Rubble Trench/Earthbag Foundation

The Fleming College Sustainable Building Design & Construction Progam project in 2008 was the Madoc Performing Arts Centre. Students of this annual summer program constructed a sustainable performing arts center in Madoc, Ontario, Canada. Since 2005, students of this program have been building sustainable public buildings for host communities. The buildings mix low impact, low technology materials with high-tech mechanical and energy systems.

The basis for the foundation for the Madoc Performing Arts Centre is a rubble trench. This type of foundation uses compacted stone in an excavated trench to provide bearing capacity for the building above. They used a grade of crushed limestone called “3 inch minus” which includes any aggregate that would fall through a 3 inch screen. The bottom of the trench has a drainage tile running in it, which slopes to carry water away from the building.


The earthbag grade beams that supports the main octagonal space are composed of woven polypropylene sack material that were obtained as a continuous tube. These tubes were filled with a site soil mixture of gravel, sand, silt and clay and then compacted firmly. Barbed wire is run between each course of earthbags to prevent slippage. They used two parallel runs of narrow bags with insulation (hempcrete) between the runs. Earthbag foundations are very low cost, and are strong enough for a large building like this. They are quite easy to build as they don’t require formwork.

For more photos and description of this project and its foundation, see

18 thoughts on “Rubble Trench/Earthbag Foundation”

  1. I am building in a tribal community where sustainable building seems to be frowned up in lieu of concrete. I have a rubble trench in clay/sand soil. A 16″ trench 2.5 feet down. The sand was a surprise! Now I’m reconsidering adding in some concrete piers for additional support as welll as widening the trench? Have u ever experimented with combining cement piers with a rubble trench for additional support? I have a drainage system in place.

    I have heard of many techniques in selecting the best gravel/rubble sizes. Which do you recommend?

    I am planning on building a post and beam frame Using a combination of techniques centered around earthbags. Strawbale could also be used. I’m just worried about the home sinking or collapsing. Roof would be a 32 rafter system- plywood and metal roofing.

    • Talk to a local engineer about your soil. It’s hard to make recommendations on specific cases because there are vast differences in soil properties. Sometimes the soil on these reservations is really, really bad. Most of the time what you’ve built so far would be fine if you keep water away from the building, but there’s no way to know without proper soil analysis. If you can’t afford an engineer then strike up some conversations with builders at building supply centers, etc.

      Precise gravel size in the trench is not important. A range of sizes will work fine.

  2. I am beginning a straw bale home in Nebraska. It will be a hybrid of a Nebraska style and a pole/ infill (to support roof load) I am doing a rubble trench with a concrete grade beam for box beam and bales. I am wondering if I can get by with a 8″ wide rubble trench, 48″ deep? the roof lod will be on post frame with separate concrete pier footings. the concrete grade beam will have the rubble trench on the outside edge. Any suggestions? I cannot afford a backhoe to dig the trench.

    the soil is a sandy clay with load cap of 3000psf

    • I think so. Better yet, study the free online doc about Frost Protected Foundations that use exterior foam board insulation and shallower rubble trenches.

  3. A friend and I are going to try a small cabin in West TX. However the soil is heavily compacted due to overgrazing, and there is bedrock and sandstone less than a foot from the surface.

    Would it be possible to just put the gravel bag layers down on a pad of gravel, or (tamped level) on the ground directly with a 6 mil moisture barrier? The road base we will use can be pushed up against the walls to somewhat emulate the sub grade gravel foundation and walls.

    Otherwise we might try gravel filled tires for a earthship style foundation. Any insight would be a great help. There are others out here that are looking to do something similar, but have the same question.

    Thank you.

    • You can do it either way. You can dig down to bedrock and start gravel bags there. Or you can dig down to bedrock, make a rubble trench and start the gravel bags at or near the surface. Making a rubble trench is faster and easier than placing gravel bags all the way. My preference would be to start the gravel bags 6″ below the surface on a rubble trench and then continue them until you’re above risk of moisture damage.

