When the California Bearing Ratio of subgrade is checked to be below a certain percentage (e.g. 5%), a capping layer is normally provided to reduce the effect of weak subgrade on the structural performance of the road.
It also provides a working platform for sub-base to be constructed on top in wet weather condition because the compaction of wet subgrade is difficult on site. The effect of interruption by wet weather can be reduced significantly and the progress of construction works would not be hindered. Most importantly, the cost of capping layers is low because the material can be readily obtained locally.
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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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