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

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

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.

Using natural materials: A comparison

by Malcolm Worby

Using natural materials for construction of dwellings and community buildings, is the oldest method of building since humans moved away from caves. In fact, more people in the world live in houses built of natural materials, than any other type of building material. It is therefore the most common method of building in the world.
There are many different types of natural building materials, and to describe and go into detail about each one would only serve to confuse many who are unfamiliar with natural building practices, as some are only applicable to certain areas of the world. So, in order to keep things simple, the more popular, and commonly known and used types, are outlined below, describing the basic methods of building with the material, along with its advantages and disadvantages. This is by no means meant to be comprehensive, and is intended solely to explain the basics to those who are unfamiliar with the different types of building and material. There is of course far more detail, and also other options of how to build than is outlined below, and it is hoped that anyone who having read this information, and is interested in pursuing building using natural materials will seek a professional to assist them to make the right choices.

ADOBE (MUD BRICK)
Adobe bricks comprise of a mixture of clay, coarse sand, fine sand, silt, and water, (the ideal clay content is no more than 20% of the mixture) which is placed in a form made to the size of the bricks required, and then removed to allow the bricks to bake in the sun until hard. A binder such as straw is added only if the clay content is low. The dried bricks are built on a solid foundation, ideally stone, built to a minimum of 200mm above ground level, which also acts as a ‘Damp Proof Course’ as it raises the adobe bricks wall from absorbing moisture from the ground. A similar clay-sand-water mix as used to make the bricks is used as mortar between the adobe bricks. Fairly large roof overhangs (600mm minimum) help prevent the walls being eroded over time, and on the north wall it helps keep the building cool in the summer. Usually a 2-coat earthen or lime plaster is applied as a final finish. Adobe wall structures lend themselves to having load-bearing walls, however, a wood or concrete ‘ring beam’ is recommended to support the roof structure.
The soil for making adobe bricks is usually of local material, and ideally from the property itself. This therefore makes adobe one of the most affordable building technologies, and is often completed without the use of engineers or architects, hence the term ‘non-engineered construction’. The walls are usually a minimum of 250mm thick for a single storey and 600mm double-brick thick for added thermal mass, and for two-storeys. Ideal areas for building with adobe would be temperate climates with hot and cold seasonal swings, cold climates, and hot dry climates which fully utilise the thick thermal mass for heat storage in winter, and for cooling during the summer. In hot and humid climates, narrower thickness walls could be used, providing sufficient roof overhang is provided for shade.

Advantages:
• High thermal mass is very energy-efficient in both summer and winter, and ideal for passive solar heating and cooling. Indoor temperatures vary only about 5 degrees between summer and winter (17-22 degrees), making it naturally cool in summer and warm in winter.
• Environmentally friendly: Low carbon footprint and embodied energy
• Ideal material for owner builders and unskilled labour
• Relatively inexpensive for a long lasting building
• Lends itself to creative and free-form walls
• Rondavel (round) type Adobe buildings are capable of withstanding seismic activity
• Fireproof
• Excellent sound insulation
• Can easily be built up to 3 stories
• Can be recycled
• Approved by many local building departments.

Disadvantages:
• Fairly labour intensive
• Obtaining a bond from lending institutions is extremely difficult
• Adobe cannot be laid during very wet or freezing weather
• Insects, notably termites and small rodents can burrow into the walls weakening them. The use of dung in the mud mix, and lime plaster can negate this problem

COB
Cob, like adobe, is also comprised of a mixture of clay, coarse sand, fine sand, silt, and water; it also uses a binder of fibrous or organic material such as straw, or dung. The cob once mixed, can either be used ‘as is’ and installed in ‘lifts’ of about 600mm, or can be rolled into balls about 200mm in diameter. The building is a process of laying the straw-clay mixture or balls in layers on top of the foundation walls, which are built ideally with stone, to a minimum of 200mm above ground level. The walls start wide at the base (600mm+) and taper in as one builds up. Each layer of cob must be allowed to dry before laying the next. As with adobe, large roof overhangs (600mm minimum) help prevent the walls being eroded over time, and on the north wall it helps keep the building cool in the summer. Cob wall structures, due to their width, lend themselves to having load bearing walls, however, a wood or concrete ‘ring beam’ is recommended to support the roof structure. Usually a 2-coat earthen or lime plaster is applied as a final finish. The soil for making cob and cob bricks is usually of local material, and ideally from the property itself. Therefore cob is also one of the most affordable types of building material, and can be built often without the use of engineers or architects, as ‘non-engineered construction’. Ideal areas for building with cob would be temperate climates with hot and cold seasonal swings, cold climates, and hot dry climates which fully utilise the thick thermal mass for heat storage in winter, and for cooling during the summer. In hot and humid climates, narrower thickness walls could be used, providing sufficient roof overhang is provided for shade.

