Capping layer
The SuDS Manual published by CIRIA in 2015 defines a capping layer as: ‘A layer of unbound aggregate of lower quality than sub-base, that is used to improve the performance of the foundation soils before laying the sub-base, and to protect the subgrade from damage by construction traffic.’
Where the sub-base is: ‘A layer of material on the subgrade that provides a foundation for a pavement surface.’
And subgrade is: ‘Material, usually natural in situ, but may include capping layer, below formation level of a pavement.’
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