In the summer of 2013 we built a cob outdoor fireplace. As soon as it was first fired, it became an instant gathering site and the large manure pile immediately behind it that collected from our barn of 7 horses became an increasingly troublesome eyesore (a shortcoming we had somehow overlooked in the fervid planning of the fireplace construction). We use the manure extensively on our property, to fertilize our pastures, build our hugelkultur gardens, and fertilize our berry patches and flower beds so getting rid of it was not an option. The solution to this problem obviously entailed another cob structure, and so was born the idea of the cob garden wall.
The plans for this wall were extremely vague at start. We had a vision of a wall that would run around three sides of the manure pile area, blocking it from view (and thought) while sitting by the fire but as far what that wall would look like, we had no idea. We decided to just go with the flow, as we had with the fireplace, and trust in the magic of cob.
To start we had to clear the area which had become overgrown with vicious blackberry and morning glory after years of unsupervision. This was a project in and of itself!
Once clear, we began by marking out the footprint of the wall, free hand, with spray paint on the ground. Previously we had mapped out the measurements of the space and doodled countless possible wall footprints in SketchUp. We settled on one that we felt flowed with the existing space, facilitated movement through the space, and adhered to the recommended guidelines with respect to buttressing features for a wall of a given height and thickness. We chose a wall thickness of 20″. Certainly more robust than the average garden wall but we had visions of this wall perhaps having mounds and mounds of manure piled against its back face during certain points of the year. As such we thought thicker would be better. Its thickness turned out to be perfect for a number of other reasons too that only became clear in hindsight.
To help ensure wall stability and prevent toppling we incorporated a number of buttress features in our footprint including a T junction at the free end, a large soft 90 degree bend, and curved alcove on the long side of the wall. There are recommendations on how many buttress features per unit length to incorporate as a function of wall thickness and height.
Following the footprint we marked out, we dug a foundation trench, roughly 14″ deep.
The trench was then filled with gravel and left alone for a few months while we finished up our other projects from that summer, namely plastering the fireplace and the tandoor.
We spent much of the winter of 2013 collecting concrete slab from demolition sites as this is what we decided to use for our foundation. Our hope was to build this wall out of earth and reclaimed materials as much as possible. During our scavenging hunts we scored a big load of cut granite slabs that were precisely 20″ wide. This was an absolute windfall for us as they stacked beautifully and made one section of the wall foundation materialize in a single day!
In January of 2014 we began building the concrete foundation. After trialling a number of different solutions we settled on puzzling the concrete slab together flat, like Lego bricks, overlapping the seams between the various layers. This proved to be frustrating work, always searching for the perfect chunk of concrete to create a nice outer face and stable structure. We became authorities on wielding a sledge hammer during this process and learned to get concrete to break in ways that were useful to us and that allowed us to puzzle chunks together to sculpt inside curves, outside curves, and flat faces. During the foundation construction we decided to incorporate an opening in the wall that would ultimately act as a gate in the wall. We spent a lot of time nursing bruises, scrapes, and sore muscles and slowly the foundation began to take shape. We worked hard to get nice clean seams between the concrete chunks and to keep an even vertical face. We built up until the foundation was 16″ tall. Slabs that didn’t sit stably were shimmed with rocks so that by the end we had a rock stable dry stacked concrete chunk foundation ready to support a cob wall.
2 Replies to “Building the Cob and Green Roof Garden Wall- Part 1: The Foundation”
Hello! How did you know how deep of a trench to build? As well as how high the foundation should be before starting the wall?
The depth of the trench depends on your area. We went with 2 feet here because the frost line, seldom if ever goes deeper than that. In other colder areas the trench would have to be deeper. We build up our foundations to roughly 18-24″. We find this is high enough to prevent significant splash back from rain/water hitting the ground. Gutters on the eves also makes a big difference in preventing the splash back.
Hope that helps.