Advantages:
• High thermal mass is very energy-efficient in both summer and winter, and ideal for passive solar heating and cooling. Indoor temperatures vary only about 5 degrees between summer and winter (17-22 degrees), making it naturally cool in summer and warm in winter.
• Environmentally friendly: Low carbon footprint and embodied energy
• Relatively easy to build for owner builders and unskilled labour
• Relatively inexpensive for a long lasting building
• Lends itself to free-form walls
• Excellent sound insulation
• Cob buildings are capable of withstanding seismic activity, but must have a ring beam.
• Fireproof
• Can easily be built up to 3 stories
• Cob can be easily recycled

Disadvantages:
• Labour intensive
• Relatively slow to build
• Obtaining a bond from a lending institution is very difficult.
• Cob walls cannot be laid during wet or freezing weather
• Insects, notably termites and small rodents can burrow into the walls weakening them. The use of dung in the mud mix, and lime plaster can negate this problem

RAMMED EARTH
Rammed Earth construction is done by using a mixture of sand, gravel, clay (the proportions depend on the available soil), and water. The mixture is placed into formwork made of plywood supported by steel frames (or similar), placed on top of the foundation wall. The amount of mixture placed in a form at a time, known as a ‘lift’, is typically about 150mm deep, which is then compacted either manually, or by a pneumatic backfill tamper. This process is repeated until the desired wall height is reached. Door and window openings are created by using formwork, with lintels placed on top of the forms prior to compacting. The final result is a sculpted earth wall of exceptional strength. A stabiliser, preferably lime, but cement can be used, can be added prior to mixing, and is typically between 5% -13% of the mixture. Note: If cement is added as a stabiliser, a rammed earth wall 300mm thick, will have more cement content than a 115mm wide concrete block wall, and therefore the carbon footprint and the embodied energy is increased dramatically. For more creative builders, Rammed Earth offers the opportunity to mix colours of soil and when the lifts are done in different colours, it provides ‘stratification’ in the walls. This process when sealed with beeswax or similar, provides a beautiful finish with minimal maintenance. The thick earth wall is structurally very sound, but it is recommended that a wood or concrete ring beam be installed at the top of the walls. Ideal areas for building with rammed earth would be temperate climates with hot and cold seasonal swings, cold climates, and hot dry climates which fully utilise the thick thermal mass for heat storage in winter, and for cooling during the summer. In hot climates, it is essential that the walls are shaded at all times.

Advantages:
• Low carbon footprint and embodied energy.
• The thermal mass is ideal for passive solar heating and cooling.
• Rammed earth walls are extremely strong
• Excellent sound insulation
• Fire proof
• Insects and rodents are not a problem
• Does not need to be plastered
• Can withstand seismic activity providing it has a concrete ring beam.

Disadvantages:
• Rammed earth walls are very labour intensive.
• Building the walls is a slow and precise process.
• The formwork adds considerable cost.
• Obtaining a bond from a lending institution is very difficult.
• The walls cannot be built during wet or freezing weather
• Difficult to recycle

SANDBAG
Sandbag construction consists of lightweight plastic bags filled with sand or other earth mixes. Ideally the soil or mix is locally available on site. Typically sandbag construction utilises the pillar and beam type of structural framework construction, whereby the full bags are used as ‘in-fill’ by laying in courses, on a foundation wall, between the pillars. If pillar and beam construction is not used, the building will need to have curved walls to create added strength. Once the walls are completed, typically chicken-wire is then attached to the sandbags, which will allow the walls to be plastered, ideally with lime plaster. Sandbag walls are relatively quick to build, and due to the pillar and beam framework, they are also strong, and relatively inexpensive. Sandbagging requires very little water especially compared to adobe, cob, or rammed earth, which can be an important factor in some areas. Sandbag walls are also a viable option for constructing temporary buildings, as the materials are mostly reusable. The thick walls offer a good thermal mass which helps regulate the interior temperature of the building during both the summer and winter months. Ideal areas for building with sandbags would be temperate climates with hot and cold seasonal swings, cold climates, and hot dry climates.

