This engineering trick can prevent washouts and structures from falling. Why plants are important.

This description is worth a read...

We take the kids outside for a science experiment. This trick has many uses, from building foundations, to massively strengthening retaining walls, to simply building better sand castles with your kids on vacation and showing off to all the losers building sand castles like peasants, leading potentially to a career as a world champion sand castle legend.

...but mostly for retaining wall gardens.

It is 3AM and I am updating the description because my explanation of the science was sloppy and its bothering me enough to wake me up. Chalk that up to doing it on the fly, and having mild dyslexia. It is important to get this clear because the implications can reach beyond soil science and towards many topics such as building construction, sunken greenhouses, earthships, retaining walls, or creating foundations on slopes. These are all important for permaculture designers who sometimes function pretty close to what civil engineers do.

How gravitational force is resisted, is by normal force perpendicular to the surface of the soil, I.e. pointing upwards.

When the surface is flat, the forces (gravity vs the normal force resisting it) line up directly opposing eachother. When there is a hill, the normal force has an angular aspect to it. In the video I keep saying that the gravity force component changes, but that's only because when the camera turns on, I become a bumbling idiot. It's the normal force (the resistive force) that rotates as the hill slope increases. It is always perpendicular from the surface. This means that as the surface gets inclined, only part is vertical, but part is now horizontal, pushing the particle down the hill.

The only thing resisting that pushing component of the normal force is now the frictional force, which runs parallel to the surface of the hill.

As the slope on the hill increases, 2 things happen:

1) the pushing force increases (the horizontal component of the normal force that resists the body being pushed down by gravity)

And

2) the horizontal component of the friction force stopping the slide decreases as the hill gets more vertical.

This means, for every hill, there is an angle at which the magnitude of these forces pass eachother, and the pushing force dominates the holding force, and the particle slides away.

So, the trick to stopping this, is that we need to find a way to resist horizontal pushing forces. We can do that in 2 main ways, externally and internally. Most people do it externally via a retaining wall. However, doing it internally works much, MUCH better.

This is essentially what trees do with their roots and why they are so so important. However, when designing a retaining wall due to a cut into a hill for any reason (garden bed, sunken greenhouse, earthship, etc), or simply making a mud based cob wall that will support a vertical load), we can do a LOT better than just some hay inside the mud.

There are many applications of this that are useful to a permaculture designer, and watching this video will help you understand the science behind why, so that you can design stronger walls, buildings and hill structures.