A good cob is made from 70-85% coarse sand (not beach sand) and 15-30% clay.
If you are building a big house, or need to build quickly, then you may consider making your cob with a tractor rather than by foot. I have not made cob by foot whilst making a full house but I was told by my previous manager that it takes about twice as long to make the cob as it does to use it. So by this ratio you can make cob for a full day and this will take you about half a day to use. When we used a tractor we can make enough mix in half a day to last us a week of building with about ten people.
So first, do your tests on your earth. Do your earth ratio tests first to see the sand and clay content of your earth. Next do your shrinkage test to see if your cob mix is good. Then make your dead men that you may need for doors, windows and shelves etc.
Make sure you have a good clean water source close to where you make your cob as you will need a lot of water. Go spend two days finding it in the woods if necessary
Cut your grass about a week before to give it time to dry in the sun. You are going to need a lot. It may look like a lot when you have cut it but when you are putting it in to the cob mix it is going to get eaten up and will go quickly.
Order your sand or clay (Whatever you need to add to make a nice 70% sand 30% clay mix) and tractor.
First of all the tractor needs to dump everything in a pile in your mixing area.
If you have added sand or clay to your earth to achieve the right quantities you need to mix it really well whilst it is still dry, make sure that when you turn it over you dont see patches of sand or clay and it is all a uniform colour.
Next add water. We used a lot. We had a 1000 litre tank and we emptied it with our mix which consisted of 5 big scoops of this tractor.
When the mix is well watered (but not too wet) then you need to add straw. You can do a quick test by making a ball in the palm of your hand, flatten it like a burger on your palm and if it sticks when you hold it upside down then you are on the right track.
The tractor needs to drive over the cob and spin his wheels to mix in the straw. Spin, roll and turn the cob till the straw is all intergrated. We had to do about 5 rounds of straw before we had enough. Make sure to spread the straw out to avoid having big clumps.
This mix took us about two hours and we should have enough for about a weeks work with up to ten people, so you can build really quickly with this technique.
Then take you cob to your site and get building. Happy cobbing.