Mixing Cob in Bulk - What Methods Have you Seen?

Hello,

I have a big project coming-up and I need to make a lot of cob, easily. What methods have we seen to make cob in bulk?

I know the ‘primitive’ / ‘tarp-method’ / ‘manpower only’ method. That works, and I’m glad to know it. It’s especially handy for community-building.

I have to make an adobe floor: 1,100-sq-ft x 6-inches deep. I’ll need an army of people doing the cob-dance to get that done before Winter sets-in.

The best method I’ve seen for mixing cob in bulk (so far) is using a Bobcat:


The second-best method I found is with a backhoe and a small skip:


I’ve used a Mason Mixer in the past too, and that worked fine for medium-sized batches. It’ll still be a lot of hard lifting work to make my floor.

What other methods have fellow cob-makers tried / discovered?

Thanks all!



-Scott

Friend used a LARGE mortar mixer, a couple years at a time.  

Something like this...

There seems to be more than one thread on this today, BTW...

https://permies.com/t/22593/Mechanical-Mixer-Cob

Thanks J.D.

I had seen the 'mechanical mixing' thread and had posted my experience with a Mason mixed (similar to your mortar mixer).

The reason I asked my version of the question had to do with scale. A mortar or mason mixer is certainly an upgrade from the tarp-method. It still requires lifting buckets, wheel-barrows, etc. (human power). The methods in the videos I link are even larger-scale mixing, and don't require any human-power, suitable for "an entire 1,100sqft floor in a day".

I'll run some math and see if a mortar or mason mixer is big enough for my needs.

Thanks.

I found another video showing an interesting process:
https://www.youtube.com/watch?v=X7ApWRznGKM

I asked questions to the channel owner, but haven't seen answers yet:
1) Does the tilling chop-up the straw into shorter lengths?

Maybe that's ok, but its good to know.

2) The instructions list mixing 'soil', but I know cob needs to be exclusively sand, clay, and straw (soil typically also includes organic compounds which will decay and shrink over time, leading to crumbling cob). I asked the channel owner if he used 'soil' to mean clay.


-Scott

For workability, you need the straw chopped into shorter lengths. Several inches long is adequate; as long as it is well mixed, you will get good cob. For a floor mixed in place, workability is more or less irrelevant, though.

I replied to your organics question in the video comments. Reposted here for ease of use:
You want minimal organics, but you can have a lot of silt, sand and gravel as long as you have the desired proportion of clay. My clay deposits are something like 20% clay, with the rest silt, sand, gravel and rock in very roughly equal proportions, and it makes good solid cob. You do need to get below the topsoil before you start digging your cob material.

I used a small mortar mixer to build a large cob house this year. I could produce and use, with my wife's help, 40 batches of cob in a day. I think the batches were roughly 2 cubic feet so a mortar mixer will only get you about 240 square feet of floor at 4" thick in one day.