The rubble trench is critical for a low maintenance and long-lasting cob house. When built properly, the rubble trench provides both a solid foundation for the walls as well as a place for any water near the house to be conveyed away as quickly as possible before it gets to the cob. Although water does not necessarily hurt cob, if saturated enough it does make the cob soft and it can wash away or lose shape. Basically it would turn into a sticky mess until it could dry out again, not something you want inside your house.
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