Sub-Base
The sub-base of the paving forms the designed foundation and has several functions, which can be:
- To prevent the road surface being damaged by frost and thaw
- Improvement of the sub-soil
- To enlarge the stability of the road frame
- To raise the pavement
Because of the different functions of the sub-base, its thickness depends on a large number of factors, which are determined by:
- The depth of penetration by frost
- The capacity of drainage
- The desired height of the pavement
- The expected traffic load
- Grain distribution and grain sizes of the materials
- The water penetration of the sub-soil
- The load bearing capacity of the ground
- The ground water level
It is difficult to give general values for the thickness of the sub-base with so many variables to take into account, and the same applies to the content of the sub-base. The sub-base can be built up of one layer of the same material, but it can also be recommended to use more types of material for the sub-base.
Sub-bases should be constructed from one of the materials complying with the Department of Transport Specification for Road and Bridge Works (clause 802), namely Type 1 or 2. The thickness of the sub-base is set by the designer and determined by weighing the required use of the pavement with the bearing capacity of the soil using the CBR (Californian Bearing Ratio) values.