Bunker -wood post thickness!!

[champo]

Hello, so I want to build a bunker.

Well not really a bunker more of a trench.

My question is this , after I've dug out the actual trench I'm going to lay logs across the top which would sit under a tarp.
I want it to be flat on the surface (not a mound)
So with 3 feet of earth on top of the tarp that is on the logs above the trench.

How thick do the logs have to be to prevent collapse?
Logs will be made of pine and fresh cut and untreated.
They will also be laid next to each other horizontally to cover the trench.

Thinking of makings the trench more of a square if possible so more accommodating.

Possibly 7ft by 7ft

Any help or advice would be great appreciated.

P.s can we not get lost on the design or any other detail, it's the thickness of the logs I'm concerned about.

 

[rainingcatzanddogs]

1) Water is not a friend to wood so I would highly recommend doing something to protect that wood from moisture rot.

2) Three feet of earth is not that much especially if you are laying whole logs one nestled up to the other.

The logs being used here are about a 4" diameter or about 7-8" across the entire end they taper very little. If you will note, there is more log on the soil than spanning the gap...so NOT getting into the building details, it is pretty well impossible to give you a safe answer.


Pine, fresh cut, still needs to be protected from rot and I see termites are also an issue in Southhampton England. If you are concerned about collapse, defiantly would treat those beams or encase them in concrete!

Termites can turn a damp 8" log into cork pulp with 0 structural integrity in about 6 months....

How are you going to do the walls? Dirt walls? What kind of dirt? Cement block? That will also determine how large of a diameter log you will need.

 

[Proud Prepper]

4+ inches in diameter. Remember to put a 90 degree turn in the entrance.

 

[champo]

Thanks for the reply.

The logs will be under a tarp so protected from rain and moisture in the soil.
The ends of the logs (where cut)will also be treated with a preservative
Only airborne moisture to really worry about.

We don't have termites here.
Will be using dirt walls.
Using concrete is not an option here.

I understand wood will not last as long as concrete but as long as it lasts roughly 10 years I have no issue replacing the logs then.


Cheers

 

[DrHenley]

First you might want to put a smaller pine log on the ground and cover it with a tarp and see how long it takes before it rots.

 

[GeorgiaPeachie]

This is a popular technique to cheaply and quickly protect wood from water.

 

[champo]

First you might want to put a smaller pine log on the ground and cover it with a tarp and see how long it takes before it rots.

Good idea but if it lasts 5 years then I'll have wasted 5 years where I could have had a shelter for that time
Also it wouldn't be a fair test

 

[DrHenley]

Results will vary depending you your geology and climate. Here in our area, the pine log will be mush in less than a year because the tarp will trap moisture and heat coming from the ground, and create a perfect environment for fungal growth.
Been there, done that...ruined a bunch of oak firewood that way.
But at least I was able to get some beautiful spalted oak for knife handles, LOL
Red oak should look something like this:

But after sitting under a tarp, it was "spalted oak." The black lines are boundaries between competing fungus colonies in the wood, which is what makes spalted wood. The wood had to be stabilized because it was technically ROTTEN.

 

[champo]

Wow they all look absolutely amazing.
I love the marbling effect of the spalted oak.

So ends of the logs which would be completely covered by the tarp would be treated with wood preservative.
The rest would be under the tarp on the surface but underneath would be in the open air.
I could treat all the logs with preservative for the whole length

 

[champo]

The logs being used here are about a 4" diameter or about 7-8" across the entire end"
 This doesn't make sense, if the diameter is 4" the how can it be 7-8" across the end?

 

[Proud Prepper]

The logs being used here are about a 4" diameter or about 7-8" across the entire end"
 This doesn't make sense, if the diameter is 4" the how can it be 7-8" across the end?

They mixed up radius with diameter. .

 

[rainingcatzanddogs]

They mixed up radius with diameter. .

Yes.

 

[user 7368]

Hello, so I want to build a bunker.

Well not really a bunker more of a trench.

My question is this , after I've dug out the actual trench I'm going to lay logs across the top which would sit under a tarp.
I want it to be flat on the surface (not a mound)
So with 3 feet of earth on top of the tarp that is on the logs above the trench.

How thick do the logs have to be to prevent collapse?
Logs will be made of pine and fresh cut and untreated.
They will also be laid next to each other horizontally to cover the trench.

Thinking of makings the trench more of a square if possible so more accommodating.

Possibly 7ft by 7ft

Any help or advice would be great appreciated.

P.s can we not get lost on the design or any other detail, it's the thickness of the logs I'm concerned about.

Make a big hole, throw in an old shipping container and fill the rest back up, you can always throw some logs over the container before you put soil over it that will withstand normal Russian bombs.

