Foundation for cob chicken coop

Hey guys, I'm currently building a cob chicken coop (circular shape, 4m in diameter) and had some questions.

I have already dug a 45cm deep and wide trench. I have decided I will use rubble for the foundation, since I don't have access to stones. My question is, can I just stack the rubble and compact it really well?

Otherwise I would use a lime and sand mortar, but I'm struggling to understand the point of it (I'm really new to construction).

Is there a YouTube video of a person actually doing a foundation like this? I'm struggling to find resources.

Also, do I need to dig out a separate drainage line too? How do I go about this?

If it helps, I live in a very hot climate where it doesn't rain much.

I believe the right word is urbanite, not rubble. Basically left over from construction sites. English is not my main language

Howdy, Axel, welcome to Permies!

This Cob House has some interesting articles on the topic of foundations:
https://www.thiscobhouse.com/cob-building-systems-foundations-and-walls/
https://www.thiscobhouse.com/building-a-stone-foundation/

They also have a video you might find helpful!



They mention using urbanite specifically at 3:37 in the video as well as in their article.
From that, and a few other videos on their channel, it looks like some compacted gravel/rubble in the bottom of your trench, followed by a drain pipe, then more rubble, would be a good base layer.
Stacking up some larger pieces of urbanite above the original ground level, then building your cob walls from there, might help the longevity of your structure.

Their channel seems to have a lot of information already, but if that's not enough to go on, they have an online course right here at Permies:
https://permies.com/wiki/226599/Cob-Building-Foundations-Window-Doors

Please keep us updated and post some pictures, if you can!

I know a solution that will meet your needs. You can create the building material you require for your chicken coop using soil. Last summer, we constructed a similar structure in a workshop. By using the same technique, you can build your own coop as well. The first step is to test the soil for its content of clay, silt, and sand. To form compressed blocks for construction, the soil must contain the following proportions:

Clay (15–25%): Increases the durability and water resistance of the building material.
Silt (15–30%): Provides flexibility and workability to the material.
Sand (40–60%): Enhances durability and improves the compressive performance of the material.
You can test these proportions in your local soil using the bottle test method.

Water Bottle Sedimentation Test Steps:
Soil Sample Preparation: First, take a sample of soil, approximately 100–200 grams. Be careful to ensure the sample is homogeneous during collection.

Soil Cleaning (Optional): If the soil contains large stones, organic matter, or coarse particles, remove these to obtain a clean sample. This will help ensure more accurate results.

Soaking the Soil in Water: Place the soil sample into a clear bottle (a transparent glass bottle is ideal for better observation). Fill the bottle about two-thirds with the soil sample. Then, fill the remaining space with water, up to about 80-90% of the bottle’s capacity.

Adding a small amount of detergent or soap can help disperse the soil more evenly, preventing particles from clumping together and allowing for clearer separation of the components.

Mixing: Shake the bottle well to ensure the soil components (clay, silt, sand) are evenly dispersed in the water. The mixture should be thoroughly agitated to ensure that all the soil particles are suspended.

Settling the Mixture: After mixing, place the bottle upright and let it settle for a period of time (usually 24-48 hours). The soil particles will settle at different rates, with the heaviest particles (sand) settling first, and the lightest particles (clay) settling last.

Examining the Layers: After the settling period, you can observe three main layers in the bottle:

Sand Layer: The coarsest material, which settles at the bottom of the bottle, typically within 5–10 minutes.
Silt Layer: The fine-grained material that settles above the sand, usually within 30 minutes to 2 hours.
Clay Layer: The finest particles, which are the slowest to settle. Clay particles may remain suspended in the water and will form a layer at the top after 24–48 hours.
Calculating the Proportions: Measure the height of each layer relative to the total height of the soil column in the bottle. The proportions of sand, silt, and clay can be determined by dividing the height of each layer by the total height and multiplying by 100.

Example Calculation: Let’s say the total height of the sediment in the bottle is 20 cm.

