A planum or earth subgrade is the name given to the level subsoil
In the case of a traffic area, the actual structure of the traffic area with frost protection layer, base course, bedding and pavement is located above the subgrade. In the case of a building, the subgrade is also below the actual foundation (cleanliness layer/foundation/floor slab). As a rule, the subgrade is an area of grown or filled/backfilled soil with defined technical properties, e.g. in terms of flatness, inclination, altitude and compaction.
Source: bauwion
