The sub-grade layer of a pavement is, essentially, the underlying

ground. It is also known as the "Formation Level”

Formation Level

Is the level at which excavation ceases and construction starts:

it's the lowest point of the pavement structure.

This process involves the removal of all surface vegetation along with the root zone, approximately 50mm in depth. The simplest form of sub-grade formation is a 'site-strip

It is bad practice to construct a pavement over surface

vegetation; as organic material, it will decompose, which could lead

to the formation of voids in the covering layers, which could, in

turn, lead to pavement settlement.

There are 2 common reasons for a reduction in level:

*to accommodate the finished levels of paving work

*to remove bad ground, top-soil, soft spots or other deleterious material

.

For most paving works, the required construction thickness is known in advance.

Bad ground, soft spots, and other construction works may need the

sub-grade to be reduced even further to overcome these

problems.

The picture opposite shows a typical site strip for an estate road. The ground level is established relative to finished floor levels in the properties to be built, while road formation level has been reduced down to a compact clay.

Capping layers are simply layers of a selected fill material, often a crushed rock, laid in layers not exceeding 225mm (normally) and thoroughly compacted before placing further layers, up to sub-base layer.

To keep things simple for contractors and design engineers, the table opposite has been prepared from the empirical data accumulated over decades of measurements.