I have started building a cob shelter in my garden on the country side. The structure is going to be 10 m2 and combines to the south side with a 5 m2 glasshouse. The floor level of the shelter will be about 40 cm below ground. This is just where the top soil layer turns into sand. This will give me a better foundation.
I started by laying down a row of 50 x 50 cm floor tiles as the bottom of the foundation. The tiles do though not provide a capillary insulation from below. Therefore I needed to place some big stones, natural rock, on top of the tiles in order to build my cob wall separated from the capillary forces from below.
It is a bit of a puzzle to place the stones on the flat tiles. Obviously, for a good foundation they need to get into a firm position. It is a matter of try and error.
But in the end I succeeded. Now I can walk over the stones without them moving. The rock foundation is now up to 50 cm broad, enough to build a 30 cm thick cob wall.
Finally I can start building in cob.
After a number of tests I decided to mix 2 parts of my clay rich soil fraction with 3 parts of sand. The amount of water needed to reach the proper consistency is very tricky, and has to be adjusted on the daily bases. Depending on wheather and rain fall.
After mixing by pounding the mass with bare feet on a tarp, I add straw to the mix. Not too much at a time. But the goal is to include as much straw as possible. Straw has many functions to the construction. It increases the tensile strength of the wall. But it also serves as an insulation.
Finally, I take portions of the cob to form to balls ready as building blocks.
30 cm thick, 40 cm high and about 3 m long is the result of 6 days of work. Not too bad for a start.