workshop foundations | The Natural Building Site

The Rubble Trench...
As the name implies, a trench is dug down to undisturbed earth or rock and is filled to grade with rubble (see diagram on left).

This system combines two essential functions. It provides solid but resilient support for the building - capable of distributing loads evenly while also absorbing any settling and seismic movement. Simultaneously it acts as a drain, channeling water away from the foundation and eliminating the risk of frost heave that can be a destructive force in cold climates.

An additional bonus is that it is very simple and uses considerably less processed material, thereby reducing the carbon-footprint of the building.

The trench will be slightly wider than the width of the walls above thus providing a 'french drain' to catch any water that might otherwise collect at the base of the walls. The rubble typically is drain rock ranging in size from 1 - 4 inches but can be made up of recycled concrete or other aggregates.

Rounded rock is best as the smooth edges allow it to naturally compacts within the trench, making it more stable while at the same time allowing it to move - and thus absorb energy - in an earthquake. It also creates good spaces for water to drain through. The bed of the trench is sloped slightly with perforated drain pipe installed in the bottom to channel water away from the building.

The houses of Parliament in London are founded on a rubble trench, and the architect Frank Lloyd Wright used them in many of his iconic buildings, such as the Johnson Wax Administration Building in Racine, Wisconsin, and his own home, Taliesin, in Spring Green, Wisconsin.

... return to top

The Stem Wall...
On top of the rubble trench will be a Rammed Earth stem wall that rises between 10 to 24 inches above grade and on which rests the supporting structure and walls. The rubble trench protects the stem wall from contact with ground water while the wall itself lifts the building off the ground and isolates it from rain splash.

Consequently there is very little chance of moisture penetrating the walls due to capillary action. This eliminates the need for a vapour barrier at the bottom of the walls. Low evergreen shrubs or grasses planted around the building can further protect the walls from splashing.

Rammed earth is a techniques that has been used for - and lasted - hundreds, and in some cases, thousands of years. For example: sections of the Great Wall of China were constructed with rammed earth over 2000 years ago.

... return to top

Rammed Earth...
In many parts of the world rammed earth (RE) walls are made without stabilisation and perform very effectively. However, in a wet and cold environment the addition of lime (or cement) can provide a degree of strength and durability.

RE stabilised with cement is being used extensively in Australia and other countries to build the walls of houses. Lime can also be used effectively to stabilise RE. However, my research leads me to believe that I do not need to stabilise my RE walls at all. Unstabilised RE is being used successfully in England and other parts of Europe. Although it will not have the same maximum strength and does not set as quickly, it will become sufficiently strong. This has been determined by my engineer.

RE has excellent 'thermal mass' (the ability to store and slowly release heat) but does not have a high r-value (insulation). Consequently, we may experiment with ways to use natural materials such as Pumice and Perlite to increase its insulative properties.

Furthermore, based on extensive research done in Germany, samples will be tested for durability with additive such as Linseed oil and cow manure which have both been shown to have a pronounced benefit.

RE walls are built by erecting panels - or 'forms' - to create the shape of the wall. Between these forms a layer - or 'lift' - of soil is poured. Each lifts is usual about 6 - 8 inches thick. The soil is then compressed - or 'rammed' - down by the use of either pneumatic or manual tampers until it is reduced to about half its original thickness.

Once a layer has been compressed, the next lift is poured and rammed until the wall reaches the desired height. Depending on the final height of the walls, the forms may need to be raised periodically as the wall grows.

If the soil is prepared properly, the forms can be removed immediately the wall has reached it's desired height and, although still damp, it will already have significant load-bearing strength.

Although metal rebar has recently been shown to work quite effectively in RE I hope to avoid using it for various reasons: the production of steel is a major contributor to GHG emissions; and, as has been shown in conventional concrete construction, rebar is subject to rusting which can eventually cause serious damage to the structure.

To increase tensile strength we will be using Bamboo which has been shown to be very effective in tests conducted by the US Military and of which we have a suitable local supply.

An added benefit of using RE is that, in many cases, the soil from the site can be used to build with. This not only eliminates a significant cost of materials, but also the carbon footprint of transportation!

... return to top

Soil testing...
An important part of using these materials and techniques will be to conduct a series of different tests. Some or all of these tests can be done yourself by a process of experimentation and 'trial and error', or they can be performed by a geotechnical lab with more accuracy.

The early Chinese and Spanish builders did not have technical labs available to them and still built incredible structures. So, although doing it yourself may take a lot longer, it IS possible.

The first test (or series of tests) is establish the nature of the soil available and whether it is suitable for RE construction. I will not go into detail here, but these include a Sieve Analysis to determine the particle sizes, a Sedimentation Jar test to determine the ratio of materials - particularly clay and other 'fine' particles, and various tests to establish the suitability of the clay within the soil. Since the clay is actually the 'binder' within the mix, it plays a crucial role.

The second is the 'Proctor Curve' test which establishes the optimum moisture content of the soil for maximum compaction. There is a very fine line between too little and too much water and this is one of the most critical elements of RE building.

The third test is to make samples of RE for compression testing. This will establish the strength of the RE to ensure that it will bear the weight of the walls and roof.

The minimum strength required for my RE stem wall has been determined by my structural engineer to be 1 MegaPascal (150 psi) but we are striving for at least 2 MPa or higher.