Barbed wire does in an earthbag wall what rebar does in concrete: two strands of 4-point wire laid between every course give the wall the tensile strength that earth alone lacks, and without it the courses can slide apart under wind or earthquake loads. Salvage it from fence lines, farms and agricultural stores, and use thorny branches, wire mesh strips or bamboo strips only where real barbed wire cannot be found.
Part of Earthbag Building
Barbed wire is the mechanical key between courses of smooth, tamped bags and the tension element of an otherwise compression-only material.
Earth Resists Squeezing But Not Pulling
Packed earth carries compression well and tension poorly, as masonry does; see Tensile Strength. A wall under side loads from wind or earthquake needs something to hold courses together, and barbed wire does this twice: the strand resists being pulled, and the barbs bite into the bag above and below so one course cannot slide over the next. The Earthbag Building overview calls missing barbed wire the single most critical error in earthbag building.
What You Need
- Barbed wire, 4-point (each barb has four points), as two strands for every course of every wall.
- Wire cutters and heavy gloves.
- Substitutes if the wire is unavailable: thorny branches such as blackberry or hawthorn crushed flat, strips of wire mesh or chicken wire, or sharpened bamboo strips woven between courses.
Method: Source And Judge Substitutes
Step 1: Salvage real wire first. Fence lines, farms and agricultural supply stores are the main sources. Roll each salvaged length onto a spool as you take it, which makes it safe to carry and lay.
Step 2: Inspect for rust. Heavy flaking or thinned strands will not carry load. Discard wire that bends and snaps rather than bending and holding.
Step 3: Treat substitutes as lower-confidence. None of the alternatives has the mechanical bite of a barb in the bag fabric, and none has a tested strength figure here. If you must use one, use it on a small, non-critical building such as a low garden wall or storage shed, in a location without strong wind or earthquake risk.
Step 4: Test a substitute section. Build a short test run of a few courses with the substitute, then push on the top courses along the wall. If a course slides, the substitute is not locking the courses.
Common Mistakes
- Omitting wire between courses because “the bags look stable”. Without it, earthbag walls have minimal tensile strength.
- Using smooth wire. Without barbs the strand pulls but the courses are not locked.
- Relying on a substitute in an earthquake or high-wind site. Use real barbed wire there.
Safety
Barbed wire cuts, punctures and whips
Wear heavy gloves and eye protection. A coil that is cut while under tension springs back suddenly, so cut with the coil restrained. Clean any puncture wound thoroughly, since barbed wire is a common source of tetanus exposure, and keep a tetanus vaccination up to date if you can.
What’s Next
- Learn next: Barbed Wire Between Courses — laying it, corners and splices
- Combine with: Iron Wire — how wire is made, if you must produce your own