A brief look ahead at our natural building courses for 2018

Attend our natural building course and take the first step to a sustainable future by learning hands-on natural building skills. Learn a whole range of materials and techniques while exploring questions around sustainable living based in Peter McIntosh’s experience living off-grid since 1999.

Natural building courses in South Africa 2018

If you’re serious about building naturally and sustainably then you’ll know that each technique has pros and cons. That is why our natural building course is designed around the principles of understanding earth, how it works and does not work together. You will leave with the theoretical understanding and practical grounding of a range of techniques and materials, so that you are able to make the most appropriate decisions regarding materials and or sustainability once you are ready to begin your project.

This year, Peter will be hosting two CPD accredited courses at Jakkalskloof farm, in Swellendam. Continue reading

https://www.naturalbuildingcollective.com

From the ground up ~ approaches to building a foundation for your natural building

When building with earth your foundation needs to be well considered as the integrity of your building rests here. Decisions you make about your foundation depend on the materials you have available, the type of ground you have to build on and what carbon footprint you want to leave. The goal should be to create foundations that are hard enough, move uniformly and resist cracking for the walls above it.

Foundations for conventional building have, to a large extent, a one size fits all approach regardless of the type of ground you are building on i.e. a concrete and steel foundation that works equally well on all types of earth and varies only slightly in its design. It requires little thought and has been proven to be effective. The cement in concrete provides the compressive strength, and the steel tensile strength to resist cracking. It does however come at a cost to both your pocket and the environment.

When building with earth your foundation needs to be well considered as the integrity of your building rests here. Decisions you make about your foundation depend on the materials you have available, the type of ground you have to build on and what carbon footprint you want to leave. The goal should be to create foundations that are hard enough, move uniformly and resist cracking for the walls above it. Foundations will always have a higher Mpa value than the walls, however it does not need to be excessive. A 4 Mpa foundation is sufficient for a 1.6 Mpa mud-brick wall, which most types of foundations are suitable for. Furthermore, if after levelling the site the undisturbed earth is hard enough, foundations may well be unnecessary.

There are several strategies for foundations depending on the type of ground that you are building on. In this blog post, I discuss the four types of ground, (1) uniformly hard, (2) uniformly soft, (3) hard and soft, and (4) clay, their challenges and several strategies you may incorporate into your design. The discussion is quite technical in some areas so I recommend that you read my three-part series on understanding earth first. Continue reading

https://www.naturalbuildingcollective.com

Owner-builder journey ~ Franz Muhl: Energy flows where attention goes

In this edition of the Owner-builder journey, Franz Muhl writes about a mud brick addition to his Scarborough home: “Let me build a home from fertile materials that is beautiful and healthy to live in, have a very low footprint and would grow a forest if you left it.”

Franz 1

Five years ago, Peter McIntosh gave me +- 900 sun-baked mud bricks, for an extension to my house. Franz 3With little start up money, a trickle of income, some plans on google sketch up, a pickaxe and, most importantly, plenty of time, I finally started the process a year ago.

 

At foundation level, with the skills that I had at the time, I used clay-fired bricks and a bitumen coat for damp-proofing. Franz 4In March, I headed off to Berg-en-dal for a crash course with Peter. He traded his skills and knowledge in natural building for mine in brewing beer. To take clay, sand, water and a bit of straw in the right proportions and work it into a material for building, was a big revelation for me. Continue reading

https://www.naturalbuildingcollective.com

Lebone Village launch

Imagine being outside on a chilly Free State winter morning with the sun just coming out and starting to gently warm your body. Now imagine being told to take off your shoes in order to trudge in icy cold mud. I glanced at my fellow volunteers and I saw a collective dissent quietly dawn on our group – this is not what we signed up for!

Mandela day

It was the morning of 18 July, Mandela Day, and we were all gathered at Lebone Village on the outskirts of Bloemfontein to volunteer our 67 minutes for the orphans. We were standing in a circle around Peter McIntosh, who was valiantly demonstrating to us the endeavor of making adobe bricks.

Peter McIntosh demonstrating how to make cob

Peter McIntosh demonstrating how to make cob

The mix using ingredients easily available for the project was chosen after rigorous testing. According to Peter, the mix will differ in every situation, depending on the composition of the ingredients used. The chosen mix for the adobe bricks at Lebone Village was as follows: collect two parts red earth, 2 parts sand with rubble, one part fine sand and two parts water in the centre of large piece of 25” thick canvas material.

