In the race to get the lounge room floor done other things have fallen by the wayside. The little rammed earth wall has been waiting to be back-filled for a while now, so it was high time to get onto it.
We had a bunch of broken-up concrete lying around, mostly from digging up the trampoline base while constructing the orchard. That went in. Also a bunch of little rocks that aren’t any good for wall building. Then a pile of dirt.
Finally it was off to the hardware to get some gravel.
We have little spaces on either side, which are waiting for us to extend things later, so we made them into little garden beds for the time being.
Work in the “garage patch” and in dismantling the old sand pit has yielded a nice pile of rocks for wall-building. The circular wall around the cherry tree is the one we’re most interested in seeing complete, mostly because you can see it out the kitchen window.
All the big rocks go on the bottom, and make a solid foundation for the rest. The rocks on the top need to be nice and flat, so the correctly-shaped ones get duty doing that. All the rest go in the bulk of the wall, and there’s usually a bunch of little ones just under the cap rocks to make the right profile to make solid contact with the bottom of the cap rock.
One of our desires is to build some retaining walls with reinforced rammed earth. Rammed earth can be really great in that the materials are both very cheap and environmentally-sound. A wall can be built with human energy alone. Formwork is wood and reusable. For reinforcing a bit of steel is required, and perhaps some line to improve the earth’s binding qualities, but that’s a minimal input as far as wall building goes. Tests show that adding a small amount of concrete (up to 6%) to a silty loam can actually reduce compressive strength.
The first step to rammed earth building is to assess the quality of the soil on site. The soil needs to have a good mix of sand, silt and clay to bind well. To find out what the soil is like, one of the tests is to mix a sample with water and then let it settle out. Rock, sand, silt and clay will settle out at different speeds, forming layers that can be interpreted.
It’s important to get soil that’s below the humus level, to reduce the amount of organic matter in it. Organic matter contributes to shrinkage as a rammed earth structure dries. Thankfully, we have a relatively deep sample available right in front of us, where the ground was cut out for the driveway. It’s pretty easy to chop a bit out.
The soil comes out in clumps. It’s enough to just pop it into the jar and then shake it.
There are no stones in this sample. When you do find a stone, it tends to be bigger, at least palm-of-hand sized. There’s also no appreciable amount of sand in there – the stuff at the bottom is very fine. There is no organic matter at all, which tends to float on the surface. So the soil around here is around 3/4 silt and 1/4 clay.
The large amount of silt is a good thing. Silty loam has relatively high compressive strength. By vibrating a silty loam, compressive strengths of over 4 MPa are possible, compared to 17 MPa for residential-grade concrete. This means that an earth wall needs to be around 4 times thicker than a concrete one, for the same strength, but is perfectly capable of supplying the strength of concrete if required.
Not having any sand in the sample is a bad thing. The sand grains provide anchor points for the small silt and clay particles to bind to. A bit of aggregate wouldn’t go astray also, to “fill out” the soil, but it’s not necessary, and would have to be imported to our place. We can make a silty loam by adding the same amount of sand as we have clay in this sample – about 25%. 3 parts local dirt plus 1 part imported sand will yield a silty loam with 20% sand, 60% silt and 20% clay. We should add some water repellant to the mix to make it durable, and top walls with a short concrete slab to protect the edges and corners of the rammed earth.
In house-building, water repellant in a rammed wall is not necessarily a good thing. One of the benefits of earth is that it absorbs vapour from the air, or gives it up when the air is dry, leading to a home with an almost constant internal humidity of 50-55%. Humidity at this level is good for health – not too dry so that airways dry out, and not too wet so that mould becomes a problem. Earth-wall bathrooms will even happily absorb the water from shower time. In a retaining wall, however, admitting water vapour to the structure doesn’t help, so it’s better to add a hydrophobic compound.
Step 2 is to test the strength of some standard rammed earth samples (in a lab).