Earthbag/Stone Foundations

Earthbag/Stone Foundations (click to enlarge)
Earthbag/Stone Foundations (click to enlarge)

The most recent issue of The Last Straw journal (#60) has my article about earthbag/stone foundations. The following information is just a short excerpt. You can buy the full length article from The Last Straw. The design outlined here can be used with load-bearing and non load-bearing straw bale walls, earthbag walls, cob, adobe, cordwood and other wide wall building systems. The drawing shown above has been modified to show earthbag walls instead of straw bale walls.

Polypropylene bags come in various sizes. Two common sizes are 18” and 24” wide bags. When 18” bags are stacked on top of 24” bags, there’s a ledge created that can be used to support stonework. (See drawing.)

When filled with gravel, 18” bags are about 15” wide. Filled 24” bags are about 20” wide. That leaves about 4”-5” for stonework. Consider double bagging the foundation (one bag inside another) for extra strength and durability. And, as always, protect the bags from sunlight.

Besides being extremely durable, using stone in a foundation just ‘looks right.’ A combination of stone, earthbags, and scoria as a fill material makes for a high-quality, durable, insulated foundation with many advantages.

You can purchase single issues, subscriptions and back issues from The Last Straw.

9 thoughts on “Earthbag/Stone Foundations”

  1. The Last Straw website is not available. I don’t know how long it’s been missing in action. Any chance of getting that article…?

  2. I understand you can’t give a full seminar online. Your response great. Basically everything I needed to here. Thanks a ton.

    One last thing, I read about excavating “4 inches below frost” and “down to mineral sub soil”. In the tropics, on degraded or eroded properties this might only be an inch or 2. I’m assuming that as long as its deep enough to drain and below the roots (and has the recommended modifications) it would be sufficient for the “low load” and “no load” applications I am planning. But you know what they say about assumptions…lol

    Thanks again for the help.

    • There are exceptions to the rules. You can build directly on rocky and sandy soil because they drain well and won’t heave from frost. Still, you’d have to meet building codes if present. In the tropics, I would go down deep enough so there’s a rubble trench and two courses of bags below grade so flash floods won’t wash away or undercut the wall. In dry, hot climates, some people are building a rubble trench that’s flush with grade and building directly on top.

  3. I would like to ask you some questions about no load bearing walls and foundation load bearing.

    I am trying to design a passive cooling system for a single container home in a tropical environment. The container will be shaded by a palm thatch flyroof at least twice its size giving me mostly outdoor living space. I would like to use earthbags in a variety of ways.

    First, I want to use a chair rail height wall, around the perimeter to block low angle sunlight from reaching the patio floor and lower walls.

    Second, I would like to use the earthbags to elevate the container 2 feet.

    Third, I would like to have a rubble trench or earthbag foundation around the perimeter to keep the patio dry during minor floods common during heavy rains.

    My questions are;
    1) Will water go under a foundation like this if submerged? Scoria seems like it would let the water rush through.
    2) If the wall is not weight bearing, is the foundation even needed or would a berm or row of stabilized bags work just as well to keep water out
    3) Would the earthbag supports for the container (or any support for that matter) need foundations if the perimeter wall or trench was already keeping water out?
    4) You talk about trench width and load bearing but I suspect a palm thatch and bamboo roof is light enough the trench could be much smaller.
    5) I would prefer narrow walls (9-12 inches) filled with organic matter in the upper courses (rice hulls or clay wood chip mix due to the need for lowest thermal mass). Iit appears this is ok when not weight bearing or very tall. It would also make the wall very light…Same question as #4 how much foundation is needed.
    6) The soil in the area is moderate heavy tropical clay (40-50% sand-5-15% silt and 25-45% clay). What would be the best amendment for the water proof lower areas (and below ground) if I had to chose between scoria, sand, flyash, lime, portland.

    Sorry for needing an entire lesson but I can’t find anything on decorative non load bearing low thermal mass construction for flood prone tropics. I’m also not trying to protect a house but an outdoor living area.

    It needs to be low thermal mass

    (non weight bearing to chair rail height) (and frame a garden). It will be in the true humid tropics and

    • It’s too time consuming to answer really long comments. Here’s a quick summary.
      – earthbags will work under the container. I would use gravel-filled bags, double bagged, set on a rubble trench.
      – trench size can vary like you say.
      – you want 1-2 courses below grade to resist being washed out by flowing rainwater.
      – lightweight walls: use gravel bags on lower courses, rice hulls above flood level. Make sure you reinforce the wall with curves, corners, buttresses, rebar, etc.
      – narrow walls will be less stable than standard walls made with 18″ bags
      – best amendment: add sand to clay
      – add two layers of 6 mil plastic sheeting to prevent moisture problems: 1. against the earthbags, 2. one layer out at an angle. Then add some backfill to create a small berm. This will help prevent water from going underneath.

  4. Hello–I tried to purchase #60, but it’s not available! Can you help me out since we want to build a cordwood shed & the foundation part is troubling me!! Thanks, Meredith Thomas


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