An earthen floor is not the obvious choice for a building on wheels. Heavy building materials need proper foundations. This being a fun experimental project for TV is an exception. The weight of clay was kept to a minimum as its only 75mm thick. It sits on battened plywood between the floor joists.
The main components used here were sand and chalk, with clay and milk as binders. Clay shrinks as it dries and cracks; sand, grit and chalk reduce the shrinkage. You can of course just keep filling the cracks up with more clay until it looks ok.
In the past I have made earth ovens, cob buildings, wattle and daub walls and earthen floors. Each has a different requirement for clay/sand mix and different properties in terms of thermal mass. If you are going to try something like this it’s worth doing your research. Do experiment and break the rules – sometimes it’ll work and if it doesn’t you’ll know quite quickly! In which case, re-hydrate the clay, change the mix ratios and re-use. Generally: too much sand = a crumbly mix; too little sand = a cracking mix.
Build Your Own Earth Oven – Kiko Denzer
The Hand Sculpted House – Into Evans