 

[champo]

That won't work. Shipping containers are not meant to be buried without modification.
Also it's going to cost and is in no way practical for what I need and the location.
Cheers tho

 

[Mountain trapper]

Untreated pine will start rotting in a year or 2. If you can find it use Doug fir, larch or cedar. Better yet buy rail road ties. They'll hold up.to the weight a lot better too.

 

[MNwr786]

I agree with pine rotting fast too, it doesn't take long. It would need to be treated. Do you have any cedar or tamarack?

 

[MNwr786]

This is a popular technique to cheaply and quickly protect wood from water.

I can definitely confirm that works. I used to do stump grinding and often times people would try burning their stumps just to find the parts that didn't burn become permanent lawn decorations that are 5x harder to grind out than the unburnt stump would have been. Another trick is to dry distill deciduous wood to produce pyroligneous acid and brush that on like stain. Although simple, it is a messy and somewhat dangerous process.

 

[EastenerWesterner]

I would think about RR ties. My wife used them to cover a drainage ditch across the property when she was developing it. Tin, galvanized sheet metal on top DG on top of that.
I think we had one failure in 25 years on that 100 ft ditch.

 

[Mad Dog 1]

Has anyone thought to use old cold room panels they are normally 1.2 mt wide and about 2.4 mt long I used them back in 1991 to cover a French drain 15 mt long ( covered over with soil ,some have metal channels in side if they came from a butchery ) that I dug on the smallholding for the septic tank to drain through , I sold the property in 2003 , to my knowledge it is still in place

 

[champo]

I agree with pine rotting fast too, it doesn't take long. It would need to be treated. Do you have any cedar or tamarack?

No I don't think so.
We have a mixture of woodland but I'll have to check for sure what I can access at the location.
Will treat the whole length of the logs

 

[user 6493]

use cedar if you have access to it,, it is what I would use but I sit in the middle of a large cedar glade

 

[champo]

Ha!
I sit in the middle of a pine wood!
Will have a look for cedar later

 

[Mountain trapper]

No I don't think so.
We have a mixture of woodland but I'll have to check for sure what I can access at the location.
Will treat the whole length of the logs

Unless you can pressure treat the logs just brushing something on the outside doesn't do much good. We used to be able to get creosote and fill a 55 gallon drum with 50/50 diesel and creosote and soak our fence posts for about a year. They lasted a long time. Pressure treated timbers are estimated to last 20-30 years, but I've seen some start rotting in less than 8 years.
If you have a way to do it, you can soak your logs in a mixture of used motor oil and diesel. Around here I can buy used #1 and #2 railroad ties for about $25 each. I use them for corner posts. They will for a generation or two.

 

[Mountain trapper]

Do you have any yew trees in your area? They would last a lifetime, even untreated.

 

[DrHenley]

Do you have any yew trees in your area? They would last a lifetime, even untreated.

Maybe even longer, much longer, much much much longer. The oldest known wooden artifact (400,000 years old) is a spear tip made of yew.

 

[user 7482]

What about larch? Those logs don´t get affected by water for a long time.

Dont put your "roof"logs directly on earth and coat them with waste oil or tar. Try to use dried logs. Fresh ones will split and there´s a possibility for puncturing your tarp.

 

[champo]

Do you have any yew trees in your area? They would last a lifetime, even untreated.

We do have yes, but all the big ones were used to make longbows back in the day and after the longbow was superseded by the musket the yew wasn't planted anymore.
Only a few scatterings around now and if I'm honest I'd find it difficult to cut one down, they are kind of sacred to us.

 

[champo]

What about larch? Those logs don´t get affected by water for a long time.

Dont put your "roof"logs directly on earth and coat them with waste oil or tar. Try to use dried logs. Fresh ones will split and there´s a possibility for puncturing your tarp.

Larch is a possibility!
I'll season for a year in that case.
Cheers man

 

[champo]

Should probably add this to be a bit clearer.
I own a woodland but I don't have access to the woodland by vehicle.
The woodland has no access,no road or track.
I have a right of passage so can walk in to my woodland no problem.
The walk from the nearest carpark (parking lot) takes 45 minutes.
So taking heavy supplies in can be difficult.
On top of this my woodland is in an area with an SSI (Special scientific interest).
Have to be really careful what I do.
As my woodland is overlooked by 5 other woodlands all adjoining, this is my problem.

Technically I wouldn't be allowed to build a bunker without planning permission and surveys etc etc.

Long story short I have to build a bunker incognito.

Which is kinda why I want a bunker,it's under the ground and can't bee seen see by nosey neighbors.

Cheers