Structural stability of arches with cob

I have been designing a theoretical blueprint for a future project, and incorporated into the project will be a few hall ways, in an arch shape.

My question is, when laying the cob, would it be stronger to make the whole arched hallway 1 monolithic piece, or would it be stronger to have 3 pieces.

#1) The left hand vertical wall and the beginning of the slope upwards and inwards.
#2) The right hand vertical wall and the beginning of the slope upwards and inwards.
#3) A trapezoidal "key stone" running like a backbone down the length of the hallway ceiling.

Which way would give more structural support and strength?

either would be fine. Cob is plenty strong for an arch, if it's not to wide or shallow.

I don't know that cob is fine for a vault (arched ceiling/roof), actually.  It depends on how it is made, how much the span is, and how much shrinkage your particular mix will experience.

Best thing here is to do a small test, and see how it behaves.

Vaults are usually made with earth that has completely dried (compressed).  When doing adobe vaults, you should always touch brick to brick on the inside curve, not brick/mortar/brick.  The reason being that the mortar is wet and will shrink, and could collapse.

It all depends on how much your cob will shrink as it dries.  I don't think I have ever seen a cob vault, and there may be a reason why.

Also, do you plan on having a form or something holding the wet cob in place?  Wet cob is heavy!

Great advice. I will have more than enough support on the sides to hold the vault up, if I can infact build it at all.

I'll run some mini scaled tests. Perhaps if I went the route of doing the 3 sectioned piece, I could do the 2 walls and the upward and inward curve, then once dry, use that to form the 3rd key stone piece. If the key stone dries and shrinks a little it will just slide further down into the locking position and I would imagine there is nothing wrong with that.

Thanks for the advice!

andy, what is the width of the arch you want to build?

definitely do a small test first, maybe like a 5 ft wide vault.  The more I think about it, the less confident I am with it, so be very careful.

Your form will have to be very substantial to hold the wet cob, again be very careful.

Alternatively, you could make blocks, let them fully cure, then build the vault with the blocks.  Research Nubian Vaults.

castlerock wrote:
andy, what is the width of the arch you want to build?

Something along the lines of 3.5 ft - 4 ft wide, and only around 6.5ft tall at the crest of the archway. It is going to be resembling a "Mini Tunnel" feel for accessing a play room.

ok, those dimensions make me feel a lot better about this.  I think it will be possible.  You will need to make a robust form and really compress the cob on the form, try to get it as compressed as possible while forming.

Here is a plan I had, if you guys have feed back on the plan let's hear it.

The area where the hall way will be, I am going to dump a few/a lot of loads of dirt. I can then dig 2 parallel "trenches"down to the the depth of what would be the floor of the hall way. I can stack the bricks in this trench and these will become the walls. Once the vertical stack reaches the height I am looking for, I can add more dirt in the shape of the arch. Think of a giant sand castle almost. This dirt will then be the form, to lay the remaining bricks on. Once complete I can dig the dirt out of the hall way.

Hope that makes sense.

andyjorgen wrote:
Here is a plan I had, if you guys have feed back on the plan let's hear it.

The area where the hall way will be, I am going to dump a few/a lot of loads of dirt. I can then dig 2 parallel "trenches"down to the the depth of what would be the floor of the hall way. I can stack the bricks in this trench and these will become the walls. Once the vertical stack reaches the height I am looking for, I can add more dirt in the shape of the arch. Think of a giant sand castle almost. This dirt will then be the form, to lay the remaining bricks on. Once complete I can dig the dirt out of the hall way.

Hope that makes sense.

I heard of hay bail being used... when its done kind a cow to eat it out.... Anyway, I might think using bails (straw is cheaper.... and a cow might take a while to eat the hay) as they will form a straighter surface to build the walls against. You can add them as you go so you are not working in a hole. Then use sand for the curved part. The bails might be easier to remove if you add some twine handles before you stack on top.

Just a thought.... It is not something I have tried. the used straw may not be useful for much more than mulch after.

Len wrote:
I heard of hay bail being used... when its done kind a cow to eat it out.... Anyway, I might think using bails (straw is cheaper.... and a cow might take a while to eat the hay) as they will form a straighter surface to build the walls against. You can add them as you go so you are not working in a hole. Then use sand for the curved part. The bails might be easier to remove if you add some twine handles before you stack on top.

Just a thought.... It is not something I have tried. the used straw may not be useful for much more than mulch after.

Very good idea, beats packing dirt up, Bales would certainly be easier.