I thought I should post an update on my fire bunker. I received some useful responses to my first posting “Help with design for fire bunker” in August 2009 when I was looking for ideas, then I posted an update “Bunker progress, and a few lessons” in October. I had aimed to finish it by the end of October (before the fire season), but it wasn’t functional until mid-November. Now, with the addition of some soil on top, it is pretty well finished.

At the last update we had poured the slab. Purchasing the besser blocks was fairly expensive ($1087), so before ordering them my dear wife made a colour-coded model (not to scale) of the design out of Lego. This was a blessing because we used six different types of blocks (channel, hollow, half height, half hollow, half width, & lintel), and it meant we were only out by a couple of blocks at the end (due to some design changes):
Attachment 78330

The slab had starter bars to link with the vertical reo (N16) in the walls at 400 mm centres. (Bear in mind that the wall against the dirt is basically a retaining wall 2m high, so it needs to be strong). This picture shows the first row in place:
Attachment 78331

Notice that I cut out access holes in the first row, as I couldn’t get the “20.45 Cleanout Blocks” from Boral (unless I ordered a thousand). They came in very handy when I dropped a trowel down one of the blocks later on.

I put in a gravel drain with 50mm agi pipe along the outside, as well as a sheet of 200 micron black plastic glued to the top of the blocks with Liquid Nails:
Attachment 78332

In practice I only tied in every sixth vertical bar to the starter bars, mainly so these could be tied to the curved horizontal (N12) reo. Lifting the besser blocks over your head over 2m-high vertical reo bars is not much fun (with each block weighing 16kg), so most of the vertical reo was threaded down through the wall after it was laid.
Attachment 78333

I used 20.42 “Channel Blocks” for most of the walls, with every second row inverted to provide a big gap around the horizontal reo for the grout to get a hold. This follows the guidelines in a terrific document at How to Build a Reinforced Besser Block Retaining

Lesson 1: Never ever build a curved wall from besser blocks. The reo has to be bent to a curve, the reinforcing mesh has to be cut to a peculiar shape, and it’s hard to run a curved string-line! Worst of all, the gap between the blocks is about 35mm on the outside bend, making it very hard to fill with mortar.

Channels had to be cut in some of the end blocks (using a diamond blade on my angle grinder), as Boral doesn’t make corner blocks.

I’m a complete amateur and only laid 40 blocks on my best day. The mortar mix was 9:2:1, or 14 litres of fine washed sand, 3.1 litres of GB cement and 1.56 litres lime (measured out with a bucket and a jam tin). I tried mixing it in a barrow but found I got a better blend in the cement mixer despite the extra hassle of cleaning up. One load would allow me to lay 15.5 to 22 blocks in about 3.5 hours (after cutting and carting the blocks). This is pretty slow, partly because I couldn’t get my wheelbarrow through the narrow doorway.

Gate hinges were pre-bolted into the blocks for the door and window:
Attachment 78334

, then these were mortared into place (the next picture also shows some old copper piping I set into the blocks on the right to allow wires to pass through later):
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To do the roof, corrugated iron sheets had to be placed on first, and propped with solid timbers (to allow us to walk on the iron, and to support it while the concrete set). Then came the N16 “stirrups” (which started in one wall, spanned the roof, then went down into the other wall), followed by F72 mesh:
Attachment 78336

The cleanout blocks were sealed up by mortaring back into place the offcuts I had cut out with the diamond blade (all pre-numbered). Then the grout was poured into the walls using a patented hopper that my wife made out of an old computer case:
Attachment 78337

That was invaluable as all the carrying and ramming of the grout was done by hand (hard work!). I mixed, carted and shovelled most of the grout while my wife rammed the grout up and down with a long pole to get the air pockets out. The grout mixture used was the same as the slab, about 3:2:1. One load of the cement mixer took 52.5 litres of gravel (“10/14 mm screenings”), 35 litres of washed concrete sand and one 20kg bag of GB cement. This was enough to fill between 8.5 and 9.5 blocks (the 20.42 blocks take more than the 20.01 blocks), taking about 1 hour.

Lesson 2: The best way we found to mix the grout in the cement mixer was (using 7-litre buckets):

- add most of the water (10.5 litres in this weather)
- add 3 buckets gravel
- add 2 buckets sand
- add whole bag cement
- add 3 more buckets gravel
- add 2 more buckets sand
- add 1.5 buckets gravel
- add 1 bucket sand
- top up with water to taste

If we varied from that routine we would find pockets of unmixed material at the back of the bowl that had to be scraped out by hand.

Only after the walls were grouted could we lay half-width blocks around the rim of the roof:
Attachment 78338

The roof slab was then poured:
Attachment 78339

(note the second sheet of black polythene starting 2 blocks down, and overlapping the first sheet).

Another row of half-width blocks around the rim of the roof, to hold the soil, then the roof and exposed sides were painted with several layers of Ormonoid waterproofing. Then came more polythene sheeting and aluminium flashing (to protect the poly from the sun). A sprinkler system was laid on the roof, to keep the plants alive in summer:
Attachment 78340

Some topsoil reserved from the initial excavation was placed over the top and it was planted with Myoporum parvifolium (Creeping Boobialla), which is a tough groundcover, self-rooting and fire resistant:
Attachment 78341

The door and window were made with frames welded from 25mm RHS, then with 7.5 mm Harditex sheeting screwed to them. These were then filled with a 100 mm thickness of fibreglass insulation:
Attachment 78342

A seal made from 19mm fibreglass rope was then glued to the steel frame with Stove Sealant:
Attachment 78343

(note the small safe for storing computer backups, and the solar shed light. The hole in the roof is for the periscope!)

The door has a “hob” to step over and a lintel to duck under because I wanted to make the door as small as practical (to minimise the effect of radiant heat). The homemade door latch can be opened from both sides so kids can’t lock themselves in. There is a thermocouple probe passing through the wall to measure outside air temperature (up to 700C) and a small peephole. I wanted an inward-opening door so it can’t be blocked, and to keep the heat away from the hinges and most of the steel frame.
Attachment 78344

Lesson 3: While the window was fairly easy to fit, I had not made the doorway quite square enough. It took several attempts re-welding the doorframe to make it fit my skew-if blocks around the frame. I should have made a steel doorframe first, laid the blocks to fit that, then attached the door.

All up the project cost just over A$3,000, and took about 3 months of hard part-time work. That’s cheaper than a commercial unit ($4.8k for a septic tank with a door), but I like having two exits. It would have been a lot easier if I’d been able to get machine-access into the site. The weird thing is that I hope I’ll never have to use it. It’s probably over-engineered to hell, but in the absence of any meaningful design standards, I prefer it that way.

I’m not suggesting that this is the best fire bunker design (or even a viable one), but I hope this posting may help others with similar projects. Just don’t use curved walls!

Cheers, GOM

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