A drainage layer is laid, the depth of which corresponds to the type of traffic which will access the surface. This is generally classed as type3 reduced fines. This means 20mm stone mixed with smaller stones to create a firm foundation but the difference between this and type 2 is that the ‘dust’ content is removed which makes the sub-base as permeable as possible. It is also possible to use clean 10-20mm stone. Often a plate test is carried out to ascertain the type of surface to be excavated. This means a plate which can be 250-500mm round or square is placed on the ground and a load is placed on the plate (often a simple tyre jack and a vehicle) The depth that the plate sinks will show the type of ground conditions and the corresponding depth of sub-base. This is often classed as the CBR (California bearing rate), in layman’s terms it means the firmness of the soil to be excavated.
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