Subsoil is the general term for soil below the top soil.
It is unusual for a subsoil to consist of gravel, sand or clay by itself. The majority of subsoils are mixes of various soils. Gravel, sand and clay may be combined in a variety of proportions. To make a broad assumption of the behaviour of a particular soil under the load on foundations it is convenient to group soils such as gravel, sand and clay by reference to the size and nature of the particles.
The three broad groups are coarse grained non-cohesive, fine grained cohesive and organic. The nature and behaviour under the load on foundations of the soils in each group are similar.