Saturday, March 7, 2009

Concrete Bunkers in Bushfire Prone Areas

Property design, planning and preparation are keys to saving life and property in a wildfire. However it is almost impossible to be 100% ready- there is always the chance of unexpected factors arising. These could include the fire being worst than expected, trees falling across exit routes or on top of houses, cars not starting, having a bad fall etc. In case a worst case scenario arises, having a backup plan is good insurance and provides a greater sense of security.

Our backup plan (which we fortunately didn’t need to resort to) was to shelter in our concrete bunker. It is an adapted concrete water tank with strengthened walls and roof so it can be earth covered. A metal door in the side allows access and there is provision for a vent in the roof. We installed this in 2000 and have been progressively getting it ready. Two years ago we backfilled earth around it, but have not yet covered the top with soil.
The bunker is located just metres from our back door allowing quick access in an emergency. It is next to the double brick masonry wall of the house which should shield it from fire, if the house caught alight. There is a tap located on the corner of the house to provide a water supply. The plan is to use Besser type bricks to build a retaining wall on each side of the bunker entrance and then completely cover it with soil/gravel.

There is some concern about fires consuming all available oxygen. Although we have to do further research on this aspect, we expect that the oxygen available in the structure should be adequate for the length of time we would be in there. Experts generally say that it is best to focus on fighting the fire and putting out embers until it gets too dangerous. We would only need to stay in the shelter until the worst of the front has passed, and then it would be back outside to put out what we can.

Other considerations are:-
· If it gets too hot the metal door could expand, jam or buckle. There will be no flammable material near the bunker door so this should not be an issue.
· Sprinklers could be used to continually soak the area with water.
· We will have a supply of water for drinking and fire fighting equipment inside the bunker including damp towels etc.
· Filters could be installed to prevent entry of smoke.

This view from the north shows our white 7,500 gal. water tank with the fire shelter behind. This arrangement was part of our permaculture plan. Rather than just having a water tank and a fire refuge we have achieved many extra functions from these two pieces of infrastructure.
Apart from use as a fire shelter, the bunker has an everyday use as a cellar for storage of pumpkins, spuds, fruit, wine, ginger beer etc. It has been sealed with a coating of water proof material and backfilled with gravel on all sides to ensure water drains away without causing dampness. Once it is finished it should maintain a reasonably constant temperature in winter and summer.
The backfill against the water tank helps keep the water cool and means the tank is less susceptible to damage in a fire.
The fact that this wall is largely earth sheltered sets the house down low, so embers and heat largely pass overhead.
The setup provides easy access to the roof without ladders. (Of value whenever roof access is required but particularly in emergencies.)
The earth sheltered house wall adds extra thermal mass to the house, helping to keep temperatures in the house comfortable all year.

Hopefully a concrete bunker would never be required for emergency use. However it is a good investment because, apart from providing some piece of mind, it has potential to fulfil many other functions.

1 comment:

Ramsey said...

Hi Andrew,
I have been catching up on your blog and I really like what you have written.
In particular the article on how the surrounding burnt bush affected you and how your Dearest helped. I bet that smell that lingers for months from burnt bush still brings back fresh memories.
I see from recent post pics that the regeneration is now well underway. Have the birds returned yet?
Your place and your blog are wonderful works.

I would like to learn a lot more on how you set up your fire shelter.
Can you do another post detailing the size of the tank, how you reinforce the walls and roof, how thick they are now, any precautions you took when cutting the doorway, do you have additional pipes for ventilation etc etc?
I would really benefit from as much detail as you can give to save me from having to reinvent the wheel and make the same mistakes.
Best Regards

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