Advantages:
• Relatively low carbon footprint and embodied energy.
• Sandbags can be reused or recycled
• Strong structures are erected quickly using pillar and beam construction with sandbag ‘infill’
• Minimal water needed for construction.
• Good thermal mass for regulating internal temperatures
• Excellent sound insulation.
• In rural areas, ‘Mealie’ bags can be collected and recycled as sandbags.
• Services can easily be added during the construction phase

Disadvantages:
• Labour intensive
• Care must be taken to prevent water penetrating through to the sandbags
• Sandbags are often made of plastic and are imported from China, which increases its environmental footprint, and embodied energy. Locally made bags are available, but are more expensive.
• Pillar and beam technology using steel, cement blocks, and wood from non-sustainable forests, are not environmentally-friendly
• Walls must be plastered.
• Obtaining a bond from a lending institution is very difficult.

STRAW BALE
The straw bales used for building must be of grain stalks (oats, barley, wheat, etc), as opposed to hay bales, which are made from grasses. The use of straw bales as a building material is very environmentally-friendly, as the straw if not baled, is typically burned by the famers after harvesting, creating tonnes of air pollution. Building with straw bales either utilises the pillar (or post) and beam type of construction, with the straw bales used as ‘infill’, or the straw bales built as a load-bearing wall. In the load-bearing wall method, a wood roof or top plate is installed covering the full width of the top of the wall, which in turn is attached to the foundation, typically with wire straps, on approximately 1200 centres. The top plate acts not only as a bearing plate, but also as a ring beam, distributing the roof load evenly along the walls. Window and door openings will need to be structurally framed in with wood in both methods. The walls when completed, and the roof is installed, are typically covered with chicken wire, and then plastered with either mud or lime plaster on the exterior, and mud, lime, or gypsum plaster on the interior. It is recommended that there is a sufficient roof overhang to help prevent water saturation during heavy rains.
Straw bale building, due to its high insulating value, is most beneficial in hot, dry desert climates, high desert climates with large daily temperature swings, cold climates, and temperate climates which have relatively hot summers, and cold winters.

Advantages:
• Energy efficient, as straw bales have a very high insulation value.
• Straw bales are produced from a waste product that is bio-degradeable.
• Can be built using unskilled or semi-skilled labour.
• Not as labour intensive as other natural building methods.
• Structurally strong
• Excellent sound insulation
• Relatively inexpensive material to purchase

Disadvantages:
• Straw bale walls are susceptible to mould and deterioration unless protected from moisture, and allowed to ‘breathe’.
• No thermal mass for passive solar heating and/or cooling
• Transportation increases the embodied energy/carbon footprint, and also the cost, unless building on a farm growing cereal crops.

As published on Enviropaedia under the title Green Building: Using natural materials

Disclaimer

Malcolm WorbyMalcolm Worby studied at Bristol Polytechnic in the UK and has had his own award-winning architectural design firm ‘Malcolm Worby Designs’ for over 30 years specialising in natural, sustainable, and environmentally friendly building. He now specialises in providing consulting services for natural and sustainable building projects, including mud brick (adobe), straw bale, sand and earth bag design and building, passive and active solar heating and cooling, photovoltaic (PV), grey water recycling, rainwater harvesting, and composting toilets. He has designed buildings in various parts of the world including the USA, UK, South Africa, Mexico, and the Netherlands, and has worked on low income affordable community-build projects in South Africa, Malawi, Mozambique, South Sudan, Uganda, Zambia through his Non-Profit Organisation ‘Homeless And Poor People’s Initiative’ (HAPPI).