Now mix it all into clay with your feet by walking back and forth through the cold, wet mixture. When the cob mixture starts to flatten out, pull the canvas up-and-in towards you from the corners to bring the clay mixture back into the centre of the canvas and into a manageable heap. Now start stepping onto it again. The clay is the right consistency when you can make a ball with your hands and pull it apart into two separate pieces without it crumbling. Adding straw to the mud mixture assures bricks that are well insulated against cold and heat, the more straw you add, the better insulated your bricks.

Adding water

Adding water

Adding straw binds everything together and adds insulation value

Adding straw adds insulation value

Lots of people turned up

Lots of people turned up

 

While the majority of us were still apprehensively contemplating the prospect of braving the cold and mud with naked feet, one person rose to the occasion without hesitation. In the spirit of “first being a follower in order to be a leader”, Itumeleng Santo started pounding the mud into clay with some über cool dance moves. Itumeleng is an out-patient at the University of the Free State’s Dept of Occupational Therapy’s clinic at the MUCPP offices in Rocklands location. He is severely impaired due to a brain injury that he suffered during an assault. For Itumeling, taking part in the Mandela Day activities at Lebone Village was therefore also a day of getting therapy without being given therapy. The Dept of Occupational Therapy vision is to support and treat their disabled and impaired patients in such a way that they will be able to return to their families and communities and be able to fully participate in community activities again. The aim is for such patients to become fully functional individuals who can partake in economic activity and contribute towards their own livelihoods.

The MUCPP clinic of the Dept of Occupational Therapy is not only for patient care and therapy, but it also serves the wider community as a place where youth can hang around after school and in this way be kept off the streets. Heidi Morgan and Bronwyn Kemp, who run the clinic, aspire to teach these children skills that will help them to create their own employment upon completing their school careers. Learning how to make adobe bricks and tire pounding for alternative and natural building practices are two such skills.

This notion of self-empowerment of the impaired, disabled and destitute was the golden thread that ran through the activities at Lebone Village on the morning of Mandela Day. Stakeholders from support institutions to the disabled came from all over the Free State region to learn the new green building techniques of making adobe bricks and pounding tires. These are skills that they intend to take back to their home towns and villages, skills that they hope will enable them to become self-sufficient and self-employed, able to earn money and make a living for themselves, without being a burden to their families.

Getting our feet dirty

Getting our feet dirty

Peter McIntosh demonstrating putting cob into the brick mold

Peter McIntosh demonstrating putting cob into the brick mold

With the ice now literally and figuratively broken by Itumeleng, the rest of us started to get into the spirit of the day. The extra brave ones took of their shoes and started pounding cob with their bare feet. The more modest traded their shoes for gumboots to get the job done.

Some started working the cob with their hands. Anita put on some vibey music and soon the day was in full swing. Volunteers started forming little groups, each group working their cob on their own piece of canvas. Some people would collect the pounded cob and compact it into wooden molds set out by Peter for this purpose. These mudbricks would then be left to dry in the sun for several days, where after they will be ready to use for building.

Peter McIntosh demonstrating putting cob into the brick mold

Peter McIntosh demonstrating putting cob into the brick mold

The teaching of green building techniques to the greater Mangaung community also served as the launch of the Lebone Village Climate Resilient Arts, Crafts and Cultural Hub and was initiated by Qala Phelang Tala, a non-profit organization based in Bloemfontein and associated with the Centre for Development Support at the University of the Free State. Qala Phelang Tala is Sesotho for “Start Living Green” and is the brain child of Anita Venter, a researcher at the Centre for Development Support. QPT strives to empower “change agents” through social entrepreneurship in order to create systems addressing housing, food security, water efficiency and energy independence that are resilient to climate change. Their slogan is “Learn by doing!” This means that they not only preach green building and sustainable, environmental friendly living, but they also practice, implement and teach these techniques. QPT head hunted and hosted Peter McIntosh from Natural Building Collective, who is one of only a handful of natural building experts in South Africa. His experience in sustainable living practices includes sustainable agriculture, off-grid energy systems and an array of natural building techniques, all of which is in fruition on Berg-en-Dal outside Ladismith in the Klein Karoo, a farm owned and managed by the community and educational non-profit the Klein Karoo Sustainable Drylands Permaculture Project, where he is a resident and member.

