Updated: Feb 6
Rammed earth is both a building method as well as a material. The rammed earth material is a combination of earth, gravel, silt, clay and lime/cement as a stabilzer combined into a damp mix and is then rammed into forms in order to create a monolithic wall.
The perfect mix
I always liken the rammed process to baking a cake. You must get the ingredients, the right consistency of the mix and a sturdy baking tin in order to get the perfect cake.
When possible we try to get the soil from the site we are building on, and depending on the composition of the soil available on site we may have to add a few ingredients in order to make the mix perfect.
For example if we are building in the north of Ghana the soil there is likely to be dry on the site, we may have to source for a more clay rich soil to add to the mix to make it good for ramming, if we are building in the Eastern region of Ghana the soil there tends to have a high clay content, because of this we may have to source locally for a more gravel textured soil to add the the mix.
Stabilized vs unstabilised rammed earth
Today a lot of commercial rammed earth construction is stabilized with cement. We tend to add on average 2-3% cement to our walls, the main reason we add cement to our walls is so that over time the maintenance of the wall is minimal to withstand the harsh west African climate.
We are looking into other ways where we can replace cement entirely in our walls ,we have looked in agro waste products like coconut husk, palm kernel ( which we have used in the past for our walls ) as a stabilizer but until then cement is what we will use until we can get the perfect mix without it.
Unstabilized walls are also commonly used, but it takes a very skilled and experienced Earth Builder in order to get the perfect mix. Unstabilized rammed earth walls have no cement but can have a natural more eco friendly stabilizer such as Limestone.
The absence of cement in no way makes the walls not structurally strong but on occasion unstabilized walls can be more prone to damage over time in harsher climates and may need maintenance after some time.
Unstablized rammed earth walls for Mud cafeteria, Sang, Ghana
What soil can be used?
Where possible we test the soil on the building site first to see if we can get a good soil mix to build with.
When looking for soil you want to avoid the top soil, because it is nutrient rich you you will find that this will not give the right compressive strength needed to construct a rammed earth wall.
A good mix guideline to go with is roughly 20-30% clay, 70-80% earth and gravel, then you add your cement or other stabilizer.
You will then need to add your water to the mix, but you don't want it too wet otherwise it won't be able to bind.
There is a basic test that you can do to determine if you have the right mix, this is the ball drop test.
The first step of the ball drop test is to grab some of your mix in your hand, secondly squeeze and compress the mix as hard as possible.
When you have your compressed sample mix in your hand you can drop it from 1m-1.5m high, this is about waist length.
Once you drop your sample it should disintegrate into clumps. In the image on the right you will see 3 samples. If your sample looks like image 1 the sample is too dry and will not hold once it is rammed, Image 3 the mix is too wet or has too much clay and will not hold once rammed. Image 2 is the best mix.
The formwork & Ramming
Once we have the perfect mix we then pour it into the formwork. We build our form works from scratch as this allows us to be very versatile with our wall design and dimension. If you don't have the expertise in building earth forms, there are formworks that you can buy that are premade ready for assembling, although these can be quite costly.
When building your own formwork you will need to have sturdy strong plywood boards, strong wood and clamps to reinforce the plywood in order to make the forms tight and strong enough to withstand the compaction of the ramming.
When the formwork is ready we then pour our mix in ready to compact. This is where we play with designs. The ramming technique determines the lifts, layers, or waves in our walls.
We like to ram a lot of layers and intricate designs in our walls so we tend to use manual rammers as this allows for us to have more control of the this.
Mechanical rammers is widely used in Earth construction and allows for a faster and consistent lift design, to use a mechanical rammer you will need to connect it to a compressor.
In terms of the strength of the wall there isn't any difference with the outcome from manual or mechanical ramming.
Building with rammed earth can can be laborious but the beauty in the finished product is a result of the labour in the finding the right mix, building your form and ramming. Not to mention the many advantages rammed earth has as compared to conventional building materials and methods.
Would you consider building with rammed earth?