  4. I want to build an earthbag house in the Tropics on deep Black Cotton Soil. This is a very expansive clay soil. My question is how deep do I have to dig my foundation trench that I plan to fill with rubble with two/three layers of double bags filled with gravel?

    My instinct says to excavate until the soil is permanently damp, i.e. it will not expand or shrink as it never gets dry. Possibly 4 feet or more deep.

    Am I right and can I build double storey, perhaps using a rice hull/ earth mix in the upper storey?

    Many brick/concrete block walled buildings in this area are built on floating reinforced slabs with 2 foot piers.

    Any good suggestions?

  5. ok, thank you, unfortunately we don’t have that here, our ground is pure dust… I assume I need to try and build as far away from trees as possible and hope for the best. There are “poisons” available that kill all plants, but that can’t be very environmentally friendly…

  6. Helolo, I have started an experimental earthbag cottage here in Tanzania and have double-bagged the gravel foundation as suggested on top of a gravel trench – my question is, how would plant roots affect the bags over the years? As the gravel is not stabilized I guess the durability of the bags in crucial…

    • Plant roots, particularly tree roots, are a serious concern. Roots routinely destroy concrete and so obviously they can penetrate bags. So steer clear of tree roots as much as possible. The optimum site is solid, rocky ground.

  7. Hi Kelly, I’ve been doing some research into building a round pole frame/straw clay/straw bale hybrid studio and I’d love to use an earthbag foundation but have a number of questions about how to best make it work. First, I’m thinking the best way to set the posts would be to build the foundation from earthbags, gravel filled for first courses and road mix for upper courses, then pour a grade beam, and place the posts on this..I want the walls to be pretty plumb, do you recommend a grade beam or not? I live in the Pacific Northwest where the ground is wet! and I want to keep the bottoms of the posts above ground level, do you think bringing the lime plaster to the grade is a good idea here? Also do you have a preferred bag mix or external system for providing foundation insulation? Thanks for being a great resource!

    • I think that your idea of placing your poles on top of a grade beam poured above the earthbag foundation is good. This should adequately spread the forces on the poles over a larger area. Of course the pole frame structure will need to be well braced into place in its design, and I also suggest that the poles that bear on the grade beam be pinned firmly in place with pins embedded in the cement.

      Lime plaster is pretty durable and should last quite awhile, even in contact with the earth at grade; cement stucco is even more durable and will last longer, especially if a mesh is used with it.

      One of the easiest ways to insulate the bag foundation is by filling the bags with crushed volcanic stone (like scoria) if it is available. Ordinary gravel alone will provide some insulation because of all the trapped air space it provides. Commercial foam insulation board is a less natural solution.

  8. I like the idea of gravel bags (double-bagged) for the stem/foundation wall on top of a rubble trench, seems very simple and straightforward. But how do you finish them? Since they would be in contact with the ground (and water), I’m guessing not with an earthen plaster. So would you use lime- or cement-plaster then? Would this stem wall need wire for such a plaster to hold onto? And if you wanted to use an earthen plaster for the rest of the building, would you need some sort of edging or lip for it to rest on and separate it from the stem wall below, so moisture couldn’t migrate up the outside of the wall?

    • You are right that a stabilized plaster with either cement or lime is a good idea for a stem wall foundation, and using a stucco mesh is also a good idea. I don’t think it is customary to place an edging between stabilized plaster and earthen plaster. I might suggest that you bevel the top of the stem wall plaster, so when you apply the earthen plaster it overlaps just a bit, like a shingle does. This should at least discourage moisture from draining behind the lower plaster.

  9. Hi Kelly,
    Thank you for posting these pictures. I was wondering if Earthbag foundations would be a safe solution for tropical soils, taking humitidy and instability into account. Thanks for any information regarding earthbag building in the tropics!

    • Alexandre, I see no reason why a rubble trench/earthbag foundation similar to the one described here can’t work just fine in the tropics. You might want to fill the first course or two of bags with gravel to further insure that moisture will not migrate up into the wall. And also for insurance, you can use two bags (one inside the other) on these courses for added durability.


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