Some of the mudbricks that were made on the day drying in the sun

Some of the mudbricks that were made on the day drying in the sun

Contributed by Amanda de Gouveia on behalf of QPT. Photos courtesy of QPT. Please visit their Facebook page for more photos of the day.

Amanda de Gouveia

Amanda de Gouveia has been a research assistant at the Centre for Development Support at the University of the Free State since 2010, where she has mostly been involved in research projects on social development and local economic development. This has refined a unique repertoire of research skills, both qualitative and quantitative. She has also Masters degree in Research Psychology.

 

 

https://www.naturalbuildingcollective.com

Owner-builder journey ~ Building ‘home’

In the first edition of the Owner-builder journey, Laurie Simpson writes about the challenges of building with mud in on the edge of Hwange National park, Zimbabwe.

Seven years ago, after years of travelling, looking for the next adventure and never feeling like I belonged, I followed my partner to live in his home, Zimbabwe. The moment I arrived I knew I had arrived ‘home’, even though I had never set foot there before. It was a strange and beautiful feeling and one that kept me from leaving despite many difficult times over the years.

Dance like an elephant

Dance like an elephant

Two years ago, I started building ‘home’ using mostly materials that are found around us. It’s been an amazing journey of self-discovery. It started with reading books and articles that inspired me to live a life that was in sync with nature. Previously, I felt like we were just spectators watching nature go by as if we were not a part of it. From all this research I quickly realised just how destructive modern building techniques were and how much sense it made to build with natural materials.

 

I decided I needed some hands-on experience before I could start building ‘home’ for our family. I discovered Berg-en-Dal eco-village and enrolled myself on the natural building course. The course was both practical and theoretical and I had an amazing time with the two facilitators Peter McIntosh and Neil Smith. Other than this short one week course I had no building experience at all, but I felt it really gave me the confidence I needed to throw myself into my own project.

I went home and started to test soils and plan my building project. I wanted to build this home totally by myself as it felt so personal and I loved the process. I also saw how in the community around me some women were still building traditional huts using natural materials. The huts are made from very high clay soil, usually from a termite mound and so crack a lot. Since restrictions were set for where people could live (there are no fences, so wild animals move freely) homesteads are no longer temporary, and the longevity of these buildings began to matter. These days, people opt for more modern materials that are costly both financially and environmentally. I wanted to prove to myself and to others that it was possible to build a home from natural materials that was comfortable in our climate, could last a long time and meet all our needs.

Sculpted spiral plaster detail

Sculpted spiral plaster detail

 

From the beginning, I fell in love with the process of cob building, mixing sand, clay and straw with my feet and making big balls of this mix to sculpt the walls. I was so in love with cob in fact that I was blinded.  I had made up my mind even before trying cob building on the course; and once I started ‘mud dancing’ that just sealed the deal for me. I was also set on doing everything 100 % natural and making no concessions, that I ended up making some mistakes.  I started to see this after a year of building a somewhat large round cob house. I had built the stone foundations and half of the cob wall, but as I was building alone it was very slow and the rainy season was approaching. I had to cover the walls to protect the cob as there was no roof yet.

 

One of my biggest fears when I was first researching cob building was termites. There are many termites where we live and they go everywhere. Still, I didn’t want to put a metal termite barrier between the foundation and the walls as this was both an added financial and environmental cost. Yet, when I covered the walls the dark moist environment was perfect for them, and they moved in to the walls. It was very difficult to face, but I had to reconsider everything!

I had started to be interested in natural building because of Permaculture, a process of designing systems that work with nature rather than against nature. I realised I should have done a Permaculture Design Course (PDC) before doing a natural building course, as this would have given me the tools I needed to think through all the elements and design a home with nature in mind.

So back to Berg-en-dal I went to do a PDC. I had to re-think and re-build my confidence to continue, and after two amazing weeks, a lot of emotion and good advice from Peter McIntosh and the PDC facilitators, I had the energy to go back home and rethink and re-design. It has been just over a year now, and I am almost finished building what has changed into a small home. It has just two small bedrooms and a small living space, the rest are verandahs and outdoor spaces. I will still use the previous structure, but in a different way.

