Friction and Tensile Strength of EB Components

Patti Stouter has published another earthbag technical report. It is a summary of the state of testing to date, and also covers some of the codes related to earthbag. The following information is quoted from this report.

According to the new international earthen building standards, if earthbag walls with some reinforcing have shear strength equal to or greater than reinforced adobe they can be designed for building structures even in the most risky seismic regions. Since shear strength is related to friction between courses, soil filled poly bags were tested by a simple tilt table method to discover the approximate coefficient of static friction between the bags with barbed wire. Additional tests used barbed wire and metal pins, or pins alone. The force needed to pull barbed wire out axially from between bags was also tested.

– Results from more than 20 tilt tests indicated that full, moderately firm bags have coefficients of static friction between 1.5 and 1.9, up to 3 times higher than those needed to create an unreinforced geo-cell wall.
– Higher coefficients of friction (up to 2.7) resulted from providing small metal pins with 2 points. Length of barb did not appear to affect the coefficient of friction.
– Results from two axial tension tests indicated that barbed wire does not pull out until the barbs bend. Although static friction was overcome at 60- 80 pounds force, 120 pounds was required to remove the wire completely from between two partially cured bags.
– Instead of tearing the bags, all of the embedded barbs appeared to have bent.
– Barbed wire contributes to both the friction (related to shear strength) and the tensile strength of earthbag walls.

The full report is accessible from and Here, you can find all earthbag testing reports for easy reference.

2 thoughts on “Friction and Tensile Strength of EB Components”

  1. Looking over this, I can tell that without the barbed wire the structure will stand and the barbed wire is added almost entirely due to earthquake considerations. Correct me if I am wrong.
    That being said, I’m not sure if the testing is realistic. I would imagine that during an earthquake the wall would be the weakest when a perpendicular force is applied to it, not a parallel force. So then in the testing should the barbed wire be located differently? I think it should be rotated 90 degrees, my reference is the first picture.
    Just a suggestion, I hope this makes sense.

    • Some people are building without barbed wire, so it is possible, especially on small structures with vertical walls in non-seismic areas. But barbed wire adds an element of strength and prevents slippage, and so we typically encourage it.

      You’re right about earthbag walls being weakest in out of plane (perpendicular) forces. Barbed wire down the length of walls helps hold everything together by providing tensile strength. You’d lose the tensile strength by turning it 90 degrees.


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