Building rammed earth walls to improve my underground bunker
Why rammed earth?
Traditionally, rammed earth houses were constructed from earth dug from the site of the building.
They were built without cement added, and were load bearing. They cost next to nothing to build, yet many are still standing centuries later.
All those qualities appeal to my husband and I, which is why we are building our very first rammed earth walls the traditional way.
As I explained in my original article about building my underground fire bunker, this is supposed to be an inexpensive and temporary solution to the threat of catastrophic fire warnings.
Our first rammed earth wall.
Experimenting with rammed earth
We have never built with rammed earth before. We do have a cinva ram press, but I'm concerned about all the potentially weak spots if we compress the cinva ram blocks at a different pressure to the clay-based mortar we would use to join them, which would hardly be compressed at all.
There's a lot of careful thought going into the design of our rammed earth walls because they will, after all, be expected to be safe and perform effectively in the event of a fire.
I have researched rammed earth construction as much as I can, but most of my questions are not addressed in existing literature. So, we have no choice but to experiment.
Here are some of the unique problems we are seeking to address, and the approach we are taking. If anyone has any helpful suggestions or personal experience we could gain from, now is the time to leave your comments because we are continuing to build in every spare moment we can find. Please don't hesitate to contribute your ideas!
Solution to air flow problems
Effective ventilation that can be closed during fires
Ventilation is very important. We have to be confident we won't be killing ourselves in an effort to stay alive. I have read some disturbing information about the lack of ventilation in some commercially manufactured underground bunkers.
I addressed ventilation issues in the original design of my fire bunker, but all that is about to change when we complete the rammed earth walls around my fire bunker entry.
There will no longer be direct access to the outside world. That's a good thing in the event of a wild bushfire. I am happy to have protection from radiant heat. The wooden door into the underground fire bunker is a small target, and was recessed into earth on two sides but there was still a danger that radiant heat from directly opposite the door could cause us some grief.
I dug a channel into the clay to hold water near the door in the hope that any flying embers would be caught there, and to help lead the way to the fire bunker entry in the event of thick smoke.
Even after the water evaporated, I hoped the channel might catch flying embers and burning debris. But it is an acknowledged fact that fire storms create their own winds, so there was always going to be an element of danger.
A rammed earth wall offers so much more protection from heat and smoke during a bushfire, but without fresh air entering the enclosed area during peaceful periods, the air could go stale.
These photos show our solution to providing adequate ventilation. We have constructed the first ventilation hole and consider it a success. You will see how we managed to insert metal fly-screen mesh part way along the hole, leaving enough room for padding and sealing against smoke if needed.
Emission-free home construction
Good for research
Questions about strengthening the walls
This is a problem for us when building our rammed earth walls. There are lots of ideas about ways to reinforce the strength of rammed earth walls in conventional rammed earth house building. But our situation is different.
When preparing for an emergency like a catastrophic bushfire, it makes sense to prepare for the worst case scenario. Best case obviously is there won't be a fire. One of the worst possible outcomes is that the door to the rammed earth section surrounding the underground firebunker, becomes blocked or compromised.
The main entry door will be facing our home. If the house burns and collapses, it could in theory fall and block the doorway. Not likely, but possible. A hot metal house frame, roof and burning debris could prove problematic.
So we need to be able to dig our way out, if necessary. We keep a shovel inside the fire bunker, so we have the tool. We are not using concrete in our rammed earth walls, so we have the ability.
But if we run barbed wire lengths between rammed earth levels, for instance, as at least one group of rammed earth house builders suggests for extra strength, a quick and safe exit through a rammed earth wall becomes less likely.
Their reasoning behind incorporating lengths of barbed wire is that if extreme pressure is exerted on one part of a rammed earth wall by a car that fails to stop and crashes into your structure, for instance, then the integrity of the entire wall should help prevent collapse.
I can see validity in their proposition, but I am not anticipating high speed traffic nearby. If families down the road retreat to our home in a fire or the local fire brigade should miraculously decide to visit our home in a fire above all the others without a fire safety plan in place, they know to keep their vehicles pointed straight ahead and not make a 90 degree turn into our fire bunker.
We have barbed wire and considered using it, but decided instead to opt for rammed earth walls made of rammed earth, nothing more.
At one end of each section, we are creating an indent for the next section to set into, a bit like attaching a lego block. Much of what I've read during my research says we don't need it, but I see no harm in the extra effort. It cost us nothing to add an extra piece of wood to the frame design.
To waterproof the rammed earth ... or not seal the walls at all?
We need our rammed earth walls to be waterproof during rainy periods, but we don't want to spend money on formal waterproofing. We are hoping that the clay and earth mix we are using from our own land can be compacted enough to discourage rain from washing it away without using a waterproofing compound.
Obviously a mechanical ram would be the first choice for most people building a rammed earth house, but we are just building a few rammed earth walls and decided to stick with the traditional method of ramming. We live in the back of beyond and would spend hours in the car to travel to a hire service, so the inconvenience and expense of obtaining modern technology are key factors.
Here's the process we have adopted in an attempt to compress the earth enough to avoid the need to waterproof.
First, we put our earth mix into our home-made frames. Then my husband walks on it. His weight and pressure makes an obvious indent in the earth.
Next we smack it with the flat end of a log splitter. It makes a further improvement, bit by bit.
After that, we slam the earth with the end of a plank. It is relatively heavy, and with some extra muscle it compresses nicely in small, compact sections.
The easiest tool to manipulate is the brick. We slam the brick along the edges in particular, so the sides will not be loose when we remove the frame.
By the time we've finished stomping and whacking, what began as about 100mm of earth becomes 50mm of rammed earth wall. Then we add a further 100mm of earth mix and do it all again.
Integrating a living roof into the design
The biggest challenge for us will be adding the roof. It has to be snug enough to exclude all smoke and burning embers during a fire storm, and strong enough to hold the soil and plants I will put on top.
We are still considering our options but at this point I suspect we will create stronger uprights close to the actual bunker, then run beams across that are somehow buried into the rammed earth walls. We might have to make extensions in width to the rammed earth walls at the points where the beams meet the wall.
Because metal is easily distorted during extreme heat, I am determined to figure out a way to integrate a living roof that exposes no part of the structure to potential fire.
I haven't quite figured out how, but I'm working on it!
The story behind our rammed earth walls ...
- How to build a Fire Bunker
I built a fire bunker by hand to protect my family in an emergency because we live off the grid in a high fire danger area. In this article I describe how I built my bunker without spending money, plus options I chose including ventilation.
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Living off the grid can be less stressful and more profitable than the lifestyle you are leaving behind. Here's my top tips to get you moving towards off-grid living.
- Successfully living off the grid
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