Sun baked mud-bricks for the wall and termite barrier between rock stemwall and brick wall.

Sun baked mud-bricks for the wall and termite barrier between rock stemwall and brick wall.

I learnt my lesson and adapted: I could still do mud dancing but now made sun-dried bricks. I made many new tests using the same clay that my neighbours were using. The difference being that I added sand to stop the bricks cracking and made very thick walls. I made sure I had finished these in the dry season and put the roof up on poles before the rains started. I had one person helping me some of the time and I called for help whenever I needed more specialised information. The foundations are stone again and this time I made a metal termite barrier between this and the walls.

Cob bench

Cob bench

Exterior

Exterior

Local clay tiles for the floor.

Local clay tiles for the floor.

To see our dream come to life is amazing. It’s been an exciting journey thus far and it’s only just begun, in the next couple of months I hope to move ‘home’ with my family and carry on testing and promoting natural building and Permaculture in our community. There are many challenges living with wild animals such as elephants and lions and surviving from the land. The soils are very sandy and the dry season can stretch out for very long periods, but I believe that there are simple and practical solutions so that we can take care of both people and wildlife.

 

You can follow what I am doing on my blog

http://dancelikeanelephant.blogspot.com

 

 

https://www.naturalbuildingcollective.com

Understanding Earth II: Testing earth

By Peter McIntosh

(Please note that in order to understand what is written here you will need to have read my previous post on understanding earth)

 

Earth requires two properties to make it strong enough for building, compressive and tensile strength. In much the same way that steel works in concrete they can’t be looked at in isolation as they work together. For example, even though concrete when supported can take an enormous amount of pressure / compression without disintegrating, if you were to cast a concrete lintel without steel and suspend it between two points and apply pressure / tension, it would snap. Steel has enormous strength in tension while concrete has enormous strength in compression.

Compressive strength is measured in Megapascal (MPa). One atmospheric pressure is 101 325 Pascal; a Megapascal is more-or-less one million Pascal, or 10 times atmospheric pressure. In other words, one MPa is 10 times stronger than it needs to be to resist the force of gravity on earth, stand on its own and not be crushed.

A good mud-brick has a MPa strength of around 1.6 to 1.9 MPa, while a clay-fired brick has an MPa strength of around 14. Concrete ranges between 15 and 25 MPa. Obviously these figures vary widely, but these are good averages. A mud-brick at 1.4 MPa is 14 times stronger than gravity, a clay-fired brick at 14 MPa is 140 times stronger than gravity or 140 atmospheric pressures.

Tensile strength is found in all material, just in varying degrees. Concrete as we have seen has high compressive strength but relatively low tensile strength. The addition of steel (reinforced concrete) increases its tensile strength. Mud bricks can handle 14 atmospheres, but like concrete they have poor tensile strength. However, as clay is somewhat plastic in its behaviour it’s not as poor as one may think. This is why the addition of straw to a mud brick is essential as it not only increases the insulation value of the mud brick but also acts like steel in concrete. (I am told that weight-for-weight straw is stronger than steel or at least in the same realm.)

In short, the tensile strength of a material is its ability to resist snapping and cracking. Increasing the hardness of an earthen material, for example by adding lime, may not increase its tensile strength or resistance to cracking, as it may end up becoming less plastic and more brittle. Thus, clay buildings are often more resistant to cracking because they can absorb the movement that harder more brittle materials may not.

When building with earth, strong enough is what you are aiming for. At 1.3 MPa, a double-storey building is already seven times stronger than it needs to be. However, given window and door openings and the fact that the gravitational forces need to be transferred around them, 1.3 MPa just covers it with a safety margin. It is important to grasp that it does not matter at all if you used clay bricks at 14 MPa, once something is strong enough, the extra strength means nothing at all.

Testing of the material

Tensile testing

–          Make a brick using the cob method (that is using sand, clay and straw ) and a 2 litre ice-cream tub as mould. Number each mix and mark your bricks and balls.

–          Allow the bricks to cure for 3 weeks minimum in the sun. A brick is considered cured after 3 months, but I have found that 3 weeks gives you a really good idea, after all it will only get stronger.

–          Drop the brick from waist height, onto a very hard and flat surface and observe how it breaks up. If it shatters it is no good; breaking into a few large pieces is acceptable. Often enough it does not break at all, which is fantastic.

