Comments on: Rubble Trench/Earthbag Foundation

By: Kelly Hart

Kelly Hart — Fri, 03 Jul 2009 03:11:50 +0000

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.

By: GreeneGardens

GreeneGardens — Fri, 03 Jul 2009 00:36:48 +0000

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!

By: Kelly Hart

Kelly Hart — Sat, 27 Jun 2009 13:33:28 +0000

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.

By: chaburchak

chaburchak — Sat, 27 Jun 2009 11:42:24 +0000

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?

By: Kelly Hart

Kelly Hart — Wed, 17 Jun 2009 22:31:55 +0000

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.

By: Alexandre Cealac

Alexandre Cealac — Wed, 17 Jun 2009 21:39:13 +0000

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!