The ground base is a layer of soil on which the underlying layer or supports under the logs are arranged.
If you look at it from the point of view of loads, then the ground base is an array of soil located under the foundation and perceiving the load from the entire building. The load transmitted by the foundation causes a stressed state in the base and deforms it.
When installing floors, if the original foundation is unpaved, it is necessary to find out what it is in its structure. How strong are the differences and in most cases, its alignment is required.