A failed tensile strength test after being dropped on a hard surface; the brick should not disintegrate. Four big pieces is just a pass, but one is happiest when the brick bounces and does not break at all. This often happens.

A failed tensile strength test after being dropped on a hard surface; the brick should not disintegrate. Four big pieces is just a pass, but one is happiest when the brick bounces and does not break at all. This often happens.

Observe the cracking. Surface cracks, no deeper than a centimetre are fine. Cracks that run deeper compromise the material. They may be due to a very aggressive clay or because there is too much clay in the material. There can be other causes of the cracking such as the addition of too much water or uneven drying of the material.

Compressive testing

–          Make tennis ball size balls using the cob method and allow to cure, as above. A ball has a point and you are testing the point load. Remember to mark the balls.

–          Place the ball on a hard and flat surface. Stand on the ball with your heal and slowly increase your weight on the ball until all your weight is suspended on it.

My weight is around 80 kg and I know that if the ball crushes just before all my weight is suspended the MPa strength is 1.3. If it takes all my weight then the MPa strength is at least 1.4. As you gain more experience and your frame of reference increases you can quite accurately gauge greater MPa strengths by gently bouncing with your heal on the ball. At around 1.8 MPa the balls are very resistant to crushing with the heal, even with repeated bouncing; but then it does not matter because the material is already more than strong enough.

Both the compressive and tensile strength tests need to be passed for the material to be good enough to build with. Of course, if the material fails these tests it does not mean it can’t be used, especially if cracking is the result of failure. You can try excluding water and instead try ramming the material as a way of lining up the particles and see if that will works; or try making compressed earth bricks or even a sand-bag house?

Bottle, tongue and touch are all good indicators of how an earth is composed, but nothing beats compressive and tensile testing.

Bottle: place 4 cm of the earth in a 400ml bottle, add water and a teaspoon of salt to help it settle and shake it all up. It will give you an indication of the particle ranges you are dealing with and their ratios. However beware you will not be able to tell the difference between sand and silt.

To check if clay is present, make the material very wet and rub between your hands, then dip your hands in water, if the material sticks then there is clay present if it falls away then there is mostly or only silt.

Resistance to water erosion is dealt with separately in the plaster stage which will be dealt with later.

Below is a list of tests I made for Magic Mountains retreat as an example of a comprehensive earth test.

First walk the area you have to source your materials and then collect samples from various sites. Here I located 2 distinct earth types. White building sand was located close to the farm. Make observations of the material so you can begin to make rational choices for you mixes.

Earths ready for blending at Magic Mountains Retreat. Note the 2 litre ice-cream container for making a brick.

Earths ready for blending at Magic Mountains Retreat. Note the 2 litre ice-cream container for making a brick.

Red earth located in the South East corner of the property. This earth appears to have a high clay content. It is also attractive in colour. Made up of fine sand clay and unspecified amount of silt

Brown earth located to the North. This earth appears to have a higher sand content although very fine. Certainly has a lower clay content than the red earth.

White sand located to the South on a neighbours farm. This sand has a particle range that excludes finer particles and is angular and not rounded.

The following test samples were made to deduce the tensile and compressive strength of the material, clay content of the red earth, and cracking of the material will also be noted:

A100: 3 x 2l 100% earth bricks red earth and test balls

A100: 3 x 2l 100% earth bricks red earth with straw and test balls

3 x 300mm x 300mm x 170mm red earth bricks with straw

 

B100: 3 x 2l 100% earth bricks brown earth and test balls

B100: 3 x 2l 100% earth bricks brown earth with straw and test balls

3 x 300mm x 300mm x 170mm brown earth bricks with straw

 

50/50: 3 x 2l earth bricks 50%/50% red and brown earth and test balls

50/50: 3 x 2l earth bricks 50%/50% red and brown earth with straw and test balls

2 x 300mm x 300mm x 170mm 50%/50% red and brown earth bricks with straw

 

W80: 2 x 2l earth bricks 20% red earth 80% white sand and test balls

W66: 2x 2l earth bricks 33% red earth 66% white sand and test balls

W50: 2 x 2l earth bricks 50% red earth 50% white sand and test balls

 

C4:     2 x 2l earth bricks 50% red earth 50% sand and test balls

C66: 2 x 2l earth bricks 33% red earth 66% sand and test balls

 

2x compressed earth bricks from red earth

The completed bricks and balls should be left to cure in the sun for at least 3 weeks, and turned a few times to ensure even drying whilst keeping an eye on the weather.

