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What are the differences between capping layer and sub-base?

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For weak and poor quality subgrade, there is a need to increase the thickness of pavement to compensate for it. In order to save cost for sub-base which is relatively expensive, the concept of capping layer is
introduced in which capping materials of cheap but strong nature are used to cap the weak subgrade. In this way, the thickness of expensive subgrade is not required to be increased.

Crushed gravels and rockfill may be suitable options of capping material. In essence, capping material should be readily available at low cost. The capping layer not only serves to strengthen the subgrade, but also protect the road formation during construction. It serves as haul road for construction traffic during construction stage. Moreover, it protects the subgrade from weathering such as wetting.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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