There are hundreds of uses for cob, so this technique is something good to learn about even if you’re not going to build a cob house.
“I have mixed many tons of cob with a tiller. When I built a cob wall in my house I mixed and moved 8 tons in eight days with one or two assistants, but these were not consecutive days. Mixing with a tiller requires that you make cob that is a little wet, so I mixed and placed a ton then let the wall dry or set for a couple of days, Then I mixed and placed another ton and let it set. One time I had to let the wall set for seven days before I could continue building, but this is a small price to pay for being able to mix up 2000 pounds of cob in one hour with just two people.
The technique that I used was place the sand and clay on a slab create a shallow well in the center filled with water, then dive into it with the tiller. One person mixed and spread the mix while the other person piled up the clay using a sand shovel to scrape the mix off the slab. In short order I had a mix of heavy wet slop to which I added chopped straw.
Note: I use a cheap five horse power front tine tiller. I can crash the tiller into a pile of dirt and pull it apart as I move around the pile while my assistant recreates the pile with a shovel so I can tear it apart again.
The straw was coarsely chopped by laying flakes out next to a wall and running a lawn mower over the flakes to throw the flakes against the wall. Note: Wear an N-95 or better dust mask and eye protection when you do this. The chopping is to keep the straw from winding its self up on the tines of the tiller.”