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Earthbag terraced garden - advice on fill?

 
pollinator
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Location: Gulf Islands BC (zone 8)
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I'm looking for advice on a durable fill for burlap earthbags that will remain after the burlap decomposes.

We're helping some friends set up the water management earthworks and plantings on a parcel of land they bought last year. They had intended originally to terrace and plant in one area that is a fairly steep hillside but it turns out there is only a thin layer of soil over rocks. However, they've recently dug out a pond in a lower part of the lot so there's a large pile of mostly clay available. So we put our heads together and came up with a plan to make shallow earthbag terraces, and then backfill with the topsoil from the pond excavation.

The earthbags are burlap, as they didn't want to use plastic that would break down over time and remain in the soil.  The idea is that the burlap will break down after a year or so, and by then the filling will have hardened and will remain. The mix we started with for filling was about a 5:1 mix of clay soil: some kind of bagged cement mix, I'm not sure exactly which.  It's dry mixed, more or less - there's been some rain so the soil isn't completely dry, and the idea is that the fall and winter rains will fully saturate the bags and the filling will harden.

The problem is the amount of cement mix that is being used in the filling. So far, it's taken 2 bags of cement for 11 earthbags. I read online that a 10:1 mix would work, but it would be even better if we could eliminate the cement altogether and use something environmentally friendlier. But it's going to have to be something that will hold together after the burlap rots away and handle a lot of winter rain.

Any ideas?
 
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I believe you will need to do more research.
Terracing steep inclines needs care so you work does not collapse.
Using soil with portland cement mixed in the garden will make that soil pretty useless for anything else.

Some of the factors that need consideration include;
- angle of slope
- width and height of each terrace
- will you cut and fill or just fill?
- rainfall in the area
- quality and depth of soil
- will plants grow in the proposed area?
It could be that you need to plant very small plants into the existing soil and let them grow into the ground, terracing may create shallow roots etc.
You mention water activity, I work a lot with moving water, irrigation and storage.
What do your friends actually have in mind to do?
 
Andrea Locke
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Hi John,

I probably shouldn't have mentioned the water management bit in connection with the earthbag terracing as I think it just confuses the issues. The earthworks consist of a few swales where they will plant fruit trees on a different part of the property. Its just that we brought the tractor over to give them a hand with that and are also using the tractor for the other bit with the terrace so the two parts are mingled in my mind equipment-wise.

The slope that I referred to as steep is a short stretch that slopes at maybe about 10 percent. Not hugely steep, but the steepest part of their lot. They want two or three broad shallow steps that will be at most two sandbags deep with no cut and fill, just filled in behind with a few inches of topsoil to level it out. The only cement involved would be in the sandbags not in the planting soil. Rainfall is about 36 inches and this particular spot receives very little runoff from above. They are trying to get a few inches more soil to make planting pockets for trees. The back part of each step will be the very shallow and rocky native soil which will be planted with herbaceous plants that don't need a lot of water in summer, like lavender. They've just removed a lot of invasive gorse and broom from this area and need to get other species established for competition to keep those from taking over again.

I appreciate that you want to make sure this is safe from an engineering point of view. I think it will be.

The earthbags are being used in a way that is equivalent to stacking a few rocks to make a shallow wall. Although the soil is rocky there isn't anything really suitable for stacking without bringing rocks from off site. At the moment though the problem is to make the material filling the earthbags stable after the burlap eventually rots away, a lot of cement is having to be brought to the site to mix with the soil for earthbag filling. Is there an alternative approach to filling the bags that would minimize or eliminate the cement in the bags while retaining the long term stability the cement imparts?



 
John C Daley
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I will think about some alternatives.
But I have to comment, it seems a pity to take out the unwanted plants because they were growing and now they are trying to find an alternative!
- can soil from somewhere just be found and spread out with seedballs for regrowth?
- any rocks available even small ones?
-

 
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