The completed bricks and balls should be left to cure in the sun for at least 3 weeks, and turned a few times to ensure even drying whilst keeping an eye on the weather.

The bricks ready to be tested on a hard surface

The bricks ready to be tested on a hard surface

Results of the brick testing above

It was established that the red earth has a high clay content. Certainly above 60% as the bricks with 20% red earth and 80% white plaster sand were only just below minimum building strength. As soon as the ratio of red earth reached 33% it was obvious that the bricks passed both a compressive and a tensile strength test. It is estimated that the MPa strength at 33% is 1.4. Above 33% red earth and the bricks harden a lot.

The brown earth from below the dam could be used as a filler with the red earth, but this was decided against as it is in valuable agricultural land. It is not suitable on its own.

The addition of straw added to the tensile strength of the material in all cases.

The red earth bricks displayed deep cracks indicating a high clay content, once 50% sand was added the cracking was acceptable. The addition of sand will ensure that this does not happen and is a good enough reason to not use the red earth on its own.

The tests done with the white sand and red earth were strong enough from 33% red earth. A second test was also done with 50% red earth and 50% white sand which delivered a brick over 1.6 MPA.

 

Compressed earth bricks using red earth only, are strong enough and has no cracking. It is interesting to note that the red earth was suitable as a building material on its own if it were not for excessive cracking due to the swelling of the clay with water and that if one uses compression as a method of lining up the particles and so exclude water the earth can be used as it is.

It was decided that, because the white sand was easy to access with little environmental damage and because it would eliminate cracking, that the addition of 60% sand was the most favourable option; 40% red earth just to remain clear of the 33% mark that we know is good, in case the earth varies slightly. So 60% white sand and 40% red earth.

A series of tests made in Groot Marico. All these tests passed and although the red earth was most attractive it was decided to go with the brown earth as the red earth was further away and good for agriculture. The red earth was however used in the final plaster coat where the quantities are very small and not in the walls themselves. When a number of tests pass you are given the freedom to make choices around sustainability, and ease gathering the material when one compares them to each other.

A series of tests made in Groot Marico. All these tests passed and although the red earth was most attractive it was decided to go with the brown earth as the red earth was further away and good for agriculture. The red earth was however used in the final plaster coat where the quantities are very small and not in the walls themselves. When a number of tests pass you are given the freedom to make choices around sustainability, and ease gathering the material when one compares them to each other.

In conclusion, often when doing tests with different earths you will find that a number of your samples will pass both compressive and tensile test. This allows you the freedom to make choices affecting sustainability or aesthetics; such as how far the material has to travel, how easy is it to gather the material and what environmental damage is being done. Remember that you are not looking for the strongest sample but rather the one that makes the most sense after it has passed the tests. Strong enough is strong enough.

In my next blog post I will look at plastering of a building where the walls are able to resist the erosion of rain and the beauty of the material shines through.

https://www.naturalbuildingcollective.com

Understanding Earth: The beginnings of a Natural Builder

By Peter McIntosh

One of the challenges of working with earth is that no two sites are the same. The recipes one learns on one site may not work on another, because the earths’ found there are composed differently. Most earth building relies on a mix of sand and clay, which may be present in a single earth or need to be blended together.

Sand has a particular particle size and is like a rock only smaller. You go from boulders to rocks to stones to gravel to sand to silt, or something like that. Each is a smaller representation of the one before it and just like you get many sizes of rock so you get a range of sand size. Sand particles range in size between 2mm and 0.0625mm which is a huge deviation.

The shape of sand in an ideal world should be shattered rather than rounded, such as beach sand. River sand is considered better because it tends to be more fractured so the sand particles do not slip past each other but rather build bridges and lock in together.

Ideally you also want the sand to have a range of particle sizes and not just lumped on the large side, 2mm or the small 0.06mm. This is because when the larger sand particles are packed together you will have spaces in-between and you want those gaps closed with smaller sand particles. Of course as you look closer you will see that there are spaces in-between the smaller particles and it really is like a fractal. The next range down is silt and ranges between 0.06 mm to 0.0039 mm, this particle would be able to close those gaps and so you go. So with sand you are looking for two things primarily, a shattered particle and a good distribution of particle sizes.