Sand layer: 8 cm
Silt layer: 6 cm
Clay layer: 6 cm
Proportions:

Sand:
8
20
×
100
=
40
%
20
8
​
×100=40%
Silt:
6
20
×
100
=
30
%
20
6
​
×100=30%
Clay:
6
20
×
100
=
30
%
20
6
​
×100=30%
Interpreting the Results:
These proportions help you understand the composition of the soil sample. The ratios are typically used for soil classification:

Sandy soil: Contains more than 60% sand, with low silt and clay content.
Clay soil: Contains more than 25% clay.
Silty soil: Contains more than 25% silt.
This technique provides a simple and practical method for analyzing soil. However, for more precise and detailed analysis, laboratory tests and equipment such as a hydrometer may be used.

Once you have identified the suitable soil, you can prepare the mixture. The composition of the mixture will be as follows:

1. Water Content:
Water is essential for shaping the soil mixture and providing the necessary moisture for compaction. The amount of water added to the soil varies depending on the soil type and its moisture content. However, a typical water content range of 10–15% is generally recommended.

The moisture content of the soil mixture should be adjusted to a level suitable for the compaction process. If the water content is too high, the soil will soften, and the compaction will weaken. On the other hand, insufficient water can result in a mixture that is not homogeneous enough.

The water used during the molding and compaction process should be adequate to ensure that the material achieves sufficient strength.

2. Cement Content:
Cement adds binding properties to the soil mixture and enhances the material’s durability. However, due to the environmental impact of cement production, its quantity should be kept to a minimum.

Typically, a cement content of 5-10% is recommended. This proportion improves the material’s strength while still maintaining an environmentally friendly approach.

Increasing the cement content will increase the rigidity and durability of the mixture, but this also makes the material more expensive and less sustainable from an environmental standpoint.

3. Lime Content:
Lime introduces chemical binding properties to the soil mixture and enhances the compaction of the soil components. Additionally, it can improve the material's strength by breaking down organic matter in the soil.

The lime content is typically maintained between 5-8%. This amount is sufficient to increase the strength of the soil without overuse, as excessive lime can lead to the loss of the material's elasticity.

Lime is particularly effective in clayey soils because it helps prevent the swelling and shrinking of the clay, thus enhancing the long-term stability of the material.

Example Mix Ratio:
A typical mix ratio for soil could be:

Soil: 85%
Water: 10% (of the total mixture)
Cement: 5%
Lime: 5%
This ratio generally increases the durability of the soil blocks while considering the material's sustainability. However, the proportions may vary based on the type of soil used and the specific purpose of the project. For example, adjustments may be necessary for very soft or very hard soils.

Application Steps:
Preparing the Soil Mixture: Once you have obtained your ideal soil mixture, adjust the moisture content by adding water as necessary.

Adding Binders: Then, add the cement and lime in the recommended proportions. Mix these materials thoroughly to ensure they are evenly distributed within the soil.

Compaction: Once the mixture is ready, pour it into molds and compact it thoroughly to achieve the desired strength.

Notes:
Environmental Impact: The environmental effects of cement should be carefully considered, and the amount of cement should be minimized wherever possible.
Soil Type: The type and characteristics of the soil you are using (such as the clay content) can influence the proportions of binders to be added. Clay-rich soils may require more lime, while sandy soils may require higher cement content.

Next, pour the mixture into molds and compress it in layers, ensuring that each layer does not exceed 3 cm in height. If the layers are too thick, the outer part of the soil will become compacted while the inner portion may remain loose.

Finally, you can use the blocks you’ve produced in the same way as regular bricks.

And you can watch this video

webpage

Welcome to Permies!

Axel Oehrli wrote:My question is, can I just stack the rubble and compact it really well?

Yes, although it's nice to have smaller stones at the top of the trench to make leveling the foundation easier.

Axel Oehrli wrote: Also, do I need to dig out a separate drainage line too? How do I go about this?

It depends on how the water flows around your site. With large roof overhangs and if you divert water away from the foundation you will probably be fine without it.

Here is a video of a rubble trench foundation I did a few years ago. I use large stones and pieces of urbanite we had on the property in addition to to some smaller stone I brought it.

If you do use rubble, is it small enough pieces to not allow passage through it to snakes, rats, or any other smaller predators you may have in your area?  That would be my main concern about this type of foundation.  (Unless you plan to have that part of it completely below grade and don't have burrowing predators.)