Trimming the Fat or Ruination?

I winced when I watched this video titled “Trimming the Fat” about how the folks at Konbit Shelter were using a machete to cut off the poly bag material and some of the stabilized fill to smooth out the interior wall of a dome they have been building in Haiti before plastering.

I suppose I shouldn’t be surprised at this practice, since Nader Khalili and the folks at  CalEarth Institute have long maintained that once the fill material (adobe soil or cement stabilized soil) has set up, the polypropylene bag material is unnecessary. I remember touring the CalEarth demonstration compound and seeing lots of tattered, sun scorched bags flapping in the breeze. I have even noticed that some CalEarth trained people will use a torch to burn off the exposed bag material, believing that the plaster will bond better with the hardened soil.

The problem is that this practice would not only weaken most earthbag buildings, it could lead to collapse, especially if the fill were at all loose. Even with stabilized soils, the bags provide a great deal of tensile strength, something that the fill specifically lacks, and in a seismic event this could spell the difference between wall failure and not. I consider the earthbags themselves to be an integral part of the whole system, and caution people to always protect them from sun exposure, and abrasion.

We don’t really know how long polypropylene will last if protected from the UV in sunlight. One report said 30-year-old polypropylene bags were recovered from a landfill (which is approximately how long poly bags have been in existence) and they were still in good condition. Another study concluded that the half life of polypropylene fabrics in benign environments could be 500 years or more. We know that moisture does not seem to adversely affect the poly material.

So please be kind to your earthbags so they can continue to be a structural part of the building!

5 thoughts on “Trimming the Fat or Ruination?”

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  2. Last week when i clicked through the link to have a look at the Konbit shelter’s work from your last post, i noticed this ‘technique’ and showed it to my better half, who was basically amazed they would do this. I am very surprised to hear that CalEarth is teaching this.

    Its good to hear that protected poly will last at least 30 years as we had to just leave our walls unplastered due to bad weather and the end of our work/vacation time. We covered the structure with two layers of UV stabilized 6mil black poly tarp and said a few prayers to the gods of compacted earth structures.

  3. It’s far better to tamp the high spots before the soil dries instead of cutting away the bags. That’s what we do. This reduces plaster work and yet retains wall strength.

    I just saw a YouTube video that showed workers flattening earthbag walls with big mallets. You can also use tampers for this purpose.

  4. I totally agree. Poly bags are incredibly strong with high tensile strength. Obviously they contribute a great deal of strength to the structure. It doesn’t make sense to intentionally weaken a structure — even more so when you’re building on the epicenter of a major earthquake that killed hundreds of thousands of people.

  5. Yes, Kelly, glad you brought it to our attention.
    So many are learning and trying earthbag. And there is so little precedent for building it in severe seismic areas.
    We don’t want to see anything done to weaken buildings that may be vibrating within a slight margin of their capacity at some time in the near future.
    Many Haitians are still refusing to return to buildings that have been declared safe but have cement ceilings or roofs.
    If lighter weight roofs can be used in earthquake areas, that may be a good idea.
    But whatever the shape of structure above our heads, lets keep it all intact.


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