Clay is the magic that does the binding in earth building. Clay is completely different to what has been mentioned above except that there is some relation to particle size with silt. If you went to the beach and made a sand castle and then when it was dry a little pressure would flatten it, especially with those rounded particles. Do the same with clay and once it is dry it is immensely strong. This is because clay is not just a smaller sand particle but rather a flat platelet that is held together by electrostatic force. It works in a similar fashion to a drop of water between two pieces of glass, you can slide them apart but you can’t pull them apart. The trick with clay is to work the material until the particles are lined up to allow the electrostatic forces to work. There is always enough humidity in the air and retained in the clay to allow this process to continue, even in very dry conditions. Clay and silt are often found together in the same deposits and are hard to tell apart if they are mixed together. Mostly what is termed as a clay earth is a mixture with silt. I consider 60% a reasonable clay content . Clays also all behave differently. Some clays swell considerably when water is added and are great for the sealing of dams and the like but no good for building with cob or mud/adobe brick, as this leads to cracking in the drying process. Really fine clays also tend to be brittle, such as Kaolin, a fine white clay. So with clay you are looking for a nice high percentage with as little silt as possible, not too fine and one that does not swell to the point of compromising the strength of the material with excessive cracking once dry.

Now to create a building material both sand and clay are blended together, to get the benefits of the structure of the sand with the binding properties of clay. Basically you just want to add enough clay to coat the sand particles and close the last of the gaps left between them and allow the electrostatic force to hold it all together. You certainly do not want silt as that is competing for the space between the sand particles and is just where you want the clay to be. At around 18% there would be just enough clay to do the job. If there is silt present with the addition of 18% clay you would begin to force the sand particles apart and you would have a more brittle material, as the material is strongest when the sand acts as a bridge over each other, locking together.

But that’s the theory, in reality you are dealing with what is available and that is always going to be less than perfect. Your sand may have only large and small particles and nothing in between or any number of permutations, depending on how nature left its deposits. Your clay may be a mixed bag of various amounts of silt and swell in a less than perfect manner. So what you are looking for is not the ideal, that does not exist, but rather something that is suitable and strong enough for your needs.

Different methods can help with how the material behaves so choice of approach is important. Blending and lining up of the material can be done in essentially two ways both have their benefits and drawbacks. The first is to add water and mix the material until well blended to achieve a good lining up of the particles. Different methods allow for different quantities of water however the addition of too much water can lead to avoidable cracking or a material with less compressive strength. Cob is often the standard most people refer to and also has some added straw (straw adds to the insulation value and tensile strength of the material) The cob mix needs to be stiff enough to resist slumping when placed on a wall to the height of 300 to 500 mm. One of the benefits of using water is that different earth can be easily blended and straw can be added, a drawback may be that if the clay is aggressive or of a high overall percentage it could lead to cracking and a weakening of the material.

Adding clay to the sand on a tarp

Adding clay to the sand on a tarp

Add some water

Add some water

 

Mix cob with your feet and add some straw

Mix cob with your feet and add some straw

Stitching cob onto a wall

Stitching cob onto a wall

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The second method of lining up the particles is to put the material under pressure and not to add water beyond just slightly damp. The material can be stamped such as for rammed-earth or compressed such as with a compressed earth block. A benefit could be that as you are not adding water there will be less cracking even if the clay content is high and a drawback is that earths are not easily mixed together without water unless you have other machinery so a single earth is often used and the addition of straw is not possible.

Add your single earth into the compressed earth brick machine

Add your single earth into the compressed earth brick machine

Put the earth under pressure

Put the earth under pressure

Out pops a brick

Out pops a brick

Building with CEBs

Building with CEBs

Understanding how earth behaves is key to choosing a method of approach that supports the materials you have on hand.

In my next blog post I will talk about the qualities of earth what it means to say that a material is strong enough and how it performs (compressive and tensile strength, insulation and thermal mass) and how to test for these properties.

 

If you’re interested to learn how to put the theory of earth into practice, learn more about our natural building courses.

See Disclaimer

Previous posts by Peter McIntosh:

Getting a feel for Light Earth

You might also enjoy: Using natural materials: a comparison, by Malcolm Worby

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