’d love to see how much stronger mortar anchors make it, and vertical rebars inside the wall and outside it. I hope that university students will explore all this soon. The sooner we know, the more safe buildings in the world.
See our newly updated Earthbag Testing page for details.