What is a sub-base and how does it differ from an embankment?

In terrace and path construction, the sub-base is the lowest layer of the pavement construction, between the formation level and the slab surfacing, while the embankment is the layer below the formation level.

A sub-base must be level, load-bearing and not susceptible to frost. It takes the loads from the surfacing and distributes them so that the ground can carry them.

Technically, an embankment is the built-up earth body below the formation level. Undisturbed natural ground lies beneath it. In everyday usage, the whole construction below a slab surfacing is often called the embankment. Here, the term is used in this wider sense.

Three groups of sub-base are distinguished. Unbound sub-bases consist of sand, gravel, chippings or crushed stone. They are laid in a loose state and compacted as far as practicable. Compaction increases their load-bearing capacity without binding the aggregate.

Grid sub-bases consist of gravel grids, also known as grass grids, gravel cells or plastic cellular grids. Their chambers are filled with chippings. Metal open-grid grating is another type of grid sub-base and can be found, for example, on balconies.

Bound sub-bases form a rigid, composite load-bearing layer. They include hydraulically bound sub-bases such as concrete, bitumen-bound sub-bases such as asphalt, existing surfacings such as concrete block paving, tiles, natural stone slabs, screed or parquet, and waterproofing layers such as roofing membranes. Under road construction standards, only concrete and asphalt are classified as sub-bases. Existing surfacings and waterproofing layers are not. All three groups are treated as sub-bases here because they all support rubber slabs.